B&B Electronics 3PCIU4 - Manual (Page 80)

DIP Switch / Mode Settings
A-2 Appendix A Manual Documentation Number 3PCIoUx-1008
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 3 (RS-232/422/485 ports)
The bottom DIP switch (3) 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 receiver is continuously enabled,
allowing it to receive all data on the communications line. Since the
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 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.
Setting the DIP Switches on RS-422/485 Only Ports
The 3PCIOU4 MIport card provides a combination of RS-232/422/485 and
RS-422/485 only ports. Ports that do not include RS-232 operation use
double DIP switches rather than triple DIP switches. These DIP switches
operate the same as the two bottom DIP switches in the RS232/422/485 ports
TX On
RX On
TX SD
RX SD
Switch 1
Switch 2
RS-422 Mode
TX On
RX On
TX SD
RX SD
4-wire RS-485 Mode
TX On
RX On
TX SD
RX SD
2-wire RS-485 Mode
Figure 55. RS-422/485 only DIP Switch Settings