Moxa Technologies C168HS user manual RS-422/485 Impedance Matching

Connection Option (Opt8x) and Cable Wiring

RS-422/485 Impedance Matching

For RS-422/485 serial communications, when an electrical signal travels through two different resistance junctions in a transmission line, the impedance mismatch will sometimes cause signal reflection. Signal reflection causes signal distortion, which in turn will contribute communication errors. The solution to this problem is to establish the same impedance at the line ends as in the line itself by terminating them with resistors.

The value of the termination resistor should equal the characteristic impedance of the transmission line. The resistors should be added near the receiving side.

Opt8J/F/Z		RS-422/485 Device
3	TxD+(B)	RxD+(B)
16	TxD-(A)	RxD-(A)
2	RxD+(B)	TxD+(B)
14	RxD-(A)	TxD-(A)
5	RTS+(B)	CTS+(B)
Opt8J only 13	RTS-(A)	CTS-(A)
4	CTS+(B)	RTS+(B)
19	CTS-(A)	RTS-(A)

Note:

1. Stands for termination resistor near the receiving side

2.The suggested termination resistor for AWG #26 cable is 100 ohm.

3.The suggested termination resistor for phone cable is 600 ohm.

Smartio C168H/HS User’s Manual 5-11