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Connection Option (Opt8x) and Cable Wiring
Smartio C168H/PCI User’s Manual 5-11
See Section “RS-485 Programming” of Chapter “Serial Programming Tools” for
more Opt8J RS-485 programming details.
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)
13 RTS–(A) CTS–(A) Opt8J only
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.