PLC Digital Input

03_PLC_1_Digital Input.doc

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INDUSTRIAL AUTOMATION AND NETWORKING SECTION

Programmable Logic Controller_1

==== DDIIGGIITTAALL IINNPPUUTT MMOODDUULLEE ====

Function:

Receive and convert field signals into a form that can be used by the CPU.

Input devices that send on/off signal to the PLC:

� Push buttons

� Limit switches

� Proximity sensors

� Photo-electric sensors

� Level switches

� Pressure switches

� Auxiliary contacts of a contactors

� Relay/contactor contacts


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The module recognizes an input signal as being ON if the input voltage level is at 40% of

the supplied reference voltage.

The module recognizes an input signal as being OFF if the input voltage falls under 20% of

the supplied reference voltage.

NB: The value given is only a guideline. For exact value always refer to specifications of

the PLC input module supplied by the manufacturer.

There are several types of input interface/module:

� AC

� DC

� TTL

� AC input module


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AC input circuit:

The input circuit responds to an input signal in the following manner:

� An input filter removes false signal due to contact bounce or electrical interference

� Opto-electrical isolation protects the input circuit and back-plane circuits by isolating

logic circuit from input signals

� Logic circuits process the signal

� An input LED turns ON or OFF, indicating the status of the corresponding input

device


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� DC input module

There are two types of DC input module:

� Current sourcing

� Current sinking

If a field device provides current when it is ON, it is said to be sourcing current. A

sourcing field input device is compatible with a current sinking input module.

If a field device receives current when it is ON, it is said to be sinking current. A sinking

field input device is compatible with a current sourcing input module.


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NB: Always refer to input wiring diagram supplied by the manufacturer. Check whether

the PLC uses built-in/internal sensors supply.


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� TTL input module

It can receives signals from:

� TTL-compatible devices

� 5 VDC-level control devices

� BCD thumbwheel switches operating at TTL levels

� Specifications of discrete input module

PLC Digital Input

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