3 AT Commands
CommPlete Communications Server 29
RS-232 Interface Commands
These commands define how an MT3334HD8 modem will use and respond to standard RS-232 sign als.

&CnCarrier Detect Control

n
= 0, 1, 2, or 4
Default: &C1
The &C command lets you control the Carrier Detect (CD) signal on the RS-232/V.24 int erface. Normally,
CD goes high when the modem detects a carrier on the communications link, and drops when it lose s the
carrier. By using &C, however, you can force the signal to stay high, or to drop momentarily when the
remote modem disconnects. (This option is useful with some CBX phone systems and mainframe fr ont
ends, which require CD to act in this manner.) You can also force the modem to reset when the CD signal
drops.
&C0 forces CD high.
&C1 allows CD to act normally—to go high when the modem detects a carrier, and to drop when it loses
the carrier.
&C2 causes CD to drop for one second on disconnect, and then go high again. (If you want to set the drop
time to something other than one second, change register S24.)
&C4 resets the modem when CD drops.

&DnData Terminal Ready Control

n
= 0, 1, 2, or 3
Default: &D2
The Data Terminal Ready (DTR) signal on pin 20 of the RS-232/V.24 interface must be high, or “on,” in
order for the modem to operate. A high DTR signal tells the modem that the computer it is connected to is
ready to communicate through the modem.
The DTR signal can also be used to cause the modem to reset to its default parameters, as if you had given
the modem an ATZ command.
&D0 (or &D) causes the modem to ignore DTR.
&D1 causes the modem to hang up when DTR drops. While DTR is low, the modem accepts commands,
but will not dial or auto-answer until DTR goes high again.
&D2 acts the same as &D1.
&D3 causes the modem to reset when DTR drops . It will also hang up if it is onl ine.