Nematron OptiLogic Series - Manual (Page 7)

Optimation, Inc. (256)883-3050 7
www.optimate.com
OptiLogic Series
Digital Inputs
Digital I/O modules are used to either
monitor (input) or control (output) the
“state” of
something.
“State” being on or off, active or
inactive, open or closed - etc. In the
“real world”
digital I/O requirements come in a variety of
shapes and sizes. Therefore, there are a variety
of available modules designed to meet the
variety of needs.
Typical digital inputs are connected to
switches, buttons, digital outputs from other
equipment, discrete level sensors, thermostats
and other on/off sensing devices.
Digital status is sensed by a controller,
such as an OptiLogic system, by passing current
through an input sensor. When the current is on,
the input state is active. When it is not there, the
input state is inactive.
Input Isolation
In most cases, it is important to
“isolate”
the real world inputs from the internal
electronics of the controller. You want to
prevent some external situation from
“zapping”
the controller
’s electronics.
An effective means of providing such
electrical isolation is optical isolation. The
figure below illustrates the basic concepts of
optical isolation of a digital input circuit. In the
figure shown, when the digital input contact
closes, the circuit path is complete and current
will flow. On the input module this circuit path
passes through a device which emits light when
current flows through it. The light emitter is in
very close physical proximity to (actually in the
same chip) a photo sensor, which will turn on
when it senses light. In this way, a digital input
module can sense whether the input device is
closed (current flow) or open (no current flow)
without a direct electrical connection between
the external sensor and the internal electronics.
DC Inputs
DC digital inputs are typically supplied
by a DC power supply. The most common DC
supplies used in industry are 12VDC and
24VDC.
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.
Power
source
Input module
optical
isolation
To
OptiLogic
processor
sensor
Input module
Input module
optical
isolation
optical
isolation
To
To
OptiLogic
OptiLogic
processor
processor
sensor
Sourcing DC input
Sinking DC input
common
common