ABB ECA06 - Communications Signal Wiring, RS485; Communications Signal Wiring, RS232; Connections in an Electrically Noisy Environment

2.4 Electrical Installation Chapter 2: Installation
38 493-0736-11 (6-2)
Communications signal wiring, RS485
For serial communications (RS485) screened twisted-pair cable must be used
with the screen grounded at each device. RS485+ should be connected to terminal
b8 and RS485- should be connected to terminal b6. The last device on the data
bus must be terminated by a 120 Ω impedance. This is achieved by connecting a
120 Ω resistor between terminals b6 and b8.
Communications signal wiring, RS232
For serial communications (RS232) screened twisted-pair cable must be used
with the screen grounded at each device. RS232Tx should be connected to
terminal b4 and RS232Rx should be connected to terminal b2.
Connections in an Electrically Noisy Environment
When making connections in an electrically noisy environment the controller
should be connected to a phase which suffers the minimum of interference, i.e.
contactors or thyristor controlled equipment should not be used on the same line.
If this is impossible an isolating transformer or a magnetic voltage stabilizer
should be installed. This type of equipment can of course be used to supply a
number of controllers, however you must ensure that no electrically noisy
equipment is connected to the secondary side – the same side as the controller.
Follow the manufacturer’s recommendations concerning grounding of this type
of transformer.
Screening
The input and output wiring to field mounted equipment, e.g. sensors and input
converters, is connected to the controller using screened twisted pair cable.
If the connections are made via an intermediate terminal block in an enclosure the
screen should be connected to the grounding terminal with as short a length of free
screen as possible, preferably less than 10 cm.
Note: In cases where analog and digital cables share the same cable runs within
an enclosure, the digital cables should also be screened. The screening should be
grounded at the controller chassis.
All screening on incoming field cables directly connected to the controller must
be grounded at the controller chassis. When separate cable runs are used twisted
pair cables will suffice. In interference free conditions single wires can be used.
However signal wiring should not be run together with power cabling unless the
signal wiring is shielded.