Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 17)

Trusted
TM
AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 17
Line Monitoring and Marshalling for Volt-Free Inputs
Field loops providing digital input signals to a Trusted
TM
system need line monitoring components if
they are safety related inputs or require assurance of operation.
Defining the open and closed states nominally at 16 and 8 volts (as above) simplifies the selection of
line monitoring components.
There are two types of 24V dc digital input module; T8402 (dual) and T8403 (TMR). These use
different field termination assemblies, which affects the selection of line monitoring components.
Using both the circuits shown below, an open switch on a healthy line will signal 8V to the module, and
a closed switch will signal 16V. Thresholds between each state should be placed to allow at least 2
volts error, an indeterminate state between open and closed, detection of open and short circuit and a
gap between each of the rising and falling thresholds to prevent rapid cycling between states.
Other configurations may be used, but the thresholds should be set around the expected voltage levels
in each state.
T8402 Dual Inputs
The diagram below shows the recommended configuration of a volt-free contact field loop monitored
by a Trusted
TM Dual 24V dc Digital Input Module T8402, connected through a T8802 FTA. This
requires a 60 channel cable, TC-601, 603, 701 or 703. The T8402 has two input circuits for each of the
sixty channels.
Field devices forming inputs to T8402 input modules may be conditioned using resistors. This is
because the equivalent input impedance is located on the Field Termination Assembly (FTA), therefore
when the input modules are paired in a hot swap arrangement, the effective impedance seen by the
field loop remains unchanged.
It is also possible to use the zener circuit shown below with T8402 module inputs using a T8802 FTA.
If free wire cables (TC-602, 604, 702, 704) are used to wire to terminals, the resistor must be wired in
the terminals. 5K is a nominal value and the more standard 4K7 could be used.