Data Terminal Ready Control &D

Chapter 4 - Command Mode

Data Terminal Ready (DTR) on pin 20 of the RS-232C interface is required in order for the modem 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 RS-232C interface, you can force DTR high with DIP-Switch #1.

DTR has some other modem functions. DTR can 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, enter the command AT&D3 and hit RETURN.

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.

Entering AT&D0 or AT&D causes the modem to ignore DTR. Entering AT&D1 causes 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. Entering AT&D2 causes the modem to go on hook with loss of DTR. The modem enters command mode when DTR goes high again.

CTS Control &R

The &R command allows you to control the Clear to Send signal (CTS-Pin 5) on the RS-232C 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, enter the command AT&R0. To force CTS on, enter the command AT&R1 (this is the factory default setting). When the modem goes on-line, CTS still provides flow control. To set up CTS so that it drops for one second on disconnect and then comes up again, enter the command AT&R2. (If you want the drop time to be something other than one second, use S- Register S24 to change this value.

Data Set Ready Control &S

The &S command allows you to control the status of the Data Set Ready signal (DSR - pin 6) on the RS-232C interface. You have three choices. You can force the 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 DSR to act in this manner.

To allow DSR to act normally, enter the command AT&S1 (the default setting). To force DSR on, enter the command AT&S0. To set up DSR so that it drops for one second on disconnect and then comes up again, enter AT&S2. If you want the drop time to be something other than one second, use S-Register S24 to change this value.

CTS/RTS Interaction Control &RF

In typical operation, Clear to Send follows 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 RTS, and is the factory default. If this is the case, refer to the &R command for control of the Clear to Send signal.