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5.5 RS422/485 Serial Communications
The serial interface fitted to this instrument 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
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 120 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.
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– –
Table 5.1 Thermocouple Compensating Cable
Module Position C
16
17
18
19
20
RS422
Module Position C
16
17
18
19
20
Fig. 5.8 RS422/485 Serial Communications
C
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9
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– –
B
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/ e
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d
5 INSTALLATION...
Host Computer
'B'
+5V
Tx+
1.8k Pull-up
Tx–
Resistor
Tx+/Rx+
Rx+
'A'
Rx–
Tx–/Rx–
COM
'B'
1.8k Pull-down
Resistor
GND
0V
A – 2-wire Connection
Host Computer
+5V
1.8k Pull-up
0V
Resistor
1.8k
Tx+
Pull-down
Tx–
+5V
Resistor
Rx+
1.8k
0V
Pull-up
Rx–
Resistor
COM
1.8k
Pull-down Resistor
0V
B – 4-wire Connection
See Warnings on Page 98
b
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7
P
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3 .
0
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B
a l
k c
i s
a c
y l l
a s
e f
c
c r i
u
s t i
– –
101
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