Pull-Up And Pull-Down Resistors; Termination Resistor; Serial Connections - ABB SM3000 User Manual

2.3 Pull-up and Pull-down Resistors – Fig. 2.1
To prevent false triggering of slaves when the master (host computer) is inactive, pull-up and pull-down resistors must be fitted to the
RS422/485 interface in the host computer.
Resistors are normally connected to the interface by means of hard-wired links or switches – refer to the manufacturer's instructions.
2.4 Termination Resistor – Fig. 2.2
For long transmission lines, a 120Ω termination resistor must be fitted to the last slave in the chain – see Fig. 2.2.
Master
Host
Computer
2.5 Serial Connections – Figs. 2.1 and 2.2
Connections to the Modbus serial board must be made as shown in Fig. 2.1. Connections on links with multiple slaves must be made
in parallel as shown in Fig. 2.2. When connecting cable screens, ensure that no 'ground loops' are introduced.
The maximum serial data transmission line length for both
determined by the total line length:
– standard screened or twisted pair cable.
Up to 6m
– twin twisted pair with overall foil screen and an integral drain wire.
Up to 300m
– twin twisted pair with separate foil screens and integral drain wires.
Up to 1.2km
First Slave
Rx+ 16 Tx+
Rx– 17 Tx–
18
Tx+ Rx+
Tx– Rx–
19
20
COM COM
Fig. 2.2 Connecting Multiple Slaves
RS422
Last Slave
and systems is 1200m. The types of cable that can be used are
RS485
2 ELECTRICAL INSTALLATION
16 Tx+
17 Tx–
18
Rx+
Rx–
19
20
COM
120Ω

Termination Resistor

3