Rs422/485 Serial Communications - ABB SM1000 User Manual

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SM1000

5.5 RS422/485 Serial Communications

The optional serial interface module has been designed to
operate using the Modbus Remote Terminal Unit (RTU) Master/
Slave protocol.
An appropriate RS422/485 communications driver must be
fitted to the host (Master) computer. It is strongly recommended
that the interface has galvanic isolation to protect the computer
from lightning damage and increase the signal immunity to noise
pick-up.
The maximum serial data transmission line length for both
RS422 and RS485 systems is 1200m. The types of cable that
can be used are determined by the total line length:
– standard screened or twisted pair cable.
Up to 6m
– twin twisted pair with overall foil screen and an
Up to 300m
integral drain wire.
– twin twisted pair with separate foil screens and
Up to 1.2km
integral drain wires.
Note.
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.
For long transmission lines, a 120Ω termination
resistor must be fitted to the last slave in the chain.
Connections on links with multiple slaves must be
made in parallel.
When connecting cable screens, ensure that no
'ground loops' are introduced.
IM/SM1000 Issue 11
Module Position C
16
17
Tx+/Rx+
18
Tx–/Rx–
19
COM
20
A – 2-wire Connection
Module Position C
Tx+
16
Tx–
17
Rx+
18
Rx–
19
COM
20
B – 4-wire Connection
See Warnings on Page 98
Fig. 5.8 RS422/485 Serial Communications
5 Installation
Host Computer
'B'
'A'
+5V
Tx+
1.8kW Pull-up
Tx–
Resistor
Rx+
'A'
Rx–
'B'
1.8kW Pull-down
Resistor
GND
0V
Host Computer
+5V
1.8kW Pull-up
0V
Resistor
'B'
'A'
1.8kW
Pull-down
+5V
Resistor
1.8kW 0V
Pull-up
Resistor 'B'
'A'
1.8kW
Pull-down Resistor
0V
101