5.1.2
Operation
The following features are common to the distributed I/O modules:
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I/O Module Common Diagnostic Alarms
5.1.2.1
Input Signals
The discrete input acquisition board provides the second stage of signal conditioning and level shifting to interface the
terminal board inputs to the control logic. Initial signal conditioning is provided on the terminal board. The discrete input
acquisition input circuit is a comparator with a variable threshold. Each input is isolated from the control logic through an
opto-coupler and an isolated power supply. The inputs are not isolated from each other. Each of the twenty-four inputs has
filtering, hysteresis, and a yellow status LED, which indicates when an input is picked up. The LED will be OFF when the
input is dropped-out.
5.1.2.2
Variable Threshold
The input threshold is derived from the contact wetting voltage input terminal. In most applications this voltage is scaled to
provide a 50% input threshold. This threshold is clamped to 13% to prevent an indeterminate state if the contact wetting
voltage drops to zero. If the contact wetting voltage drops below 40% of the nominal voltage, the under-voltage detector
annunciates this condition to the control. A special test mode is provided to force the inputs from the control pack. Every four
seconds, the threshold is pulsed high and then low and the response of the opto-couplers is checked. Non-responding inputs
are alarmed.
YDIA Discrete Input Modules
GEH-6855_Vol_II System Guide 117
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