Rs422/485 Serial Communications - ABB SM1000 User Manual

5.5 RS422/485 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 increase signal immunity to noise pick-up.
The maximum serial data transmission line length for both
and systems is 1200m. The types of cable that can be used
RS485
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.
Notes.
• 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.
Terminal Block D
16
17
18
19
RS422
20
Terminal Block D
16
17
18
19
20
Fig. 5.8 RS422/485 Serial Communications
5 INSTALLATION...
Host Computer
'B'
+5V
Tx+
1.8kΩ Pull-up
Tx–
Resistor
Tx+/Rx+
Rx+
'
Rx–
Tx–/Rx–
COM
'
1.8kΩ Pull-down
Resistor
GND
0V
A – 2-wire Connection
Host Computer
+5V
1.8kΩ Pull-up
0V
Resistor
1.8kΩ
Tx+
Pull-down
Tx–
+5V
Resistor
Rx+
1.8kΩ
0V
Pull-up
Rx–
Resistor
COM
1.8kΩ
Pull-down Resistor
0V
B – 4-wire Connection
'A'
'
'
'
'
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