Pull-Up And Pull-Down Resistors; Termination Resistor; Serial Connections - ABB SM500F User Manual

SM500F
Field mountable paperless recorder

2.9.3 Pull-up and Pull-down Resistors

To prevent false triggering of slaves when the master (host computer) is inactive, pull-up and
pull-down resistors are fitted to the SM500F's Modbus/digital input module.

2.9.4 Termination Resistor

For long transmission lines, a termination resistor must be fitted to the last slave in the chain – see Fig. 2.15.
Master
Rx/Tx +
Rx/Tx –
Host
COM
Computer
Fig. 2.15 Connecting Multiple Slaves

2.9.5 Serial Connections

Connections to the Modbus serial board must be made as shown in
two-wire systems with multiple slaves must be made in parallel as shown in
connecting cable screens, ensure that 'ground loops' are not introduced.
The maximum serial data transmission line length for both RS485 systems is 1200m. The types
of cable that can be used are determined by the total line length:
Up to 6m – standard screened or twisted pair cable.
Up to 300m – twin twisted pair with overall foil screen and an integral drain wire.
Up to 1.2km – twin twisted pair with separate foil screens and integral drain wires.
IM/SM500F Rev. Z
First Slave
Rx/Tx +
1
Rx/Tx –
2
COM
3
4
5
6
2 Installation
Last Slave
Rx/Tx +
1
Rx/Tx –
2
3
4
5
6
120 
Termination Resistor
. Connections to
Fig. 2.14
. When
Fig. 2.15
23