Serial Card Setup
Document Number 3PCI-0712m
Chapter 2
9
DIP Switch 1 (RS-422/485 only)
The top DIP switch (1) configures the port for RS-485 or RS-422 operation. For RS-422
operation (which uses two wire pairs and sends point-to-point or point-to-multipoints)
the transmitter can be enabled all the time. Placing the middle DIP switch in the TX ON
position accomplishes this.
For RS-485 operation the middle DIP switch is placed in the TX SD position. In this
position the transmitter is only enabled when data is being sent. The transmitter is tri-
stated when not sending data, allowing other transmitters on the communications line to
transmit without interference.
DIP Switch 2 (RS-422/485 only)
The bottom DIP switch (2) configures the port for half-duplex (two-wire) RS-485
operation or full-duplex (four wire) RS-422/RS-485 operation. Placing the bottom DIP
switch in the RX ON position configures the port for four wire operation. In this mode the
port‘s receiver is continuously enabled, allowing it to receive all data on the
communications line. Since the port‘s transmitter sends data on the other wire pair the
port does not receive its own transmissions.
Placing the bottom DIP switch in the RX
position configures the port for two wire
operation. In this mode the port‘s transmitter and receiver are connected to the same
wire pair. The receiver is disabled when its transmitter is sending, preventing the port
from receiving its own data.
