AKI-CDA-MOD-04T | ADDITIONAL INSTALLATION TIPS
7.6
Galvanic Isolation
USB
USB
RS232
RS422/485
+24 V DC
0 V
Filter
DC/DC
galvanic isolation
VCC
0 V (GND)
DC/AC
CFL
Ethernet
Internal electronic
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
Printer
PC
PC
Different ground potential
*
= Internal 0 V (GND) connection
Power CPU
COM COM2
RS422
RS232
USB
When a PC is connected to the operator panel, the panel’s internal 0 V (GND) will be 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 operator panel’s 0 V (GND) 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.
Note:
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 will disrupt 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.
ky
Kollmorgen | October 2014
24
