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PN 88300150
3.9.3 RS232 Interface Commands
&C
Carrier Detect Control
The &C command lets you control the status of the Carrier Detect signal (CD - Pin 8) on the
RS232C interface. You have three choices. You can force the signal high, allow it to act
normally, or set it up so that it will stay high until the modem disconnects, go low
momentarily, and then go high again. The last option is useful with some CBX phone systems
and mainframe front ends, which require CD to act in this manner.
To allow CD to act normally, type the command AT&C1 (this is the factory default setting). To
force CD on, type the command AT&C0. To set up CD so that it drops for one second on
disconnect and then comes up again, type AT&C2. (If you want the drop time to be
something other than one second, use S-Register S24 to change this value. Refer to Chapter
4 for S24 instructions.)
&D
Data Terminal Ready Control
Data Terminal Ready (DTR) on pin 20 of the RS232C interface is required for the MT2834MR
to operate. A high DTR signal tells the modem that the device to which it is connected is
active, or "ready" to communicate through the modem. If the signal is not being provided on
the RS232C interface, 12-position DIP-Switch #1 can be used to force the DTR signal on.
DTR can also be used to trigger a dialing sequence, called DTR Dialing. The condition of
DTR can also be used to cause the modem to reset to its default parameters, just as if you
had given the modem an ATZ command. To do this, type the command AT&D3 and hit
ENTER. The modem will now reset itself whenever DTR is dropped from On to Off, and will
also go on-hook (hang up) if it is on-line.
Type AT&D0 or AT&D to cause the MT2834MR to ignore DTR. Type AT&D1 to cause the
modem to go on hook (hang up) with loss of DTR. The modem enters Command mode when
DTR goes high again. Auto-answer is disabled while DTR is low. Type AT&D2 to cause the
modem to go on hook with loss of DTR. The modem enters Command mode when DTR goes
high again.
&R
Clear to Send Control
The &R command lets you control the Clear to Send signal (CTS - Pin 5) on the RS232C
interface. You have three choices. You can force the CTS signal high, allow it to act normally,
or set it to stay high until the modem disconnects, go low momentarily, and then go high
again. The last option is useful with some CBX phone systems and mainframe front ends,
which require CTS to act in this manner.
To allow CTS to act normally, type AT&R0. To force CTS on, type AT&R1 (when the modem
goes on-line, CTS still provides flow control). Type AT&R2 to set up CTS so that it drops for
the S24 setting on disconnect and then comes up again. (If you want the drop time to be
something other than one second, change the S24 value. Refer to the Chapter 4 instructions
for S24.)
If you type the &R2 command, be sure that the CTS DIP-Switch on the modem circuit board
is set to allow CTS to act independently of Request to Send (RTS). (Refer to Chapter 7 for
details.)
&RF
CTS/RTS Interaction Control
In typical operation, Clear to Send will follow Request to Send when the modem is on-line. In
other words, if RTS goes off, CTS goes off in response. The &RF0 command enables CTS to
follow RTS. In some applications, however, it may be necessary for CTS to operate
independent of RTS. &RF1 allows CTS to operate independently regardless of the state of
4/3/01
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