SM500F
Field mountable paperless recorder
2 Installation
IM/SM500F Rev. Z
23
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.
2.9.5 Serial Connections
Connections to the Modbus serial board must be made as shown in
. Connections to
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.
Fig. 2.15 Connecting Multiple Slaves
1
2
3
4
5
6
1
2
3
4
5
6
Master
Host
Computer
First Slave
Last Slave
120
Termination Resistor
Rx/Tx +
Rx/Tx –
COM
Rx/Tx +
Rx/Tx –
COM
Rx/Tx +
Rx/Tx –
