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Cable and Bus Termination RS485
When a PC is connected to the operator panel, the internal 0 V (GND) of the panel is
connected to the protective ground via the PC.
A number of USB devices can have the shield connected together with the protective
ground. Here, the 0 V (GND) of the operator panel is connected to the protective
ground when, for example, a USB memory stick, keyboard, or similar device is
plugged in.
If a number of units are connected that have a 0 V and a ground connection,
and these 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 operator panel, and can cause errors.
Use external units to improve communication and achieve galvanic isolation.
Westermo has good industry-standard insulators that are also insulated from the
24 V DC feed.
It is very 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 disrupts the communication.
Using this type of unit solves one problem but creates a larger problem! A
substandard installation may work now, but problems may arise when other
devices are connected.
Cable and Bus Termination RS485
•
If maximum transfer distance and maximum transfer speed is needed, shielded
and twisted pair cable should be used. The mutual capacitance may not exceed
52.5 pF/m, and the cable area should be at least 0.25 mm
•
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 µF/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.
42
Section 7 Additional Installation Tips
2
(AWG 24).
3BSE069462-600 Rev D
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