User Manual

Additional Installation Tips
7.6 Galvanic Isolation
USB
USB
RS232RS422/485
+24 V DC
0 V
Filter
DC/DC
galvanic isolation
VCC
0 V (GND)
DC/AC
CFL
Ethernet
Internal electronics
1.5 m
The operator panel has galvanic isolation against the 24 V DC feed but no galvanic
isolation between the communication ports for RS232, RS422/485 and USB.
Only the Ethernet connection has galvanic isolation.
* *
*
***
**
*
Operator panel Modular controller Printe
r
PCPC
Different ground potential
*
= Internal 0 V (GND) connection
Power CPU COM COM2
RS422 RS232 USB
When a PC is connected to the operator panel, th e internal 0 V (GND) of the
panel is connected to the protective ground via the PC.
A number of USB d evices 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 U SB memory stick, keyboard, or similar
device is plugged in.
If a n umber of units are connected that have a 0 V and a ground connection, an d
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 p anel, and can cause errors.
Use externa l units to improve communication and achieve galvanic isolation.
Westermo has good industry-standard insulators that are also insulated from the
24 V DC feed.
Note:
It is very important to make sure that the 24 V feedin the external insulation unitis not
connected to one of thecommunication outlets. If it does not have 100% insulation
againstthe 24 V feed,disturbancesand groundingcurrentsfrom the0 Von the24 V
side disrupts the communication.
Using this typeof unitsolves o ne problem but creates alarger problem! A substandard
installationmaywork now, but problemsmayarise when other devicesare connected.
Beijer Electronics, MAEN202
60