Campbell Scientific SDM-SIO1 Serial Input/Output Module - Manual (Page 17)

SDM-SIO1 Serial Input/Output Module
3.3.3 RS-485 Half Duplex Wiring Example
1 C1
2 C2
3 C3 To logger
4 G
5 +12V
6 G
7 RX-A
8 CTS-B
100 R or higher
(Termination resistor may not be
needed for many applications)
9 RTS-Y R
10 TX-Z
11 0V RS-485 0V
12 Z RS-485_B
13 Y RS-485_A
14 B RS-485_Z
15 A RS-485_Y
RS-485
equipment
3.3.4 RS-485 Internal Circuit Diagram
22R
22R
22R
22R
100R
R
R
TX
RX
0V
0V
Internal External
100R or higher*
100R or higher*
Z
Y
B
A
* Note the external termination resistors are optional. Some applications can use much higher value resistors
and some may not need resistors at all depending on the distance and wire quality involved.
If the termination resistor is primarily needed for impedance matching then a value matching the line
resistance will have to be used, i.e. 100R.
11