Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 22)

Trusted
TM
AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 22
Field powered inputs
These inputs appear as a voltage signal, powered by the remote device. This is likely to be another
PLC, which may or may not have isolated outputs.
There is an inherent danger with field powered inputs: the system supplies may be on different mains
phases, causing damaging potential differences, unsafe wiring, or even just massive signal noise on
semi-isolated channels. All field powered inputs must therefore be isolated. In the three circuits shown
below, note that each has a point of isolation.
All Trusted digital input modules provide isolation between the system and field voltages, but they do
not provide isolation between individual channels. Therefore an input module can be assigned to
receive all inputs from another unisolated PLC, as long as there are no other devices connected, or all
other devices are isolated. However, the input wiring must be physically protected as if it is mains
wiring, because it may be at a high potential with respect to the system.
If a whole input module cannot be assigned to just one remote system, care must be taken to ensure
that isolation is provided between each supply source. This may include isolated outputs at the remote
end to Trusted:
Or relay/galvanic/optical isolation devices in the loop:
For relay isolation, safety related signals should use appropriate certified safety relays. These are
described in the digital outputs section.
The isolation device should be placed near the remote end, so that the digital input module can monitor
the line for short/open circuits. Line monitoring components may be required to set the measured
voltages for ‘open’ and ‘closed’ states. If the remote device is also a safety system, it should be set to
monitor its side of the circuit (and an auxiliary contact if using a safety relay).
Output
Module Input
Module
Output
Module Input
Module
Output
Module Input
Module