Figure A-3. 723Plus Rs-485 Connections With Optional Termination; Figure A-4. Rs-422 Terminator Locations - Woodward 723PLUS Technical Manual

723PLUS Digital Marine Control Manual 26482
The RS-485 connections are shown in Figure A-3. The maximum distance from
the Master Modbus Device to the 723PLUS control is 1219 m (4000 ft).

Figure A-3. 723PLUS RS-485 Connections with Optional Termination

RS-422 and RS-485 can use a multi-drop set-up where more than one device is
connected to a master device. A termination should be located at the receiver
when one or more transmitters are connected to a single receiver. When a single
transmitter is connected to one or more receivers, termination should be at the
receiver farthest from the transmitter. Figure A-4 is an example.

Figure A-4. RS-422 Terminator Locations

Termination is accomplished using a three-resistor voltage divider between a
positive voltage and ground. The impedance of the resistor network should be
equal to the characteristic impedance of the cable. This is usually about 100 to
120 . The purpose is to maintain a voltage level between the two differential
lines so that the receiver will be in a stable condition. The differential voltage can
range between 0.2 and 6 V. The maximum voltage between either receiver input
and circuit ground must be less than 10 V. There is one termination resistor
network for each port located on the 723PLUS board. Connection to this resistor
network is made through the 9-pin connectors on pins 6 and 9.
98 Woodward