Galvanic Isolation - Mitsubishi Electric E1151 Installation Manual

Additional Installation Tips

6.6 Galvanic Isolation

DC/DC
Filter
galvanic isolation
+24 V DC
0 V
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
PC
*
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.
24
Internal electronic
VCC
0 V (GND)
1.5 m
RS422/485 RS232
RS422 RS232 USB
* *
Not same ground potential
* = Internal 0 V (GND) connection
DC/AC
CFL
USB USB
Modular controller
Power CPU COM COM2
* * *
*
Mitsubishi Electric, MA00780C
Ethernet
5356
Printer
*
PC
*
5357