Modem Configurations; Full Duplex 4-Wire Mode; Half-Duplex 2-Wire Mode - Black Box 202T User Manual

MODEM 202T

3. Modem Configurations

3.1 Full Duplex 4-Wire Mode

To operate in full duplex, the Modem 202T must use a 4-wire connection and can
have only two units in a point-to-point connection. In full duplex, the Modem does
not use carrier control since both modems will have their carriers transmitting at
all times. No handshake signals are used by the modems, but the signals can be
asserted by the users' equipment and fed back to the connecting equipment if
needed.
When operating, the Carrier Detect LED of both modems should be on at all
times. The carrier reflects the presence of the FSK frequency on the connecting
wires. The Data In LED (Yellow) and the Data Out LED (Green) of the modem
will follow the data at the RS-232 9-pin connector. The two modems are cross-
connected via the "2-wire" and "4-wire" connectors.

3.2 Half-Duplex 2-Wire Mode

The Modem 202T can operate in half-duplex 2-wire mode with one master and
one or more remote modems. The setup configuration is the same for the master
and remote modems. The user's systems equipment must provide a timing delay
between a received message and a transmitted answer to allow the FSK carrier time
to switch directions.
The data flow control can be either DataSense (DSCC) or the RTS line from the
connected equipment.
In DSCC mode, the modem senses the characters from the user's equipment, turns
on the FSK carrier, and then sends the buffered characters. All receiving modems
detect the carrier, then send the received data out the RS-232 port to the user's
equipment. In DSCC mode, a minimum line turnaround delay of 40 milliseconds
is required.
With RTS control, the assertion of the RTS line by the user's equipment tells the
modem to turn on the transmission carrier. Only the carrier at one modem can be
on at any time in a half-duplex system, or carrier collision will occur and no data
can be transferred. The minimum RTS timings are 20 milliseconds pre-transmit
delay, 60 milliseconds RTS to data, and zero milliseconds RTS off delay at the end
of the data. If you are adding the Modem 202T to an existing system with previous
models of Modem 202Ts, a minimum of 100 milliseconds RTS to data delay is
required.
10