Specifications

Appendix A l l Pinouts and Cable Wiring
46
2. Multidrop
Multidrop configuration means that more than two devices can be linked
together to communicate one another via RS-485 interface. In this configuration,
one of the device serves as master device while the rest of the devices as slaves.
Multidrop RS-485 Half-Duplex Multidrop RS-485 Full-Duplex
C114HI site Remote site C114HI site Remote site
1 TxD- RxD- 1 TxD- RxD-
4 RxD- TxD- 4 RxD- TxD-
2 TxD+ RxD+ 2 TxD+ RxD+
3 RxD+ TxD+ 3 RxD+ TxD+
5 GND GND 5 GND GND
Remote site Remote site
RxD- RxD-
TxD- TxD-
RxD+ RxD+
TxD+ TxD+
GND GND
A.4 Impedance Matching and Termination Resistors
When an electrical signal travels through two different resistance junctions in a
transmission line, the mismatched impedance between cable and receiver 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 receiver ends by connecting a termination
resistor in parallel with the same impedance as the cable’s characteristic impedance.
The value of termination is determined by cable. Please refer to your cable supplier
for more information regarding the characteristic impedance for your cable.
It is normally sufficient when the value of the termination resistor equals the
characteristic impedance of the transmission line. The resistors should be added near
the receiving side. See below RS-422 example.