2-4. Interface Connections
U3-1021
2-24
2/99
Westinghouse Proprietary Class 2C
2-4.5. Cable Selection
Cable for RS-232 serial communication should be multiple conductor with shield.
The shield drain wire should be used for the ground connection.
Cable for RS-485/RS-422 serial communication should be multiple twisted pairs,
overall or individually shielded. Each signal’s two differential connections should
be made within a twisted pair; signals should not cross pairs.
The RS-485/RS-422 Group 2 personality module includes terminating resistors
which can be placed across each signal pair by jumpering the base unit terminal
block between B14 and B15 for transmit and between A14 and A15 for receive.
Refer to Figure 2-2.
Some field devices will have resistors which can be switched in. For other devices,
terminating resistors may need to be added. A typical value is 120 ohms across the
+ and - pins. In particularly noisy environments, the resistor can be split into 60
ohms from + to ground and 60 ohms from - to ground.
Refer to the “Cable Guidelines” section in U3-1000 for detailed information about
cable usage and installation.
