Nematron Pointe Controller - Manual (Page 87)

Pointe Controller User Guide Chapter 4: System Design and Installation
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Typical DC digital input circuits are shown below:
As shown, the physical optical emitter on the input module is an LED (light
emitting diode). OptiLogic DC inputs use bidirectional LEDs — i.e. Your inputs
may either source or sink current. The top figure shows a sourcing input. The
figure below it shows a sinking input. When inputs are connected to a “common”
(most instances), inputs must be either all sourcing or all sinking.
AC Inputs
AC digital inputs are typically supplied either directly from line voltage or
transformed down from line voltage. The most common AC inputs are 120VAC
and 24VAC, although any voltage range is possible.
A typical AC input circuit is shown below:
As shown, the physical optical emitter on the input module consists of two LEDs
of opposite polarity. An AC (alternating current) connection flows current one
way, then the other. Light is emitted in both cases.
There is a short period when voltage, and therefore current flow, switches from
one direction to the other when no current flows. This is called zero crossover.
During zero crossover, the digital input circuit must “debounce” the signal to
ensure that the system does not provide a false indication that the input contact
is not closed when it is, in fact, closed. OptiLogic AC digital inputs handle such
zero crossover conditions.