Appendix G V.25Bis Operation - Multitech MT2834MR Owner's Manual

PN 88300150
Appendix G
V.25bis Operation
Operation of your modem in ITU-T V.25bis mode provides you with an alternate set of commands and
responses to those described in the AT Command Mode chapter of this manual. The V.25bis mode
performs dialing functions in the asynchronous or synchronous mode according to the recommendations
of the ITU-T (Consultative Committee of International Telephony and Telegraphy). Synchronous mode,
which is the more common use of V.25bis, includes both Binary Synchronous Communications (BSC)
and Synchronous Data Link Control (SDLC).* V.25bis is commonly used in IBM (and other) mainframe
and mid-range (S/3x and AS/400) environments. The V.25bis mode commands are used to establish data
connections over public switched telephone networks.
We've already covered the AT command set with its dialing features earlier in this manual. The V.25bis
commands can be looked at as a second command set used to perform synchronous dialing functions,
and, since the AT command set cannot perform synchronous dialing, it is necessary for dial-up
synchronous applications.
V.25bis mode AT commands do not include any modem configuration commands. You can execute
normal AT commands in V.25bis mode for modem configuration.
Delayed and Forbidden Numbers
V.25bis 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 are 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 are allowed. When the Forbidden Numbers list is full, no
dialing is allowed and a CFIFF indication is given. The modem in AT command mode responds with NO
CARRIER. If country regulations require that the Forbidden Numbers list be checked in AT mode, then
NO CARRIER is the response to a dial attempt in AT mode. Numbers are 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 is 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 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).
Operation
Operation in V.25bis mode is similar to AT command mode except that certain DIP Switch functions are
important to its operation. V.25bis 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 get no responses). In synchronous mode, the modem supplies the
clock, so the synchronous terminal "knows" the speed.
You must be in V.25bis mode for the commands described here to function. Most AT commands also
function, except those associated with dialing such as ATD, ATN, ATO, and ATU. To get into V.25bis
mode, type AT$V1. At this point your modem does no more speed or parity detection (things associated
with asynchronous operation). To get out of V.25bis mode and back into AT command mode, type
AT$VO. The AT$V2 command allows you to run one V.25bis command from AT command mode without
leaving AT command mode.
There is no command to select between asynchronous and synchronous V.25bis operation. The position
of 4-Position DIP-Switch #4 selects modes. For synchronous mode, thesynchronous switch must be on,
the command mode enabled and a jumper plug selects between BSC and SDLC.
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