Analogue electrical signals are used to represent process va

Analogue electrical signals are used to represent process variables such as pressure, speed, temperature, flow rate and so on. The instruments produce electrical outputs that are processed into a standard form so that standard equipment can be connected to it such as analogue to digital converters and meters. One standard form is 4-20 mA.

Give the other common standard forms used in PLC for I/O interconnection?

Why is the minimum in the above standard 4mA instead of 0 mA?

Solution

The other standard form used is a plc with 24VDC I/O,with values above 22V DC representing on,values with below 2V DC representing Off,and Intermediate values undefined.This are used mainly in Discrete Type I/O

The reason for using 4mA is initially transmitter were first introduced which was working on pneumatic pressure signal for transmission.Later electronic chips were introduced which requuire minnimum 3mA of current to function .so, a margin of 4mA is taken as reference. also if a wire is broken the current drops down and so it knows this is an error and not the value of measurement which is equal to 0 and also in addition to error detection you can supply simple remote sensors with the current even over a long distance.