Moxa Technologies C168P User Manual download pdf (Page 67)

Connection Option (Opt8x) and Cable Wiring

C168P User’s Manual 5-11

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.

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Moxa Technologies C168P User Manual Page 67