Rockwell Automation T80004 Application Note Field Loop Configuration - Manual (Page 18)

Trusted
TM
AN-T80004 Field Loop Configuration
Issue 10 Jun 13 AN-T80004 18
T8403 TMR Inputs
When using the Trusted TM TMR 24V dc Digital Input Module T8403 with volt-free contact field devices,
it is better to clamp the voltages rather than rely on potential dividers, because the loop termination
resistor is inside the module and will be paralleled with another during swapping, changing the
resistance.
The field loop configuration shown in the diagram below is recommended for T8403 (and is also
acceptable for T8402). The FTAs used for T8403 inputs do not have resistors fitted, but the modules
themselves have resistors. The T8403 has three input circuits for each of the forty channels.
Two Zener diodes are necessary to clamp the voltage at pre-determined levels when a second module
is fitted in the adjacent Companion slot or the SmartSlot position. The Zener diodes may also be 7V5;
the difference is insignificant. The 8V2 diodes will dissipate 27mW on a closed switch with two
modules, but should be rated for at least 410mW with a T8800 FTA, which is the power expected on a
short circuit to zero volts due to the T8800 fuse rating of 50mA.
Resistors may also be used for line monitoring with T8403 modules, but the effect of the parallel 5K
module impedance (above) and variations in power supply must be taken into account when choosing
resistor values and thresholds. The resistors shown for the T8402 on the previous page are a good
starting point.
The T8800 has 50mA fused supplies to each channel.