74 MultiModemISI Hybrid Series, ISIHI-2S
Chapter 4—MODEM AT Commands, S-Registers, Result Codes
Modem
Online Connection Commands
The following commands control the conditions of the online connection.
B
n
Answer Tone
n
= 0, 1, 2, 3, 15, or 16
Default: B1 and B16
The B command selects the frequency the modem uses for its answer tone. (The answer
tone is the tone transmitted by the receiving modem to the calling modem, thus initiating
the

handshake

between the two modems.) At higher speeds (2400 bps and above) there
is no conflict, because all protocols use the Bell frequency of 2225 Hz. Lower speeds
require different frequencies.
B0 selects ITU-T V.22 mode when the modem is at 1200 bps.
B1 selects Bell 212A when the modem is at 1200 bps. This is a default.
B2 deselects the V.23 reverse channel.
B3 is identical to B2 in function. It also deselects the V.23 reverse channel.
B15 selects V.21 when the modem is at 300 bps.
B16 selects Bell 103J when the modem is at 300 bps. This is a default.
C
n
Carrier Control
n
= 1
Default: C1
The Cn command provides backward compatibility with some data communications
software.
C1 enables normal transmit carrier switching.
-C
n
Data Calling Tone
n
= 0 or 1
Default: -C0
The data calling tone is a tone of a certain frequency and cadence, as specified in the
V.25 standards, which identifies whether it is remote data, fax, or voice. The frequency is
1300 Hz, with a cadence of .5 s on and 2 s off.
-C0 disables the V.25 data calling tone.
-C1 enables the V.25 data calling tone.
F Echo Online Data Characters
n
= 1
Default: F1
This command determines if the modem echoes data from the DTE. This command
provides backward compatibility with some data communications software.
F1 disables online data character echo.