Generally PLC accepts two types of input , first one is digital and secondly analog. As we all know that digital input is one which is either 0 or 1. If it is 1 it means input is present and if it is 0 then its not present OR in simple words we can say that it is either ON or OFF. Examples of Digital Input to PLC are Push Buttons, Limit Switch, Trip signal or feedback signal etc.
But the Question is what is Analog Input ?
Analog input are those whose real time value keep on changing accordingly i.e. if we talk for PLC then widely two types of analog input are accepted by PLC . First one is Voltage analog input and second one is Current analog input. Voltage Analog input is 0-10 V and Current input is 4-20 mA. All the devices whose output is in above two forms can be connected to PLC. Pressure, temperature, flow etc are a few things which can be measured and accordingly give output in current or voltage and can be read on PLC.
How to Connect Analog Input to PLC?
To connect Analog input first connect analog input card to PLC and then select whether its current input or voltage, i.e you have to define in the setup of analog input card that whether you are connecting voltage or current input. Some PLC card also have a dip-switch and through dip-switch also you can define type of input.
There will be two wires one "+" and one "-" , simply connect the "+" with "+" and "-" with "-".
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PLC Programming for Analog Input:-
First of all PLC samples the data i.e it converts the analog value to digital at a continuous sampling rate . Sampling rate differs from PLC to PLC. So from programming point of view we have to write a program to scale the value. PLC converts the received value at a fixed sampling rate to pre-defined range and after that we can convert it according to our use.
For example:- A 4 to 20 mA signal will be sampled by PLC in to 1 to 6000 accordingly and after that by programming we can convert data sampled by PLC in to any range according to our requirement.
See below image for sample program idea for analog input.
As You can see in the above picture that we have converted the data received by PLC in to 0 to 100 accordingly. So the 4-20 mA signal received is converted in to 0-100 and after this we can use it according to our application.
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