Specifications

47
Input Devices The PLC receives signals from various switches and sensors in
the controlled machine or related equipment. Many of these
signals are on/off type conditions, also called discrete or digital
signals. In some cases, the signals come from manual devices
such as pushbuttons and selector switches. However, many
discrete PLC inputs come from devices, such as limit switches
or proximity switches, that are turned on and off by machine
operations.
Pushbutton
Selector
Switch
Limit
Switch
Proximity
Switch
In addition to discrete inputs, the PLC may also receive analog
inputs from sensors that vary voltage or current as conditions in
the machine or related equipment vary.
Inputs, as well as the current condition of PLC outputs and
internal data values, are analyzed by the PLC as it executes its
stored program. The PLC uses this process to determine the
signals it sends to output devices that control the operation of
the machine or indicate machine conditions.
Output Devices Just as some inputs are analog type inputs that vary in current
or voltage, some outputs are analog type as well. Many
outputs, however, are discrete (on/off) signals. In some cases,
these signals control equipment directly. In other cases, an
intermediate device, such as a control relay or motor starter, is
used. The following illustration shows a few examples of the
types of devices that may be controlled.
Indicator
Light
Relay
Motor
Starter
Signaling
Column
Signal
Lamp