Two-Wire And Four-Wire Connection; Pull-Up And Pull-Down Resistors; Termination Resistor; Serial Connections - ABB Endura AZ40 Operating Instructions Manual

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.
1 2 3 4 5
2-wire connection
4-wire connection
Fig. 4.1: 2-wire and 4-wire MODBUS serial links
SK1
SK2
Endura AZ40 | Oxygen and carbon monoxide equivalent (COe) analyzer | COM/AZ40/MOD–EN Rev. A

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.
Last slave
1 2 3 4 5
COM
Rx–
Rx+
Tx–
Tx+
120  termination
resistor
Fig. 4.2: Connecting multiple slaves

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.
120  termination
First slave
resistor
1 2 3 4 5
COM
Rx–
Rx+
Tx–
Tx+
computer
Master
Host
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