Nematron Pointe Controller - Manual (Page 86)

Chapter 4: System Design and Installation Pointe Controller User Guide
84
4.6.1 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:
As 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.