Half-Duplex 4-Wire Mode - Black Box 202T User Manual

In a half-duplex system, the higher level equipment is responsible for recognizing
that the incoming data is valid and responds with return data. Either time delays
(DSCC mode) or handshake signals (RTS mode) must be provided to allow line
carrier turn around.
In a two-wire system, only the left connector marked "2-wire" is used to
interconnect all the modems in the system.
These modems also use audio frequency tones and the connections to
the phone wires do not have a polarity.
The LEDs on a modem reflect the status of the lines at the 9-pin RS-232 connector.
When a modem is transmitting, the Data In (Yellow) LED will flash, and at the
remote modem the Carrier Detect (Amber) and Data Out (Green) LEDs will flash
as data is received.

3.3 Half-Duplex 4-Wire Mode

The Modem 202T will operate in half-duplex 4-wire mode with one master and
one or more remote modems.
The master's carrier transmitter "4-wire" line must be connected to the remote's
receiver line "2-wire" and vice versa. The master modem's carrier is being
transmitted at all times so the remote's carrier (Amber) LED will always be on. The
remote modem's carrier is only transmitted when a remote modem is actively
sending data to the master.
No data flow control is needed at the master since its carrier is always on and ready
to transfer data. The data flow control at the remotes can be either DataSense or
the RTS line from the connected equipment.
In DSCC mode, the modem senses the RS-232 characters from your equipment,
turns on the FSK carrier, and then sends the characters. The master modem
detects the carrier, then the data, and and sends the received data out its RS-232
port to your equipment.
With RTS control, the assertion of the RTS line by your equipment tells the remote
modem to turn on the transmission carrier. Only one remote carrier can be on at
any time in a half duplex system or carrier collision will occur and no data can be
transferred. At a remote modem, your system must recognize that incoming data is
for the unit and must respond with data (DSCC) or RTS handshake line to provide
the correct timing delays to prevent carrier collisions. The minimum RTS timings
CHAPTER 3: Modem Configurations
NOTE
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