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1.1.5 Digital Input Circuitry
The Digital Input Circuitry is used to monitor digital status input signals. Several versions have evolved.
They are described in the following sections.
The digital input is divided into two primary sections, High-Voltage and Low-Voltage. All generations of
digital input assemblies have 32 inputs which accept DC field contact voltage. All generations also offer
three input configurations.
Isolated:
This configuration provides channel-to-channel isolation. It is used to monitor contacts having voltage
supplied by isolated station battery circuits.
Internally Commoned:
This configuration internally connects all the return terminals. It is used to monitor contacts having voltages
supplied by a common station battery circuit.
Internally Wetted:
This configuration internally connects field contact voltage to one terminal of each input. It is used to
monitor contacts not connected to a field contact voltage.
• HIGH-VOLTAGE DIGITAL BOARD (FIRST GENERATION)
This version is no longer used in new construction. It has been superseded by the second
generation digital input assembly. This board utilizes precision voltage dividers to reduce the input
levels and high voltage surge protection by-pass capacitors to attenuate incoming transient spikes.
Standard input levels for this generation are 48VDC, 125VDC, and 250VDC. Custom input levels
are available.
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