Additional Installation Tips
24
Mitsubishi Electric, MA00818C
6.6 Galvanic Isolation
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.
When a PC is connected to the 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 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.
5356
Internal electronic
Filter
DC/DC
galvanic isolation
1.5 m
VCC
0 V (GND)
Ethernet
DC/AC
CFL
0 V
+24 V DC
RS422/485
RS232
USB
USB
*
*
*
*
*
*
*
*
5357
*
Operator panel
Modular controller
PC
RS422
RS232
USB
CPU
COM COM2
Power
Not same ground potential
PC
Printer
* = Internal 0 V (GND) connection
