Endura AZ40 | Oxygen and carbon monoxide equivalent (COe) analyzer |
COM/AZ40/MOD–EN Rev. A
9
4.2 Two-wire and four-wire connection
MODBUS serial communications can be configured as either
2-wire or 4-wire serial links – see Fig. 4.1. The transmitter must
be added to the link configuration on the host system – refer to
information supplied with the host system.
4.3 Pull-up and pull-down resistors
To prevent false triggering of slaves when the master (host
computer) is inactive, fit pull-up and pull-down resistors to the
RS422/485 interface in the host computer.
Resistors are normally connected to the interface by
hard-wired links or switches – refer to the manufacturer's
instructions
4.4 Termination resistor
For long transmission lines, fit 120
termination resistors to
the last slave in the chain and the master receiver – see
Fig. 4.2.
4.5 Serial connections
Make connections to the MODBUS serial board as shown in
Fig. 4.1. On systems with multiple slaves, make connections to
two- or four-wire link configurations in parallel as shown in
Fig. 4.2. When connecting cable screens, ensure that ground
loops are not introduced.
The maximum serial data transmission line length for both
RS422 and RS485 systems is 1200 m (3937 ft.).
The types of cable that can be used are determined by the total
line length:
— Up to 6 m (19.7 ft.) – standard screened or twisted pair
cable.
— Up to 300 m (984 ft.) – twin twisted pair with overall foil
screen and an integral drain wire.
— Up to 1200 m (3937 ft.) – twin twisted pair with separate
foil screens and integral drain wires.
Fig. 4.1: 2-wire and 4-wire MODBUS serial links
COMMUNICATIONS
MODULE
SK1
SK2
1 2 3 4 5
2-wire connection
Common
Tx/Rx–
Tx/Rx+
Common
Rx–
Rx
+
Tx–
Tx
+
CO
MUN
ICATI
ONS
MODU
LE
4-wire connection
Fig. 4.2: Connecting multiple slaves
1
2
3
4
5
1
2
3
4
5
Host
computer
Tx–
Rx+
Rx–
COM
Tx+
Tx–
Rx+
Rx–
COM
Tx+
Last slave
First slave
120
termination
resistor
Master
120
termination
resistor
