Termination Resistors; Rs485/422 Standard - ABB C1900 Installation Manual

...2 ELECTRICAL INSTALLATION
2.4 Termination Resistors – Fig. 2.3
For long transmission lines, termination resistors are required on the last slave in the chain and the host computer/computer
terminal – see Fig. 2.3. Under normal operating conditions the resistors are required at the receive inputs only. For 3-wire
operation the slave termination resistor is fitted to the serial module terminal board – see Fig. 2.3A. For 5-wire operation the slave
termination resistor is selected using plug-in links on the serial module – see Fig. 2.3B.

2.5 RS485/422 Standard

The RS485 standard quotes connection of thirty two slaves maximum, to any single driver (computer terminal or host computer);
the RS422 standard quotes connection of up to ten slaves. However, these numbers can be increased if the driver's serial port
permits.
2
3
4
5
6
A – 3-wire Operation
Note.
• For operation an external 120Ω resistor must be on the last slave in the chain. The resistor is fitted across
3-wire
terminals 4 and 5 on the serial module terminal board.
• For operation the internal termination resistor on the last slave in the chain must be linked in.
5-wire
4
Tx+/Rx+
120Ω
External Termination Resistor – see Note
Tx–/Rx–
Fig. 2.3 Slave Termination Resistors
7 6
12 1
10 3 10
11 2
11
12 1 12
Termination Termination
Resistor
Linked In – Linked Out
see Note
B – 5-wire Operation
3
2
1
Resistor