SECTION 4D - HAYES COMPATIBLE MODEM INTERFACE
LOCAL OPERATION
While programmed for modem operation the Model 16 can be made to
temporarily deviate from the modem protocol so that the Model 16 can be
accessed directly via the RS-232 port. This makes it easy to unplug a modem
and plug in a computer or dumb terminal for on-site service. Simply send
the model 16 a "break", and it will respond by switching its serial port to
the programmed baud rate. It will then send the message "Local Access
Enabled". The Model 16 can then be accessed as if it were not programmed
for modem operation. This local access remains in effect until a timeout
occurs or until the user logs off. To send a "break" From Procomm, you
would press alt-F7 (the alt key and function key 7 simultaneously) after
switching to the local access cable. From CrossTalk you would press the End
key. On some dumb terminals you will find a dedicated "break" key. If none
of the above apply, consult the manual for your dumb terminal or terminal
emulation program.
USING DTR/DCD HANDSHAKING
When possible, it is desirable to use DTR/DCD Data Terminal Ready / Data
Carrier Detect handshaking. These hardware interface lines allow the modem
to tell the Model 16 about the presence or absence of a carrier signal from
the calling modem. This type of hardware signaling ensures that the Model
16 disconnects when the calling modem hangs up, which reduces the
possibility of the Model 16 getting "stuck" in the programming mode waiting
for a timeout should the caller disconnect without exiting the programming
mode.
The Model 16 uses the DCD line to determine when a disconnect has taken
place. It still relies on the response codes from the modem to determine
when a connection has been made to a calling modem and the baud rate.
The Model 16 supports RTS/CTS and DTR/DCD handshaking but not both at the
same time. The same input and output lines are used for both types of
handshaking. To set the Model 16 for DTR/DCD operation, you must first
program the system database communications settings for DTR/DCD. The DTR
line to the modem should be connected to Jl-7 (which is the RTS/DTR line)
on the Model 16. The DCD line from the modem should be connected to Jl-8
(which is the RTS/DCD line) on the Model 16.
If you purchased a Model 16 modem interface cable from Zetron, then the
DTR/DCD lines are already connected. If you are making your own cable, then
use the connection table below for interfacing to a modem with a standard
EIA RS-232C 25-pin connector.
DB25P Modem_____________ DB9P Model 16
2
Tx Data
3
TX Data
3
Rx Data
4
Rx Data
7
Ground
5
Ground
8
DCD
8
DCD/CTS
20 DTR
7
DTR/RTS
4D-3
