Host Computer Serial Communications; Two-Wire Connection - ABB SM500F User Manual

SM500F
Field mountable paperless recorder

2.9.1 Host Computer Serial Communications

The serial interface option module has been designed to operate using the Modbus Remote Terminal Unit
(RTU) Master/Slave protocol. An appropriate RS422/485 communications driver must fitted to the host
(Master) computer. It is strongly recommended that the interface has galvanic isolation to protect the
computer from lightning damage and to increase signal immunity to noise pick-up.

2.9.2 Two-wire Connection

Modbus serial communications must be configured as two-wire serial links – see Fig. 2.14. The instrument
must be added to the link configuration on the host system – refer to information supplied with the host
system.
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2
3
4
5
6
Fig. 2.14 Two-wire Connection
22
Terminal Block F
Rx/Tx +
Rx/Tx –
COM
GND
Host Computer
'B'
'A'
+5 V
Tx +
1.8 k Pull-up
Tx –
Resistor
Rx +
'A'
Rx –
'B'
1.8 k Pull-down
Resistor
GND
0 V
2 Installation
IM/SM500F Rev. Z