Rockwell Automation 1772-LV Mini-PLC - 2/15 Programmable Controller (Series B) Programming and Operations - Manual (Page 11)

An Introduction to Programmable
Controllers
Chapter 2
2Ć4
Indication
The input section of most modules also provides a visual indication of the state
of each input terminal with indicators. The indicator is on when there is a
voltage applied to its terminal. It is off when there is no voltage applied to its
terminal. Since the indicator reveals the status of its terminal, it’s usually called
an input status indicator.
You should also notice another important characteristic of input indicators.
They are only associated with terminals used for wiring sensing devices to the
input section. The terminal that’s used to provide a ground for the sensing
circuits has no indicator.
Conditioning
Another function of the input section is signal conditioning. The electrical
power used at the machine is usually not compatible with the signal power used
within the programmable controller. Therefore, the input section receives the
electrical signal from the machine and converts it to a voltage compatible with
the programmable controller’s circuitry.
Isolation
The input section isolates the machine circuitry from the programmable
controller’s circuitry. Isolation helps to protect the programmable controller’s
circuitry from unwanted and dangerous voltage levels that may occur
occasionally at the machine or in the plant’s wiring system.
Output Section
The output section has functions similar to those of the input section:
Termination
Indication
Conditioning
Isolation
Termination
The output section provides terminals for the field wiring going to the output
devices on the machine.
Indication
The output section of most modules provides a visual indication of the state of
each output device with indicators.
The output status indicator will be on when the output device is energized. A
common term applied to either input status indicators or output status indicators
is I/O status indicators. I/O stands for either input or output.