B&B Electronics 3PCIU4 - Manual (Page 19)

Serial Card Setup
Manual Documentation Number 3PCIoUx-1008 Chapter 2 13
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.