Black Box MD1641A Manuale Utente

Data Terminal Ready (DTR) on Pin 20 of the RS232C interface is
required in order for the Series II 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 RS232C interface, you can force DTR high with
DIP-Switch #1 (Chapter 9).

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 

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 

or 

causes the Modem to ignore DTR. Entering

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 

causes the modem

to go on hook with loss of DTR. The modem enters command mode
when DTR goes high again.

The 

command allows you to 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, enter the command 

. To force

CTS on, enter the command 

(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 

. (If you want the drop time to be

something other than one second, use S-Register S24 to change this
value. Refer to Chapter 6).