Galvanic Isolation - Mitsubishi Electric E1000 Series Service And Maintenance Manual

3.10.6 Galvanic Isolation

DC/DC
Filter
galvanic isolation
+24 V DC
0 V
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
RS422
PC
*
When a PC is connected to the panel, the panel's internal 0 V (GND) will be con-
nected 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 prob-
lems. 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.
Mitsubishi Electric, MA00758A
Internal electronic
VCC
0 V (GND)
RS422/485 RS232 USB
RS232 USB
* *
*
Not same ground potential
* = Internal 0 V (GND) connection
Ethernet
DC/AC
CFL
USB 5356
Modular controller
Printer
Power CPU COM COM2
* * *
Hardware
*
PC
*
5357
55