Telebit T3000 Reference Manual page 64

3 Only fallback enabled. Connect only at the fall back
speed of the modulation mode specified by the
S50 register. If the selected modulation mode
does not have an associated fallback speed,
connect at the primary speed.
V .22bis uses a fallback speed of 1200 bps; V.32 falls
back to 4800 bps; V.32bis falls back to 12,000 bps.
When the modem connects in automatic speed
determination mode (S50=0), the answering modem
cycles through several different answering tones in
sequence to allow the calling modem to synchronize
on the appropriate one. In general, the modem starts
with the highest transmission speed and works down
toward the lowest speed. Therefore, if a slower speed
modem is calling, the local modem may need to
extend the time that it waits to detect carrier.
The B command determines if the modem issues Bell
standards (212A and 103) or CCIIT standards (V.22
and V.21) for 1200 bps and 300 bps operation as
follows:
BO Use CCITT V.22 and V.21 standards.
B1 Use Bell 212A and 103 standards.
If you plan to use the modem in V.22 or V.22bis
mode, be aware that some modems use guard tones
during synchronization. If you experience problems
connecting, try configuring the modem with an &G
command to use one of the two available guard
tones:
&GO No guard tone.
&G1 Use 550 Hz guard tone.
&G2 Use 1800 Hz guard tone.
Beyond the Basics 4-17