Telebit T3000 Reference Manual page 121

Set to either 2 or 3, the modem checks the &G
command to determine what type of guard tone (if
any) to transmit with the data in answer mode. The
V.25 answer sequence is used while in V.22 or
V .22bis mode.
Set to 5, the modem operates using the V.23
standard. If the modem originates a call, it modulates
data at 75 bps and demodulates data at 1200 bps. If
the modem answers a call, it modulates data at 1200
bps and demodulates data at 75 bps. V.23 operation
works only in buffered data mode.
Set to 6, the modem operates in V .32 mode, and
starts any search sequence at 9600 bps.
If you set the S50 register to 7, the modem operates
in V.32bis mode, and starts any search sequence at
14,400 bps.
Set to 254, the modem automatically searches for the
correct transmission speed starting with the speed of
the last AT command. For example, if the last AT
command was entered at 2400 bps, the modem
attempts to connect at 2400 bps, followed by 1200
bps and finally 300 bps. The modem attempts a V.32
connection only if the last AT command was issued
at 9600 bps or greater.
S51 DTE Interface Speed
Range: 0-7,35,43,46,252-255
Default: 255
The S51 register determines the data rate between the
modem and the local DTE when operating
asynchronously. ·
6-14 AT Register Descriptions