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Chapter 5 - Command Mode
Multi-Tech Systems, Inc. MT2834BA/BL Series User Guide
Fallback Modes When On-Line #F
If line conditions deteriorate, the modem automatically drops its transmission speed (“fallback”). The
#F command controls the different ways the modem falls back. During operation, if the error rate
becomes too great, the modem performs a retrain. If after the retrain, the error rate is still too high for
33600 bps operation, the modem initiates a retrain at 4800 bps. If after the first retrain the modem
returns on line at 33600 bps , the modem then starts a counter and a timer. If three retrains occur
within a two minute period, the modem falls back to 4800 bps.
Entering AT#F0 (or AT#F) allows no fallback when on-line.
AT#F1 allows the modem to fallback (based on the error rate or if three retrains have occurred within
a two minute period) from:
33.6K to 31.2K to 28.8K to 26.4K to 24K to 21.6K to 19.2K to 16.8K to 14.4K to 12K
to 9.6K to 4.8K bps.
The AT#F2 command enables incremental fall back, but also enables incremental fall forward if the
phone line improves. AT#F2 is the factory default setting.
Enable/Disable Lowspeed Fast Connect $FC
Entering AT$FC0&W0<CR> sets up the data pump so that it will not transmit a 5 second 2100 Hz
signal. Entering AT$FC0&W0<CR> allows the modem to transmit a 5 second 2100 Hz signal and is
the factory default.
Cleardown at Disconnect &CD
The &CD command is used for control of cleardown at disconnect in V.32, V.32bis, V.34 and V.34
enhanced modes.
A cleardown usually adds 1-2 seconds to the time it takes for the modem to go on-hook after a
disconnect. A cleardown makes a disconnect by the remote modem easier to detect and a
subsequent re-connection more likely. Enter AT&CD0&W0<CR> to execute a cleardown at
disconnect (default); and enter AT&CD1&W0<CR> to disable the cleardown function.
Auto Speed Detect #A
The function of the #A command is to detect and select the operational data rates (the “starting”
speeds) with which the modem uses for initial handshake and speed selection.
Remember, this command does not control the originating data rate of the modem (that is done by
the Modem Baud Rate command $MBn), but only the Answer mode “starting” speeds.
Entering AT#A0 (or AT#A) causes the modem to operate starting at:
33,600 bps, with fallback to 28,800 to 19,200 to 14,400 to 9600 to 4800 to 2400 to 1200 and to 300
bps.
Entering AT#A1 causes:
33,600 bps operation only.
Entering AT#A2 causes the modem to operate at starting speeds of:
33,600 bps, with incremental fallback to 28,800, 19,200, 14,400, 9600 and 4800 bps.
The AT#A3 command causes the modem to begin operation as a V.22bis modem at 2400 bps, with
fallback to 1200 to 300 bps. AT#A0 is the factory default value.