Programming of PLC


There are various approaches for entering the program into PLC they are

  1. Ladder diagram based
  2. Low level based on Boolean expressions
  3. Functional blocks
  4. High level language

Most of the programming methods used today for PLC are based on the ladder logic diagram. Therefore the concept of ladder diagram is explained in the following sections

The PLC programming based on the use of ladder diagram involves writing a program in a similar manner to drawing a switching circuit. The ladder logic diagram is converted into PLC ladder diagram by using the conventions of PLC ladder diagram constructions. This method requires the use of simple keyboard and CRT with minimum graphic capability to display the symbols, representing components and their inter relationship in the ladder logic diagram. The components are of two types, contact and coils. Contacts are used to represent input switches, relay contacts and similar elements. Coils are used to represent load such as solenoids, relays, timers, counters etc. The programmer inputs the ladder diagram rung by rung into the PLC memory with the CRT displaying the results for verification.

The ladder diagram has two vertical sides (also called rungs) (Figure 1.1). The left side line represent line with a positive voltage and right side represent a line with zero voltage. Between these two sides are the horizontal rungs for the assumed power flow. The symbols representing the various program elements are placed on the rungs in order to realize the required control task.


There are five program elements/operations commonly used in PLC ladder diagram they are

  1. PLC Bit logic operations
  2. Timer Operations
  3. Counter operations
  4. Comparison operations
  5. Arithmetic operations.

1 PLC bit logic operations : Some important programming elements for bit logic operations are

a) NO contact
b) NC contact
c) Coil

Each of these elements can be selected from the program window. NO and NC elements should
not be confused with the hardware NO and NC contacts of switching devices.