PLC- Programmable Logic Controller from Industry Practice

4. Basic Programming Instructions

Each program statement is represented with a line, called the rung, that has all relevant inputs to the left and the output to the right.

The output device of a rung is energized if electric power can conceptually flow from the left side of the rung to the right side. Input devices are assumed to block the flow of power if they are not activated.

During the execution of a ladder diagram, the PLC reads the states of all inputs, then determines the states of all outputs starting from the rung at the top side, going down to the last rung, and finally updates the state of the output devices.

During the development of a PLC program, we must use specific names to identify the inputs, outputs, memory flags, timers, and counters.

PLC manufacturers use a variety of approaches in naming the inputs, outputs and other resources.

A typical naming convention is to identify inputs with the letter “I” and outputs with the letter “O”, followed by a 1-digit number that identifies the slot number and a 2-digit number that identifies the position of the input or output in the slot.

By manufacturers and types representation and nomenclatures differs for Inputs, outputs, memory flag and all other operands in PLC. Like Siemens uses I0.0 to I0.7 One byte to present 8 inputs and then next byte will start from I1.0 to I1.7 and further onwards. In Allen Bradley you will find I:0/0, I:0/1 etc.

1. NO- Normally Open:

NO stands for normally open contacts.

NO will check whether input / respective assigned flag is On or Off.

If assigned flag or Input is Off, then power will not flow further and further statement will be not true.

When assigned input or flag turn ON then NO will turn position to close from open and further statement or condition will be start executing to provide required output.

NO is normally used to assign condition to start sequence.

2. NC- Normally close:

NC stands for normally close contacts.

NC will check whether input / respective assigned flag is off.

If assigned flag or Input is Off, then power will flow further and further statement will be start or execute.

When assigned input or flag turn ON then NC will turn position to open from close and further statement or condition will be stop.

NC contact normally used to put interlocks of some critical conditions in sequence to prevent faults and make sequence safe.

Like putting trip interlock in sequence, low level interlock etc.

3. Coil- Flag/output:

Coil symbol is used to energize output or assigned flag as per written logic.

Coils can be used to assign reference memory flag to use the same in further programming interlock to set some condition and use the same in further program to check the same and execute.

Also coil is assign to actual Digital output to turn ON output on system / machine to start actual machine operation as per logic.

In below picture you can find example of NO, NC and Coil symbol and its basic use in PLC programming.

Positive Transition Sense (PTS)

The condition of the right link is ON for one ladder rung evaluation when a change from OFF to ON at the specified input is sensed. It is also called as positive pulse/ trigger in some PLCs.

Negative Transition Sense (NTS)

The condition of the right link is ON for one ladder rung evaluation when a change from ON to OFF at the specified input is sensed. It is also called as Negative pulse/trigger in some PLCs.

There are many more basic operators and instructions like S, R, SR, RS, first scan, Always On and Not etc. in PLC to control and develop logic, you can easily get the details of respective instructions and its use in software of that PLCs how to use and all details.

Keep Programming to develop more skills and expertise.