Graphic Operator Terminal /
INSTALLATION TIPS
24
If a number of units are connected have a 0 V and a ground connection, and these units are
connected to various grounding points, there is a substantial risk of problems. Grounding currents
go through communication cables, the rear plate of the controller, and internally in the graphic
operator terminal, and can cause errors.
Note:
It is important to make sure that the 24 V feed in the external insulation unit is not
connected to one of the communication outlets. If it does not have 100% insulation against the 24
V feed, disturbances and grounding currents from the 0 V on the 24 V side will disrupt
communication.
Using this type of unit solves one problem but may create a larger problem! A substandard
installation may work now, but problems may arise when other devices are connected.
6.7
Cable and Bus Termination RS485
Use shielded and twisted pair cable.
The pair capacitance may not exceed 52.5 pF/m and area at least 0.25 mm
2
(AWG 24), if you
want to use the maximum transfer distance and maximum transfer speed.
0 V, the reference voltage for communication should be included in the cabling. With two-way
communication use two pairs; one pair for communication and one pair for 0 V.
The shield must be grounded at one end. The other end is usually grounded, but with longer
distances or when there is a difference in the ground potential, the shield should be
connected to the ground via 0.1 uF/250 V plastic capacitor to prevent ground current in the
braided shield.
A number of manufacturers recommend that the shield be grounded at each node. Various
manufacturers have different systems for bus termination. The RS485 standard does not describe
how the Fail Safe function is carried out, just that the system should be able to handle the error.
Depending on the recipients’ design, the bus wires may be on the same level or require pull-up or
pull-down to ensure that no faulty signals are detected when the bus is in resting mode (all
transmitters are disconnected).
