GE Mark VIeS System Manual page 124

5.4.3 Operation
Filters reduce high-frequency noise and suppress surge on each input near the point of signal entry. The dry contact inputs on
TBCIS1C are powered from a floating 125 V dc (100-140 V dc) supply from the control cabinet. The 125 V dc bus is current
limited in the power distribution module prior to feeding each contact input. The other terminal board versions use lower
voltages. Refer to the section TBCI Specifications.
Note A current limit resistor is only available on the TBCIH4C.
Discrete input voltage signals pass to the I/O pack, which sends them through optical isolators providing group isolation, and
transfers the signals to the controller. The reference voltage in the isolation circuits sets a transition threshold that is equal to
50% of the applied floating power supply voltage. The tracking is clamped to go no less than 13% of the nominal rated supply
voltage to force all contacts to indicate open when voltage dips below this level.
A pair of terminal points is provided for each input, with one point (screw) providing the positive dc source and the second
point providing the return (input) to the board. The current loading is 2.5 mA per point for the first 21 inputs on each terminal
board. The last three have a 10 mA load to support interface with remote solid-state output electronics. Contact input circuitry
is designed for NEMA Class G creepage and clearance.
124 GEH-6855_Vol_II GEH-6855_Vol_II Mark VIeS Functional Safety Systems Volume II
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