15 Sept 1992 Version P2.0 Hayes Microcomputer Products, Inc. P.O. Box 105203 Atlanta, Georgia 30348 U.S.A. (c) 1990, 1992 Hayes Microcomputer Products, Inc. All rights reserved. 44-00012 AA H32 (BBS Version)Note: This version of the "Technical Reference for Hayes Modem Users" is a special version edited for bulletin-board downloading. As a plain ASCII character file, it therefore cannot contain any of the illustrations and graphic elements provided in the printed version.
Result Codes Description
------------------------------------------------------------------------
300 Smartmodem 300.
120 Smartmodem 1200, Smartmodem 1200B, Smartmodem 1200C,
Smartmodem 1200A.
240 Smartmodem 2400, Smartmodem 2400B, Smartmodem 2400P,
Smartmodem 2400Q, Smartmodem 2400M, V-series Smartmodem
2400, V-series Smartmodem 2400B, V-series Smartmodem
2400 Quad, V-series Smartmodem 2400M, Smartmodem OPTIMA
(TM) 24, Smartmodem OPTIMA 24 + FAX96,
ACCURA (TM) 2400 EC/FAX96.
960 Smartmodem 9600, V-series Smartmodem 9600, V-series
Smartmodem 9600B, V-series ULTRA (TM) Smartmodem 2400,
V-series ULTRA Smartmodem 9600, V-series ULTRA 24 with
Express 96, Smartmodem OPTIMA 9600, Smartmodem
OPTIMA 96 + FAX96, ACCURA 9600 EC/FAX96.
14400 V-series ULTRA Smartmodem 14400, Smartmodem OPTIMA 144,
Smartmodem OPTIMA 144 + FAX144, ACCURA 14400 EC/FAX144.
------------------------------------------------------------------------
Since the following tables identify features for Hayes modem products, the values included here and the number of strings are subject to change and expansion. The maximum length per string is 40 characters.
The I4 text is displayed in the following form:
a097800C204C264
bF60410000
r1031111111010000
r3000111010000000
surrounded by additional <CR> and <LF> characters as are required by the
V command option in effect. According to convention, all <CR> and <LF>
characters are defined by S3 and S4, respectively. The meanings of the
a, b, r1, and r3 strings currently defined are described below.
a D1 D2 D3 D4 D5 D6 D7 D8 D9 D10 D11 D12 D13 D14 D15 D16 D17 D18....
------------------------------------------------------------------------
D1, D2 Reserved
D3 Bit 3: Indicates modem based on SM1200FE commands
Bit 2: Indicates modem based on SM2400 commands
Bit 1: Indicates that modem supports &J commands
Bit 0: Indicates that modem supports &L commands
D4 Bit 3: Indicates that modem supports AutoSync (&Q4)
D5 Bit 3: Plug-in board modem product
Bit 2: Standalone modem product
Bit 1: Supports &H0 through &H4
Bit 0: Supports &I0 through &I4
D6 Bit 3: Reserved
Bit 2: Supports M4 command
Bit 1: Supports 32-bit FCS (frame check sequence
for AutoSync)
Bit 0: Supports AutoSync II
D7 Bit 3: Supports V.22 at 1200 bps (B0)
Bit 2: Supports 212A (B1)
Bit 1: Supports ASB in V.23 75 bps xmt/1200 bps rcv (B2)
Bit 0: Supports ASB in V.23 1200 bps xmt/75rcv (B3)
D8 Bit 3: Supports V.23 1200 bps xmt/75 bps rcv (B4)
Bit 2: Supports V.23 1200 half duplex (B5)
Bit 1: Supports V.23 75 bps xmt/1200 bps rcv (B10)
Bit 0: Supports V.23 75 bps xmt/600 bps rcv (B11)
D9 Bit 3: Supports V.21 100/300 bps (B15)
Bit 2: Supports ASB in V.23 75 bps xmt/600 bps rcv (B20)
Bit 1: Supports ASB in V.23 600 bps xmt/75 bps rcv (B21)
Bit 0: Supports V.23 600 bps xmt/75 bps rcv (B22)
D10 Bit 3: Supports V.23 600 bps half-duplex (B23)
Bit 2: Supports V.22bis at 2400 bps (B30)
Bit 1: Supports V.27ter at 2400 bps (B31)
Bit 0: Supports V.27ter at 4800 bps (B40)
D11 Bit 3: Supports V.32 full duplex at 4800 bps (B41)
Bit 2: Supports Express 96 at 4800 bps (B42)
Bit 1: Reserved
Bit 0: Supports V.29 half duplex at 4800 bps (B44)
D12 Bit 3: Supports V.29 half duplex at 7200 bps (B50)
Bit 2: Supports V.32 full duplex at 9600 bps (B60)
Bit 1: Supports Express 96 at 9600 bps (B61)
Bit 0: Reserved
D13 Bit 3: Supports V.29 half duplex at 9600 bps (B63)
Bit 2: Supports 103 110/300 bps (B16)
Bit 1: Supports V.32bis (7200/12000/14400 bps)
Bit 0: Reserved
------------------------------------------------------------------------
------------------------------------------------------------------------
D1, D2 Reserved
D14 0: Reserved
1: Personal Modem 1200
2: Personal Modem 2400
3: Personal Modem 1200 Plus
4: Personal Modem 2400 Plus
5: Pocket Edition
D15 Bit 3: Reserved
Bit 2: Supports $ dial modifier and NO DIALTONE result code.
Bit 1: Supports S95 Bit 5 for COMPRESSION: result code
Bit 0: Supports S95 Bit 4 for AUTOSTREAM: result code
D16 Bit 3: Supports S95 Bit 3 PROTOCOL: result code
Bit 2: Supports S95 Bit 2 CARRIER result code
Bit 1: Supports S95 Bit 1 CONNECT/ARQ result code
Bit 0: Supports S95 Bit 0 CONNECT XXXX (for DCE rate)
------------------------------------------------------------------------
b D1 D2 D3 D4 D5 D6 D7 D8 D9....
------------------------------------------------------------------------
D1 Bit 3: V.42 Alternate Protocol Supported
Bit 2: V.42 LAPM Protocol Supported
Bit 1: X.25 Protocol Supported
Bit 0: LAPB (Point-to-point error control) protocol supported
D2 Bit 3: Reserved (should be set to zero)
Bit 2 MNP (TM) Class 5 supported
Bit 1: V.42bis supported
Bit 0: Compression through the X.25 network supported
D3/D4 These combine to indicate the number of AutoStream
Type A channels which are supported. The formula
(D3*16 + D4) is used. Zero means AutoStream is not supported
D5 Bit 3: Reserved
Bit 2: Reserved
Bit 1: Adjustable startup (ASU) is supported
Bit 0: Negotiates adjustable startup
D6 Bit 3: &K5 not supported
Bit 2: &K5 supported
Bit 1: Unidirectional flow control
Bit 0: S105 N104 parameter supported
------------------------------------------------------------------------
The presence of the r2-string in the I4 result infers that S87 is supported and is required for autobauding at the higher speeds. To autobaud at one of the speeds indicated in the r2-string requires that S87 be set to match that speed.
The r3-string is issued if synchronous DTE speeds are supported. The map indicates which DTE rates are supported in synchronous modes. Each r- string begins with the lower case letter "r" and may be followed by as many as 39 additional characters, not counting <CR><LF>'s that will be used to separate them from other strings. Those 39 additional characters are limited to the ASCII-HEX alphabet 0-9 and A-F.
After the two lead-in characters (i.e., r
In r1, if the bit is filled with a 1, the corresponding DTE rate is
supported for sending AT commands to the device. A zero indicates that
DTE rate is not supported for AT commands.
In the r3 map, the bits simply indicate which DTE rates are supported
for synchronous operation on-line. This does not indicate, however, the
supported rates for synchronous V.25bis commands.
Split speeds if available are marked in the r1 string only when the
appropriate B command option has been selected. Future expansion of
these strings may include new speeds that are not in strict ascending
order.
The W command supports extended result codes in addition to
the CONNECT result code. When the modem is operated in error-control
mode (&Q5 is in effect), the W command and S95 together allow the user
to select these additional result codes:
&T19 in the modem works interactively with the DTE to confirm the RTS
and CTS signal. Only the DTE can detect CTS, and only the modem can
detect RTS. The modem cannot verify both signals without the active
involvement of the DTE software.
The modem performs the following algorithm in response to the AT&T19<CR>
command:
The following algorithm is used by Hayes Smartcom software in its use of
the &T19 test:
Note: For more predictable timing, do not combine other commands on the
same command line with &T19. Also, higher DTE port speeds improve the
timing resolution.
A modification to the software algorithm could be made to permit
software to detect which signal is missing from the cable. After issuing
the AT&T19 <CR> command, give the modem about 100ms to drop CTS. Then,
even if the CTS drop is not detected, proceed to toggle RTS anyway,
keeping in mind that the CTS part of the test has failed.
RTS/CTS flow control is preferred because it is "out of band", that is,
it does not consume any of the 256 serial codes from the user data.
XON/XOFF flow control uses DC1 (ASCII 17) and DC3 (ASCII 19). If RTS/CTS
is not supported, then Transparent Flow Control (&K5) is the next best
option (IF the software supports it) because it accomplishes Xon/Xoff
flow control without interfering with the user data (e.g. during binary
file transfers).
The table below shows the various formats in which modem responses can
be presented. Note that the "text" of the info-text may consist of
multiple lines of text. The formats depicted here only refer to the
<CR><LF> characters between info-texts and not within them.
The command response and call progress monitoring result codes are
defined below:
0 - OK
1 - CONNECT
2 - RING
3 - NO CARRIER
4 - ERROR
5 - CONNECT 1200
6 - NO DIALTONE
7 - BUSY
8 - NO ANSWER
10 - CONNECT 2400
11 - CONNECT 4800
12 - CONNECT 9600
13 - CONNECT 14400
14 - CONNECT 19200
22 - CONNECT 1200/75
23 - CONNECT 75/1200
24 - CONNECT 7200
25 - CONNECT 12000
28 - CONNECT 38400
Negotiation progress messages are reported in the following order:
40 - CARRIER 300
44 - CARRIER 1200/75
45 - CARRIER 75/1200
46 - CARRIER 1200
47 - CARRIER 2400
48 - CARRIER 4800
49 - CARRIER 7200
50 - CARRIER 9600
51 - CARRIER 12000
52 - CARRIER 14400
66 - COMPRESSION: CLASS 5
67 - COMPRESSION: V.42BIS
68 - COMPRESSION: ADC
69 - COMPRESSION: NONE
70 - PROTOCOL: NONE
71 - PROTOCOL: ERROR-CONTROL/ LAPB
72 - PROTOCOL: ERROR-CONTROL/ LAPB/HDX
73 - PROTOCOL: ERROR-CONTROL/LAPB/AFT
74 - PROTOCOL: X.25/LAPB
75 - PROTOCOL: X.25/LAPB/HDX
76 - PROTOCOL: X.25/LAPB/AFT
77 - PROTOCOL: LAP-M
78 - PROTOCOL: LAP-M/HDX
79 - PROTOCOL: LAP-M/AFT
80 - PROTOCOL: ALT
91 - AUTOSTREAM: LEVEL 1
92 - AUTOSTREAM: LEVEL 2
93 - AUTOSREAM LEVEL 3
Note: The bit values of S95 may be set to override some of the
characteristics of the Wn command. Setting any of the S95 bits to "1"
enables the corresponding result codes regardless of the Wn command in
effect. Changing the Wn command setting does not affect the value set
for this register.
Refer to the Wn and Xn commands for additional and related information.
Because the type of modem that will be present, certain assumptions can
be made regarding modem characteristics, such as maximum transmission
rate, support of AT commands or specific commands such as L or
X. If a more general application is being designed for an environment
about which assumptions cannot be made regarding type or brand of modem
that might be used, the software's first task should be to identify the
modem.
The I0 and I4 command options make this process simple. In the initial
versions of Smartmodem 1200, I0 returned the three digit response: 120.
Since then, responses have been extended for several groups identifying
modem supporting 2400 bps, 9600 bps, and other products. The I0 response
simply indicates the speed category of the modem.
The result of the I0 command is a number which identifies the category
of modem product. Some unique I0 values can be used to identify a unique
product which has specific behaviors. 960, for example, identifies a
Hayes product capable of 9600 bps, which has additional commands and
behaviors.
The I4 command provides a reliable means of communicating
specific features and modulation protocols to software. The responses to
the I4 command are strings delimited by <CR> and beginning with a
lowercase letter and typically followed by a hex-character bit-map. The
I0 and I4 responses currently defined are detailed in the description of
the I4 command in Chapter 1. The tables show the decoding of the hex-
map returned in the "a", and "b" bit-mapped strings. If I4 is used to
identify features of the modem, consider that new result strings are
periodically defined that may be returned in addition to those expected.
Fields once designated as "reserved" that held a zero may now have values
assigned. The strings themselves may also be of different lengths than
previously implemented.
In spite of the modifications to this command necessary to be current
with new modems, the I4 command is the best way for software to
determine the modem type and capability, if the guidelines below are
considered:
An example I4 command and response is shown below:
Software can be confused by a command echo. For example, if the
following command were sent with echo on (E1) and numeric results (V0)
on:
Turning off echo mode (E0) in the initial setup string would solve this
problem; however, do not end that command with any digits (simply E).
Another shortcoming of numeric results is that the software must
anticipate all possible responses. This requires updating controller
software whenever new result codes are added. For example, suppose a
CONNECT 115200 result were added with a numeric value of 31. If verbose
results were used instead, and the controller directed to interpret the
number after the CONNECT result as simply the connection speed in bits
per second, no changes to the driver are necessitated by the new result
code. If, however, numeric result codes were used, the result code 31
must be added to the table, and the controller modified to interpret it
appropriately.
As characters are received, they should be processed through a state
machine providing the functionality of the following one. This state
machine recognizes strings surrounded by <CR><LF> characters and stores
the string in a character array. <CR><LF> are defined by S3 and S4.
Once a result code string is returned, it can be compared against the
known result code strings. Some strings may incorporate wild-card
suffixes. For example CONNECT followed by any numeric value indicates a
successful connection at the indicated transmission rate. Even if a
result such as CONNECT 38400 is not anticipated, if the controller has
been coded for wild-card recognition, the controller will be capable of
interpreting such responses correctly. This practice also facilitates
interpretation of connection failed messages that are preceded by NO
followed by any other character string such as DIALTONE, CARRIER, or
ANSWER.
Setup commands can be issued at the highest transmission rate the modem
supports as determined from the identification process or it may be
fixed at a certain value if the modem is not identified.
Some commands will always be overridden by the controller in order to
ensure its proper functioning. Other command options should either
default to the factory setting, or simply act as the "transfer agent"
for the commands specified by the user. Menus and dialogs can be
provided to prompt the user for specific activities; the program can
then interpret these requests and configure the modem accordingly, as
Hayes Smartcom products do, or provide the user opportunity to enter AT
command strings.
Commands frequently set by a modem controller:
Recommendation for software design based on using pre-existing user
settings:
Once this setup command has been sent, and the OK response returned, the
controller can continue to the originate or answer processing.
If user-programmed settings are included in additional setup strings, or
the user is permitted to enter AT setup strings, the software should
anticipate ERROR result codes. If an ERROR is returned in response to
such a command, the result does not have to be reported to the user, but
the controller should not be prevented from continuing in either case.
Many times a connection can be made even though some setting is in error
or is inappropriate for the class of modem being addressed.
The optimum fallback control strategy depends on which Hayes error-
control modem you have. The fallback behavior is determined by S36, S46,
S48, and &Q5.
There have been two improvements in the V-series, OPTIMA and ACCURA EC
modems that affect S36: The first added &Q6 Asynchronous Speed Buffering
(ASB); ASB provides the same fixed speed DTE interface and local flow control
as in the error control mode but without the error control or remote
flow control. The second improvement added V.42 with its Alternate
Protocol (MNP).
Previously, S36 only consisted of one bit, Bit 0,
which permitted the user to decide whether the modem would hang up or
not if it did not succeed in negotiating an error-controlled connection.
The factory default for S36 was 1, which meant fall back to direct async
rather than hang up if an error control protocol is not established in
&Q5 mode.
When &Q6 (ASB) was added, a fall back option for &Q5 mode, which can be
controlled with S36, was also added. If S36 Bit 1 is set, and bit 0 is
also set, the modem will fall back to ASB instead of direct async if no
error control protocol is established in &Q5 mode. The factory default
for S36 remained 1.
When V.42 was added, it added another fall back option, MNP, to S36. If
S36 Bit 2 is set, then MNP will be attempted if the primary protocol
selected in S48 is not established. Bit 2 is evaluated before Bits 0,
then Bit 1. If MNP is established as the protocol, then Bits 0 and 1 are
ignored. The factory default became 5 and later was changed to 7 to take
more advantage of ASB.
Now that we know that there are three versions of S36, we may need a way
to tell which version our software is controlling. First, verify that it
is a Hayes error-control modem. This is done by issuing an I4 command
and looking for the existence of a "b-string". A b-string begins with a
lower case letter "b" and is followed by several upper case letters or
numbers forming a hex value.
The easiest way for software to identify whether a Hayes modem
supports Asynchronous Speed Buffering (also known as ASB, Buffered
Async, or Normal) is to issue the following AT command, AT &Q6 &Q5
<CR>. If the result code is OK then &Q6 is supported, and ASB is also
supported as a fall back. If the result code is ERROR, then &Q6 is not
supported. If this is the case, then software should be prepared to fall
back to a direct async connection.
The easiest way for software to identify whether a Hayes modem
supports V.42 (and MNP) is to decode the first character after the
leading "b" in the b-string of the I4 ID command response. The
characters following the "b" in that line are "ASCII-hex" (0-9, A-F),
which decode into 4 bits of ID code (3-0). If Bit 3 is set, then MNP is
supported; Bit 2 is V.42.
Armed with the knowledge of which S36 bits are supported, the software
may now safely configure the modem and properly anticipate the fall-back
action.
It should be safe to set S36 bit 2 to enable V.42 (and MNP) even if
those protocols are not supported in the modem.
If the modem does not support ASB, then software should be prepared to
follow the CONNECT XXXXX speed result code and change the DTE port speed
to match the indicated line speed of the direct connection when no error
controlled connection was negotiated.
If software will not change port speeds in response to the CONNECT
message, then when software has identified a 'pre-ASB' modem,
it should set S36 to 0 or 4 so that if no protocol is negotiated, the
modem will hang up.
The following table shows the order in which the bits of S36 are
evaluated: (Remember, these steps only occur after the S46/S48
selections have failed to make a LAPBased error controlled connection in
&Q5 mode.)
Type 1: Hayes Modem Operation in &Q5 Communications Mode
Applies to V-series Smartmodem 2400 up to and including Version 1.3 and
V-series Smartmodem 9600 up to and including Version 1.4.
Type 2: Hayes Modem Operation in &Q5 Communications Mode
Applies to V-series Smartmodem 2400 Version 1.4 and V-series Smartmodem
9600 Versions 1.5 and 1.6.
Type 3: Hayes Modem Operation in &Q5 Communications Mode
Applies to all V-series products newer than Types 1 and 2 and all OPTIMA
and ACCURA EC products.
If you recognize any numeric value for the baud rate after the CONNECT
result, you will have a much more robust controller able to handle many
situations. You can use the PROTOCOL result to determine if the flow
control requested by the &K command is in effect.
Note: If you want to manually dial a call and then have the modem connect,
enter the command string ATX1D (X1 disables dial tone detection) when you
hear the dial tone.
If you set S0, you may want to set it back to zero after your controller
finishes the call to prevent inadvertent answering when your software is
not running. By setting S0 to the number of rings you desire before the
modem answers, you utilize the ring detection technology already built
into the modem.
You should not use the A command to answer after counting RING results
because the command may collide with another RING result from the modem
and be missed. The RING results may be generated in pairs depending on
the ringing cadence of the phone system.
If you use CD, you do not know when the modem has given up waiting for
the carrier, or why. If the line is busy, you may want to re-try the
dial operation. If there is no dial tone, the user needs to know this.
Smartcom products send AT<CR> to abort an in-progress connect command.
This elicits a result code regardless of whether the modem were off-hook
or not. If the modem was off-hook attempting to connect, this will abort
the connect operation and return NO CARRIER. If the modem was on-hook in
command mode, this simply returns <CR><LF>OK<CR><LF>.
However, this is the most restrictive and risky choice. It requires a
properly wired cable and support of &C1 by the modem's command set or
proper DIP-switch settings.
Typically, at the low-level of the program all received data is
retrieved through one subroutine. This subroutine can be augmented or
layered to provide the service needed. As data passes through, the last
fourteen characters are buffered, typically in a circular buffer. If
more data passes in each call, only the last fourteen need to be copied.
At a time when the processor is free, such as after 100 ms of idle time
or the receive routine has returned no data for 30 to 100 calls, then
the buffer is compared against the <CR><LF>NO CARRIER<CR><LF> result
code. If a match is found, the carrier lost event is triggered.
By only checking when there is idle time, or after no data has been
received for a while, you reduce the CPU overhead and ensure that the
modem is not falsely triggered when the string is imbedded in an actual
data stream.
You can also modify the linefeed character by using S4 to a different
value such as S4=31 to make the result code sequence more unique. This
action, however, affects other result codes generated by the modem.
The controller software must be sure to wait the required guard time
before sending the escape sequence characters. Your controller may have
been called just after data was transmitted and, without the delay, your
characters will just be sent without triggering the escape sequence
recognition process. It is important that the serial transmitter be
permitted to be idle for the escape sequence guard time, plus a few
extra milliseconds to allow for error, before sending characters.
For example, if S12=10, a delay of at least 200 milliseconds is
required before sending the escape sequence characters. After sending
the escape sequence characters, the OK result will be received after
another 200 millisecond wait. This completes the escape sequence process
in slightly over 400 milliseconds. If S12=50 (factory setting) is used,
one full second must pass before the characters can be sent, then
another second delay must transpire prior to the OK result. This
completes the process in slightly over two seconds. For this reason, it
is recommended that S12=10 be issued to speed up this process.
A delay slightly longer than that stored in S12 should be used to allow
for errors in the system clock as well as in the modem clock. 100ms is
an adequate safety margin.
The minimum the controller should do when through with the modem is
issue a Z command option to ensure the modem is restored to its power-up
state.
Under DOS, the INT 1C timer tick produces an interrupt every 55
milliseconds. An ISR can be installed on this interrupt to add 55 to a
long integer every time it is called. This will provide a millisecond
counter.
On the Macintosh, the "Tick Count" function will return the number of
vertical-retrace ticks since computer power-up. Each tick represents
1/60 of a second.
This technique is independent of processor speed. A faster processor may
make thousands of trips through the loop, where a slower one would only
make a few hundred. Any anticipated result code would arrive within that
two-second real-time window.
Care should be given to considering when to start the timing loop. If an
AT command string is sent, then a loop executed, the time interval may
also include the time required to send the AT command (if data is
buffered and sent by an interrupt service routine).
At 300 bps, where each character takes 33ms just to transmit (10/300), a
40 character AT command would take over a second to transmit. This means
a two second loop spends more than half of its time waiting for the AT
command process to complete, leaving only a fraction of a second for the
modem to respond with the result (again at 33ms per character).
One way to avoid this is to wait until all data has been transmitted by
an ISR before entering the result code scan loop. Alternatively, more
time can be provided for loops to process results. Another option is to
measure idle time rather then elapsed time.
A timeout may also occur when the software receives a result code it
does not recognize. The software may continue to wait until it receives
a code it does recognize. If this is the case, the controller should
proceed as if an ERROR response were received. The only instance in
which it is not prudent to continue is when a connect (D, A, or S0)
command was issued.
Before implementing a timeout, the advantages, if any, to this level of
program interruption should be considered. For example, if the program
times out from a dial command in one minute when it may take two minutes
to complete the call, the timeout defeats the purpose of the command.
The modem always responds with a result code, whether BUSY, NO CARRIER,
or CONNECT, after some length of time.
Idle time is the time since data was received. Elapsed time is the time
since the software started looking for the result. Idle time can be
measured by resetting the timeout clock each time the software receives
a character. Rather than exiting the loop after two seconds of elapsed
time, the logic changes to exit after no further data has been received
for two seconds.
Hayes modems support the industry standard "RTS/CTS" hardware
flow control (invoked by the &K3 command) and "XON/XOFF" character-based
flow control (&K4) schemes. However, in some situations, it is not
possible to use either of these. For example, hardware flow control
cannot be used if the DTE serial port does not support the RTS and/or
CTS hardware signals, if corresponding conductors are not present in the
interface cable, or if intermediate equipment does not properly transfer
the RTS and CTS signals. XON/XOFF flow control cannot be used if the XON
and XOFF characters appear in the user data or in the control
information associated with the file transfer protocol in use, since
this would interfere with the use of these characters for flow control,
resulting in failure of data transfer and possibly locking up the
interface.
Hayes addressed these problems in the design of V-series, OPTIMA and
ACCURA EC modems. Desiring to provide a complete solution for all users,
Hayes provided a third flow control scheme, which accommodates systems
which cannot use RTS/CTS but which must transfer data containing the
XON and XOFF characters. This scheme, known as Transparent XON/XOFF
Local Flow Control (invoked by the &K5 command), uses only the Transmit
Data and Receive Data circuits, yet provides for the transfer of all
256 possible 8-bit characters over the interface without interfering
with the flow control scheme.
Transparent flow control can be viewed as a layered process, organized
as follows. Note that this process is duplicated in the reverse
direction (operates in both directions simultaneously).
If the software wants the modem to suspend delivering data, it inserts
an XOFF ($13 at current parity setting) in the transmitted data stream
at any arbitrary point, which may be between a DLE ($10) and the
following transparentized character. The XOFF should be issued BEFORE
the buffer is completely full, since there may be a lag of several
characters before the modem is able to react to the XOFF and suspend
delivery. To resume delivery of data, the software inserts an XON ($11
at current parity setting) at any arbitrary point. Characters inserted
for flow control purposes are NOT passed through the transparency
algorithm defined above.
If the character received is $13 or $93 (XOFF with either parity), it is
interpreted (regardless of the setting of the TSIP-FLAG) as a request
from the modem for the DTE to suspend transmission of data to the modem.
The receiver portion of the software must communicate this request to
the transmitter portion of the software in a timely manner, since the
modem has a limited amount of buffer space to allow for additional
characters to be received after it sends XOFF (about 64 characters
maximum). The XOFF character is discarded from the received data stream.
The TSIP-FLAG is not changed.
If the character received is $11 or $91 (XON with either parity), it is
interpreted (regardless of the setting of the TSIP-FLAG) as a request
from the modem for the DTE to resume transmission of data to the modem,
if any is available. The XON character is discarded from the received
data stream. The TSIP-FLAG is not changed.
If the character is neither an XON or and XOFF, the following steps are
performed:
DTE Rate Bit Map for r1, r2 and r3 Strings
Character Bit # DTE Rate
------------------------------------------------------------------------
3: 1 bit 0 45.45 bps
2 bit 1 50
4 bit 2 75
8 bit 3 75/600 (xmt is 75, rcv is 600)
4: 1 bit 4 75/1200
2 bit 5 110
4 bit 6 134.5
8 bit 7 50
5: 1 bit 8 300
2 bit 9 450
4 bit 10 600
8 bit 11 600/75
6: 1 bit 12 1200
2 bit 13 1200/75
4 bit 14 1800
8 bit 15 2000
7: 1 bit 16 2400
2 bit 17 3000
4 bit 18 3600
8 bit 19 4200
8: 1 bit 20 4800
2 bit 21 5400
4 bit 22 6000
8 bit 23 6600
9: 1 bit 24 7200
2 bit 25 7800
4 bit 26 8400
8 bit 27 9000
10: 1 bit 28 9600
2 bit 29 12000
4 bit 30 14400
8 bit 31 16800
11: 1 bit 32 19200
2 bit 33 21600
4 bit 34 24000
8 bit 35 26400
12: 1 bit 36 28800
2 bit 37 31200
4 bit 38 33600
8 bit 39 36000
13: 1 bit 40 38400
2 bit 41 43200
4 bit 42 48000
8 bit 43 52800
14: 1 bit 44 56000
2 bit 45 57600
4 bit 46 62400
8 bit 47 64000
15: 1 bit 48 67200
2 bit 49 72000
4 bit 50 76800
8 bit 51 81600
16: 1 bit 52 86400
2 bit 53 91200
4 bit 54 96000
8 bit 55 100800
17: 1 bit 56 105600
2 bit 57 110400
4 bit 58 115200
8 bit 59 reserved ("0")
------------------------------------------------------------------------
W - Negotiation Progress Message Selection
The W command works in conjunction with S95 (where supported) to
determine which result codes will be used to describe the type of
connection and protocol, etc., that resulted from handshaking and
negotiation.
CARRIER
PROTOCOL
AUTOSTREAM
COMPRESSION
CONNECT
Any result codes enabled by the W command and S95 will be generated in
the order indicated above. If AutoStream is not being used, no
AUTOSTREAM result code is returned. Result codes not enabled when the W
command option is in effect may be turned on by setting certain bits in
S95. The W command options below are available when S95 is configured
for any setting other than the factory-setting of 0.
------------------------------------------------------------------------
W0 CONNECT result code reports DTE speed, and if S95=0, then disable
all extended result codes.
W1 CONNECT result code reports DTE speed, and if S95=0, then enable
the CARRIER and PROTOCOL extended result codes.
W2 CONNECT result code reports DCE speed, and if S95=0, then disable
all extended result codes.
------------------------------------------------------------------------
Refer to the S95 bit map description in the next section. Note that
selecting W0 and setting S95=12 is the same as selecting W1; and that
selecting W0 and setting S95=1 is the same as selecting W2. S95 cannot
be configured to force W2 to report DTE speed in the CONNECT result
code; there is no setting of S95 that will force W1 not to produce the
CARRIER and PROTOCOL result codes. S95 extends the functionality of the
W command. The W command with S95=0 (factory setting) maintains
backwards compatibility with previous Hayes Products. However,
selecting W0 and setting S95 as required allows you to tailor result
code characteristics to your own requirements.
Z - Soft Reset Command
The modem can be reset by issuing the Z command. The command tells the
modem to go on hook and restore the selected stored profile. Any non-
storable parameters previously set by commands are returned to their
factory settings. The modem aborts execution of all commands following
the Z command on the same command line. Subsequent commands on the same
line are ignored. Refer to the &W command for description of which modem
parameters are included in a stored profile. Z0 recalls stored user
profile 0, stored with &W0; Z1 recalls stored user profile 1, stored
with &W1.
------------------------------------------------------------------------
Z0 Recall stored profile 0
Z1 Recall stored profile 1
------------------------------------------------------------------------
&T19 - Perform RTS/CTS Cable Test
Software can use &T19 to help determine whether the DTE-to-Modem cable
supports the RTS and CTS signals necessary for hardware flow control,
which is selected by the &K3 command.
Note: By itself, the modem cannot verify either signal. The DTE must
toggle RTS during the test in order for the modem to be able to detect a
change. Also, the DTE should look for CTS to drop during the test to
verify it is connected.
Note: Keep in mind that the result code may arrive while you are still
in Steps 2 and 3 above.
1.2 Result Code Listing
This section defines the result codes returned by Hayes modems in
response to commands.
------------------------------------------------------------------------
V0 V1
Information Text text <CR><LF>
<CR><LF> text
<CR><LF>
Result Codes numeric code <CR><LF>
<CR> verbose code
<CR><LF>
------------------------------------------------------------------------
1.2.1 Command Response and Call Progress Monitoring
This set of result codes includes responses to commands and call
progress monitoring responses. They are available to all modems within
the capabilities of the modem. For example, the result code CONNECT 9600
is not available to Smartmodem 2400. The factory setting for all high-
speed modems enables the extended set of call progress monitoring (X4).
When set up in this way, the modem performs and reports full call
progress monitoring (RING, NO CARRIER, NO DIALTONE, and BUSY). It also
indicates the speed of the connection (CONNECT 1200 as opposed to simply
CONNECT). The factory setting for Smartmodem 300, Smartmodem 1200, and
all others whose highest speed is 1200 bps is basic call progress
monitoring (X0).
This result code indicates that a command or command string was
executed. Note that if more than one command were included on a line and
an ERROR result code received, this means that one or more of the
commands was not processed. If one or more were executed properly, but
even one was invalid, no OK will be issued, only the ERROR.
This result code indicates a connection was made between the DTE and the
modem. If X4 (extended set of call progress monitoring) were selected,
the code indicates that a connection from at 0 to 300 bps was made.
However, if X0 (basic set of call progress monitoring) were selected,
the connection could be 0-300, 1200, 1200/75, 75/1200, 2400, 4800, 7200,
9600, 14400, 19200, or 38400 bps. If the modem is not operating in
error-control or ASB mode, this is the same as the line speed. See other
CONNECT messages and CARRIER messages.
This result code indicates the modem has detected a ring signal. No
distinction can be made as to whether this is a voice call, a modem
call, a fax call, or other type.
This result code indicates that no carrier signal was detected, or that
the signal was lost. This is the response the modem will give when no
connection is made; see CONNECT result code. The modem will also return
this message when the connection is broken, either intentionally as when
the hangup process completes, or if line difficulties break the
connection.
This result code indicates that an invalid command was issued, or that
there was an error in the command line. For example, if the command line
exceeds the character limit for your modem, this result code will be
returned. See your user documentation to determine the character limit for
your modem. This result code is also returned in response to the I1 command
requesting a ROM checksum, if the modem detects an error in the computation.
This result code indicates a connection has been established at 1200,
1200/75 or 75/1200 bps between the modem and the DTE. If the modem is
not operating in error-control mode, this is the same as the line speed.
This result code is disabled by X0. Only CONNECT is reported.
This result code indicates that no dial tone was detected when the modem
went off hook. Dial tone detection and this result code are enabled by
X2 or X4, or the W dial modifier.
This result code indicates that the modem detected a busy signal when it
attempted to connect with the modem at the number dialed. Busy signal
detection and this result code are enabled by X3 or X4.
This result code indicates no silence was detected when dialing a system
not providing a dial tone. Enabled by the @ dial modifier.
This result code indicates a connection has been established at 2400 bps
between the modem and the DTE. If the modem is not operating in error-
control mode, this is the same as the line speed. This result code is
disabled by X0. Only CONNECT is reported.
This result code indicates a connection has been established at 4800 bps
between the modem and the DTE. This result code is disabled by X0.
This result code indicates a connection has been established at 9600 bps
between the modem and the DTE. This result code is disabled by X0.
This result code indicates a connection has been established at 14400
bps between the modem and the DTE. This result code is disabled by X0.
This result code indicates a connection has been established at 19200
bps between the modem and the DTE. This result code is disabled by X0.
This result code indicates a connection has been established at 1200 bps
when transmitting data and 75 bps when receiving data between the modem
and the DTE.
This result code indicates a connection has been established at 75 bps
when transmitting data and 1200 bps when receiving data between the
modem and the DTE.
This result code indicates a connection has been established at 7200 bps
between the modem and the DTE. This result code is disabled by X0.
This result code indicates a connection has been established at 12000
bps between the modem and the DTE. This result code is disabled by X0.
This result code indicates a connection has been established at 38400
bps between the modem and the DTE. This result code is disabled by X0.
1.2.2 Negotiation Progress Messages
Hayes products report special result codes during error-control
negotiation. Whether or not these messages are displayed is selected
with the W command (not to be confused with the W dial modifier). The
factory setting is messages disabled (W0) to avoid conflict with
software programs that do not support this additional level of call
progress monitoring.
CARRIER
PROTOCOL
AUTOSTREAM
COMPRESSION
CONNECT
If AutoStream is not used, no message is reported.
This message indicates that a carrier signal has been detected at 300
bps (modem-to-modem line speed).
This message indicates that a carrier signal has been detected at 1200
bps when transmitting and at 75 when receiving (modem-to-modem line
speed).
This message indicates that a carrier signal has been detected at 75 bps
when transmitting and at 1200 bps when receiving (modem-to-modem line
speed).
This message indicates that a carrier signal has been detected at 1200
bps (modem-to-modem line speed).
This message indicates that a carrier signal has been detected at 2400
bps (modem-to-modem line speed).
This message indicates that a carrier signal has been detected at 4800
bps (modem-to-modem line speed).
This message indicates that a carrier signal has been detected at 7200
bps (modem-to-modem line speed).
This message indicates that a carrier signal has been detected at 9600
bps (modem-to-modem line speed).
This message indicates that a carrier signal has been detected at 12000
bps (modem-to-modem line speed).
This message indicates that a carrier signal has been detected at 14400
bps (modem-to-modem line speed).
This message indicates that data compression using MNP Class 5 has been
negotiated for the connection.
This message indicates that data compression using CCITT V.42bis has
been negotiated for the connection.
This message indicates that data compression using Hayes Adaptive Data
Compression has been negotiated for the connection.
This message indicates that data compression was not negotiated for the
connection.
This message indicates that no protocol was negotiated for the
connection. A standard asynchronous connection was made.
This message indicates that an error-control connection was negotiated
with LAPB protocol.
This message indicates that a half-duplex error-control connection was
negotiated with LAPB protocol.
This message indicates that an error-control connection was negotiated
using the Hayes Asynchronous Framing Technique. This protocol is used
for connections between modems such as Smartmodem 1200 that do not
communicate synchronously across the telephone line. AFT enables an
error-control protocol to be used.
This message indicates that an error-control connection using the X.25
protocol was established with a carrier speed of 1200, 2400, 4800, or
9600 bps.
This message indicates that a half-duplex error-control connection using
the X.25 protocol was established with a carrier speed of 4800 or 9600
bps.
This message indicates that an asynchronous error-control connection
using the X.25 protocol was established with a carrier speed of 1200
bps. The Hayes Asynchronous Framing Technique was used.
This message indicates that an error-control connection using the V.42
LAPM protocol was established.
This message indicates that a half-duplex error-control connection using
the V.42 LAPM protocol was established.
This message indicates that an asynchronous error-control connection
using the V.42 LAPM protocol was established with a carrier speed of
1200 bps. The Hayes Asynchronous Framing Technique was used.
This message indicates that an errorge indicates that an error-control
connection using the V.42 LAPM alternative protocol was established.
This protocol is MNP Classes 2, 3, and 4 compatible.
This message indicates that Hayes AutoStream Level 1 has been negotiated
for the connection. This technique provides for multiplexing of multiple
virtual channels.
This message indicates that Hayes AutoStream Level 2 has been negotiated
for the connectionsage indicates that Hayes AutoStream Level 2 has been
negotiated for the connection. This technique provides for multiplexing
of multiple virtual channels, with transparent control of one PAD (non-
simultaneous). Level 3 has been negotiated for the connection. This
technique provides for multiplexing of multiple virtual channels, with
transparent control of all PADs (simultaneous).
This message indicates that Hayes Autostream Level 3 has been negoitiated
for the connection. This technique provides for multiplexing of multiple
virtual channels, with transparent control of all PADs (simultaneous).
S95 Negotiation Message Options
S95 enables various result codes that indicate the sequence of events in
the establishment of an error-control connection. This register does not
affect the way in which the modem negotiates the connection; it merely
enables message options. The factory setting for this register is value
0, no bits selected. To enable any combination of the bits, add the
value(s) to the right of the bit number and set the register to this
sum.
Bit Value Explanation
------------------------------------------------------------------------
0 1 Verbose CONNECT result code indicates the DCE speed
(rather than DTE speed). Numeric result codes are also
different when CONNECT reports DCE speed.
1 2 Append "/ARQ" to CONNECT result code when an
error-control connection is made.
2 4 Add CARRIER messages
3 8 Add PROTOCOL messages
4 16 Add AUTOSTREAM messages
5 32 Add COMPRESSION messages
------------------------------------------------------------------------
For example, if you want to add the compression result code (with W1
selected), you would select bit 5 (value of 32). The command line
ATS95=32J<CR> will then enable the COMPRESSION negotiation messages.
APPENDIX D: APPLICATION SUGGESTIONS
This appendix offers suggestions for developing applications software
using the Hayes Standard AT Command Set. The techniques described apply
to Hayes modems in general except where specifically indicated. Although
provided here, this information is intended for experienced programmers
who want assistance in modem application development.
D.1 Modem Identification
The initial concern for most communications software is modem
identification. Before the software determines the type of modem (e.g.,
is it a Hayes modem, a high-speed modem, what features does it support -
error-control or compression?). By limiting the AT command controller
portion of the software to work with a known set of modems, you can
limit the complexity of your software.
AT E0 V1 Q0 S0=0 I4 <CR>
response:
<CR><LF>a087840C004424<CR><LF>
<CR><LF>bF60410000<CR><LF>
<CR><LF>cUS<CR><LF> J
<CR><LF>m0000000001001FFFF<CR><LF>
<CR><LF>OK<CR><LF>
Note: Each I4 result is surrounded by <CR><LF>; not all responses are
hex-strings; and some responses may not be expected at all.
D.2 Result Code Recognition
Modems that are equivalent to Hayes modems support verbose and numeric
forms of result codes. Unless echo may be a problem and you will be
installing the controller in a limited-growth environment, verbose
results rather than numeric results are preferred. Numeric result codes
were originally intended to make it easier for software to control the
modem, but there are two primary reasons they should not be used:
AT ... S9=20 <CR>
The resulting data, echoed by the modem, would be followed by the
numeric result code zero, meaning OK:
AT ... S9=20<CR>0 <CR>
Software may become confused by seeing a 0<CR> result which is actually
part of the command echo, then another 0<CR> which is the numeric
result. A program can become unsynchronized with the command processor
in the modem.
D.3 Modem Preparation
Once the modem has been identified, the controller can continue to
program any registers or user-defined values into the modem necessary
prior to initiating the connection process. Typically, the setup
operation is separated from the connection processing because it is
performed independently of whether the call establishment will be in the
originating or answering mode.
D.3.1 Reset
Before issuing any other commands to the modem, it is advisable to issue
a Z or &F command to the modem before the identification or setup
process. No specific response should be anticipated. The modem may be
set up to return numeric, or no result codes. If a reset will be used,
the following points should be considered:
After the modem is reset, the first setup string (e.g., verbose rather
than numeric result codes) should be issued, then the identification
command.
D.3.2 Setup
Software should generally provide some modem setup. However, the
software can be written to rely on modem configuration via a stored
profile recalled on reset, or by DIP switches set depending on the
product. In this case, any unique settings must have been set up prior
to running the software, and all the program does is send the Z command
to recall the desired profile. Even more basically, software can assume
the modem is in the power-up state. However, unless the software will be
used within a very predictable environment, these assumptions may result
in failures with the controller software.
E0 Turn off echo mode to avoid having command echoes pass
through the result code scanner.
&F Recall the factory profile.
Q1 Enable result codes to ensure that commands are being
processed, and to synchronize with the modem command
processor (except for synchronous communications where result
codes may cause the DTE confusion).
V0 or V1 Use either numeric result codes or verbose (recommended)
result codes.
S0=0 Disable auto-answer during the setup process to avoid
inadvertent disruption by an incoming call.
H0 Ensure modem is on hook before continuing to the answer or
originate process.
S12=10 Set the escape sequence guard time to 200ms to hasten the
escape for hang-up process. Also reduces the probability of
inadvertent user escapes.
S2=* Change the escape sequence character for two reasons: To
avoid inadvertent user escapes, and to provide different
escape sequence characters for answer and originate ends.
This prevents inadvertent escaping when data is echoed.
S4=* Modify the linefeed character to make the <CR><LF>NO
CARRIER<CR><LF> result code more unique if you scan for it
to detect carrier loss.
S95=60 Enables the result codes which will provide the maximum
amount of information about the connection when it is
established.
Two typical setup sequences using these recommendations are shown below:
AT E Q V S0=0 H S12=10 S2=28 S4=31 S95=60 <CR>
Recommendation for software design based on starting from a known
factory setting:
AT &F S2=28 S4=31 S12=10 S95=60
Note: Where the zero suffix is used, it may be omitted from a command.
Spaces are shown above for readability, but the use of spaces between
commands is not recommended.
D.3.3 Establishing the Desired Connection and Fallback Strategy
(S36 Developers' Tips)
S36 determines which fallback action will occur if the protocols and
procedures set by S46 and S48 do not produce a LAP-based (LAPB) error-
controlled connection.
------------------------------------------------------------------------
S36 Bit 7-3=0 reserved
First Bit 2 (4) If set, means try MNP protocol; reset means
don't use MNP.
Third Bit 1 (2) If set, means fall back to ASB; reset means
direct async.
Second Bit 0 (1) If set, means fall back based on Bit 1;
reset means hang up.
------------------------------------------------------------------------
The following table shows the meaning of each setting:
------------------------------------------------------------------------
S36=7 Try MNP, then fall back to ASB.
S36=6 Try MNP, then hang up.
S36=5 Try MNP, then fall back to direct async.
S36=4 Try MNP, then hang up.
S36=3 Don't try MNP; fall back to ASB.
S36=2 Don't try MNP; hang up.
S36=1 Don't try MNP; fall back to direct async.
S36=0 Don't try MNP; hang up.
------------------------------------------------------------------------
There are three types of fallback stratagies which Hayes products use.
------------------------------------------------------------------------
ATI3 Response contains...
V-SERIES SMARTMODEM 2400 V-SERIES SMARTMODEM 9600
Type 1 04-00005-10 04-00015-10
04-00005-11 04-00015-11
04-00005-12 04-00015-12
04-00005-13 04-00015-13
04-00015-14
Type 2 04-00005-14 04-00015-15
04-00015-16
Type 3 Does not contain any Does not contain any
of the above numbers any of the above numbers
------------------------------------------------------------------------
D.4 Connect Processing
Once the setup operation has been completed, the commands to establish
the connection can be issued. The instruction can be either to originate
(using the D command), or to answer (using the A or S0 commands).
D.4.1 Originating a Call
If the D command is issued with the desired phone number, several
possible result codes can be returned. The list below outlines some
results to expect:
Result Code Meaning
------------------------------------------------------------------------
NO CARRIER Connection failed
NO ANSWER No response to '@' dial modifier
NO DIALTONE No dial tone in X4 mode
NO ____ Connection failed for some other reason
BUSY Busy signal detected
CONNECT ____ Connection successful; you may need to change DTE
speed to the indicated baud rate. (See Wn and S95)
CARRIER ____ * DCE carrier speed. (See Wn and S95)
PROTOCOL: ____ * Error-correction protocol being used by the modems.
(See Wn and S95)
COMPRESSION:____ Compression technique in use by modems. (See Wn
and S95)
AUTOSTREAM Autostream technique in use by modems if selected by
user. (See Wn and S95)
------------------------------------------------------------------------
Ignore other responses, but continue to wait for CONNECT ___, BUSY, or
NO ___ responses. The CARRIER and PROTOCOL results are intermediate
results and precede either a CONNECT ____ or NO ____ result. These
results are only returned by Hayes modems when configured to use an
error-correcting protocol.
D.4.2 Answering a Call
The simplest technique for answering an incoming call is to set S0 and
wait for a CONNECT ___ result. You may get several RING results, and
possibly a NO CARRIER result if the caller hangs up before connecting.
These results should not cause your controller to abort. Continue to
wait for a CONNECT result code.
D.4.3 Using the CD Line
Monitoring the Carrier Detect (CD) line of the RS-232 interface is
another technique for carrier detection in answer or originate mode.
This assumes that &C1 or the corresponding DIP-switch has been set and
the cable is wired properly. Both are risky assumptions. You will have a
more robust controller if you use result code scanning rather than the
RS-232 lines.
D.4.4 Aborting a Connect Request
Once the D or S0 command has been issued, the modem goes off hook (or
may be off hook for S0) and it must be put back on hook (hangPup) before
the abort is completed. To abort an in-progress connect command, send
any character to the modem. This will typically result in a NO CARRIER
response. The result code scanner should be called after the abort
character is sent to prevent additional commands from being sent before
the controller and the modem are again in sync.
D.5 Carrier Loss Detection
You want your application to be able to detect when the carrier has been
lost so you can determine when the connection is complete. You might be
unable to put this part of the code in your controller software, since
the controller is typically running only during the connect or hang-up
process. Once the application has detected the carrier loss event, it
can call the modem controller and restore the modem settings.
D.5.1 Using the CD line
If you are confident of the communications environment and cabling
requirements, and have access to the RS-232 signal status, then
monitoring the CD line is the easiest carrier loss detection method to
implement. This requires &C1 to be programmed at setup time, or be
stored in the modem as the value recalled on reset or power-up.
D.5.2 Scanning the Incoming Data Stream
In cases where you cannot depend on 100% Hayes compatibility or want to
be independent from the cable wiring, then scanning for the NO CARRIER
result code is more reliable. It is also more complex to implement.
D.6 Escape and Hang Up
When your controller has been instructed to terminate the connection,
you must put the modem back in command state and issue the hang-up (H)
command. In addition to hanging-up, you will also want to restore
settings you changed to their factory-set values, or issue an ATZ<CR> to
undo the effects of your changes. In any case, restoration of the modem
settings is necessary even if the connection was terminated due to loss
of carrier.
D.6.1 Escaping the Modem to Command State
To escape the modem, the controller must first delay the escape sequence
guard time (specified by S12), then issue the escape sequence character
three times (specified by S2); then wait for an OK result. Waiting for
the result also enforces the required guard time after the escape
sequence. Once the OK result is received, the modem has entered command
state. The controller can then hang up and restore the modem.
D.6.2 Using DTR to Escape or Hang Up
The DTR RS-232 signal can be used to escape the modem to command state,
or to reset the modem depending on the &D command set or DIP-switch
settings. This also requires the cable to be properly wired. Unless the
software will operate in a highly-controlled environment, this technique
is discouraged over using the escape sequence process because of the
requirements to make it function properly. Leaving a call connected
simply because the cable was not properly wired can be potentially
expensive. The escape sequence is reliable in all environments if it is
properly utilized.
D.7 Modem Re-configuration
When a call has been completed, a "clean-up" command should be issued to
return the modem a more known configuration. For example, if verbose
result codes were selected when the modem was reset, and the controller
selected numeric result codes, on completing the session, the controller
should reset the modem to re-select verbose result codes. In the same
way, if the linefeed character were changed to suit the software or
environment, the character should be set to its former value. Any other
command options that were modified, should be restored to their factory-
set values.
D.8 Timing Considerations
A modem controller inherently has a sense of time. Usually all that is
needed to utilize the timing part of the controller is access to a time
reference. For example, the number of milliseconds since power-up or
program launch, or a "system tick" value can be used.
D.8.1 Programming for Time
The time value is used to determine relative time. For example, if a
loop should be executed for only 2 seconds it could be coded as:
D.8.2 When to Consider Time
The use of timing varies from command to command and operation to
operation. Some commands take longer to execute. The guidelines below
can be used to determine the best amount of time to wait.
If the software times out, the modem may, in fact, not be connected to
the computer, disconnected, or turned off. If this is the case, enforce
a reasonable timeout to the first setup or identification command. That
will determine whether a modem is attached and functioning.
D.8.3 Recovering when "Out Of Sync"
Another disadvantage of timing out is that an early timeout can put the
software out of sync with the modem command processor. The controller
may be interpreting results sent in response to previous commands as the
response to later commands. To avoid this condition, any pending receive
data should be flushed before the next command is issued.
D.9 Transparent XON/XOFF Local Flow Control
The use of flow control during error-control modem connections is
essential to avoid loss of data during error-control protocol
retransmissions. Use of flow control allows the DTE-DCE interface to be
run faster than the line speed, permitting the additional benefit of
increased throughput due to stripping of start and stop bits by the
error control protocol and data compression techniques such as V.42bis.
When the interface is run at high speed, data loss due to modem buffer
overrun is quite likely to occur if flow control is absent or not
functioning properly.
D.9.1 Summary
Transparent XON/XOFF Flow Control functions on the interface between the
local DTE (computer or terminal) and local DCE (modem). When the data is
placed on the link between the modems, the original characters have been
restored. Neither the remote DTE nor DCE are aware, or need to be aware,
of the fact that Transparent XON/XOFF Flow Control is in use.
D.9.2 Transparentization
Transparent XON/XOFF Flow Control operates by scanning the data stream
for XON ($11), XOFF ($13), and DLE ($10) characters ("$" indicates
hexadecimal), with either 0 or 1 in the high-order bit position. When
one is found, a DLE with same parity character is inserted ahead of it.
The original character is then "transparentized" by exclusive-ORing the
character with the value $21 (hexadecimal 21, decimal 33, binary
00100001), which preserves the parity bit value. XON and XOFF characters
are thus "hidden" in the data stream.
D.9.3 Flow Control
Once the XON and XOFF characters have been transparentized (hidden), the
DTE and DCE are free to insert XON and XOFF characters for flow control
purposes at any point in the data stream. This includes the possibility
of inserting XONs and XOFFs between a DLE and the following
transparentized character. XON and XOFF characters in the user data will
not interfere with the flow control scheme. The operation of this
XON/XOFF flow control process is identical to traditional XON/XOFF flow
control.
D.9.4 De-Transparentization
In the de-transparentization process, the receiving device scans for
DLEs. When one is found, it is discarded, and the following character is
once again exclusive-ORed with the value $21, recovering the original
character. Note that XONs and XOFFs that appear between the DLE and the
following transparentized character are used for flow control purposes
and do not affect the detransparentization process.
D.9.5 Examples
The following discussion is written in terms of the actions performed by
the computer software. The modem performs identical actions on the
reverse directions of transmission. This text is not meant to constrain
implementation or to imply that better implementations are not possible,
but serves simply as an example.
D.9.5.1 Transmitter Example
Each character in the user data stream is individual examined. It it is
any of the six characters $10, $11, $13, $90, $91, or $93, it is
exclusive-ORed with the value $21, and prefixed by a DLE character
($10). This can also be viewed as a replacement function according to
the following table:
Char Replaced By
------------------------------------------------------------------------
$10 $10 $31
$11 $10 $30
$13 $10 $32
$90 $10 $B1
$91 $10 $B0
$93 $10 $B2
------------------------------------------------------------------------
The characters are transmitted using the parity setting then in force on
the DTE-DCE interface; the XOR with $21 preserves the correct value of
the parity bit.
D.9.5.2 Receiver Example
Each character received from the modem is individually examined. A
"transparency sequence in progress" flag (TSIP-FLAG) is maintained as
part of a simple state machine. The initial value of the TSIP-FLAG is
OFF. The following tests and actions should be undertaken in the order
listed.
D.9.6 Conclusion
When properly implemented, Transparent XON/XOFF Local Flow Control
permits a fully-functional flow-controlled interface even when only
three conductors are present (Transmit Data, Receive Data, and Signal
Ground). Versatile software may use the &T19 cable test feature of Hayes
modems to determine whether or not RTS/CTS flow control can be
used, and, if RTS/CTS is unavailable, automatically select &K5 operation
to use Transparent XON/XOFF flow control, and allow the user to continue
to transfer data using any protocol or data contents without concern for
possible interference with the XON/XOFF flow control scheme.
D.10 General Tips and Techniques
The following are tips and techniques that may help in the exchange of
information between the software controller and the modem command
processor.
Sample Controller/Modem Exchange
Clock Controller Speed Modem
(1200)
bps)
------------------------------------------------------------------------
00000 ATZ<CR>
00034 ATZ<CR>
00068 (one second to do
reset)
01068 0<CR> (V0 stored as
default)
01084 (delay additional 600ms)
01684 ATEQV1S0=0S12=10S4=3HI<CR>
01884 ATEQV1S0=0S12=10S4=3HI
<CR> (echo)
02084 <CR><LF>960<CR><LF>
02142 <CR><LF>OK<CR><LF>
(19200
bps)
02192 ATM0X4L1S12=10S2=1&Q5W1S36
=7S37=9&K5<CR>
02206 <CR><LF>OK<CR><LF>
02209 ATDT9W14045551212<CR>
38000 <CR><LF>CARRIER
2400<CR><LF>
45000 <CR><LF>PROTOCOL:
NONE<CR><LF>
45010 <CR><LF>CONNECT
2400<CR><LF>
(2400
bps)
(Connection Established)
-----------------------------------------------------------------------
(2400
bps)
00000 (delay 300ms, need 200, add
100 for safety)
00300 (escape char is ^A, S2=1)
00313 (waits 200ms)
00513 <CR><LF>OK<CR><LF>
00538 ATHE1S2=43S12=50W0&Q0<CR>
00630 <CR><LF>OK<CR><LF>
------------------------------------------------------------------------