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MultiModemBA User Guide
Appendix D - V.25bis Operation
Chapter 4 described a set of commands which let the modem dial, hang-up, and be configured for
various applications. However, these commands, the AT command set, are only functional when the
DTE transmits data asynchronously. That is, they cannot be used with synchronous equipment such
as that found in IBM's Binary Synchronous Communications (
BSC
) and Synchronous Data Link
Control (
SDLC
)* environments.
The ITU V.25
bis
commands provide you with an alternate set of commands and responses to those
described in Chapter 5 for applications in which the DTE is synchronous. V.25
bis
commands support
dialing functions in asynchronous or synchronous mode according to the recommendations of the
ITU; however, the AT commands are understood to reign in the asynchronous world, while the
V.25bis commands are relegated to the synchronous world of datacomm. If you need to establish a
datacomm link over Public Switched Telephone Network (
PSTN
) dial-up services, and your
equipment outputs a synchronous data stream, V.25
bis
commands are essential. V.25
bis
mode AT
commands do not include any modem configuration commands. You can execute normal AT
commands in V.25
bis
mode for modem configuration.

Delayed and Forbidden Numbers

V.25
bis
provides the facility to delay failed call retry attempts by putting numbers that failed to
connect on a special Delayed Number list. Subsequent dialing of these numbers will be delayed (time
specified by a country regulation) and an appropriate message displayed. If the number is retried
more than the maximum allowed number of times (number is also specified by country regulation), it
is placed on the Forbidden Numbers list, and no further retries will be allowed. When the Forbidden
Numbers list is full, no dialing is allowed and a CFIFF indication is given. The modem in AT command
mode will respond with
NO CARRIER
. If country regulations require that the Forbidden Numbers list
be checked in AT mode, then
NO CARRIER
will be the response to a dial attempt in AT mode.
Numbers will also be put on the Forbidden Numbers list if the Delayed Numbers list is full and a new
number fails for the first time. In that case, the new number will be added to the Delayed Number list
and the oldest existing number added to the Forbidden Numbers list. Numbers are removed from the
Forbidden Numbers list by after a certain time has past (also by country regulation).
Some country regulations have numbers remain on the Forbidden Numbers list permanently. The
Delayed Number and Forbidden Numbers lists are eight numbers long (20 characters each).

V.25bis Operation

Operation in V.25
bis
mode is similar to AT command mode in that certain DIP-Switch functions are
important to its operation. V.25
bis
does not include any speed detection for asynchronous mode, so
when you are giving commands, you must stay in your initial speed (i.e., if you change your terminal
speed while entering an AT command, you will get no responses). In synchronous mode, the modem
supplies the clock, so the synchronous terminal
“knows”
the speed.
You must be in V.25
bis
mode for the commands described here to function. Most AT commands will
also function, except those associated with dialing such as
ATD
,
ATN
,
ATO
, and
ATU
. To get into
V.25
bis
mode, you type
AT$V1
. At this point your modem does no more speed or parity detection
(things associated with asynchronous operation). To get out of V.25
bis
mode and back into AT
command mode, enter
AT$VO
. The
AT$V2
command allows you to run one V.25
bis
command from
AT command mode without
leaving AT command mode.
There is no command to select between asynchronous and synchronous V.25
bis
operation. The
position of DIP-Switch #12 selects between modes. For synchronous mode, DIP-Switch #12 must be
UP, and the AT command mode enabled.