Beijer Electronics T15BR EN - Manual (Page 26)

Additional Installation Tips
6.6 Galvanic Isolation
Internal electronics
VCC
0 V (GND)
Filter
+24 V DC
0 V
galvanic
isolation galvanic
isolation
RS422/485 RS422 USB USB USB
Ethernet
10 nF
The communication ports COM1 and COM2 have built-in galvanic isolation
from the terminal and the input power supply. The communication port grounds
are bonded to protective earth ground and therefore to one another. Both Ethernet
ports have galvanic isolation as well. There is no galvanic isolation between the
terminal and the input power supply. If galvanic isolation is required between the
terminal and the power supply an external isolation device is required.
Caution should be taken when connecting peripherals to the terminal. Many
peripherals including USB devices will bond the terminal’s signal ground (0 V) to
the protective earth ground. Bonding signal ground and protective earth ground
may increase electrical emissions and introduce communication errors.
Operator panel Modular controller Printer
PC
Different ground potential
= Internal 0 V (GND) connection
PC
galvanic
isolation
RS232 RS422/485
USB
USB device
Power CPU COM COM2
=
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.
Beijer Electronics, MAEN165 26