Programmable Logic Controller (PLC)

A programmable logic controller is a computer based solid state device that controls industrial equipment and processes. The typical method of programming PLCs is using ladder logic.

Rules for Ladder Logic

1.   Vertical lines indicate power supply ===> Power flow is from left to right.
2.   Ladder diagram is read top to bottom and left to right.
3.   Electrical devices are normally shown in their de energized condition.
4.   Devices that indicate a start operation for a particular item are normally wired in parallel.
5.   The contacts associated with coils, timers, counters and other instructions have the same numbering convention as their control device.
6.   Devices that indicate a stop operation for a particular item are normally wired in series.
7.   The operation of latching is used where a momentary start input signal latches the start signal into the ON condition so that when the start input goes into the OFF condition, the start signal is energized ON.
8.   An output address status is readily available to rungs or branches which follow its generation.

The Different Ladder Logic Instructions

1.   Standard Relay Logic Type
2.   Timers and Counters
3.   Arithmetic and Logical
4.   Move
5.   Comparison
6.   File Manipulation
7.   Sequence Instruction
8.   Specialized Analog (PID)
9.   Communication Instructions
10.  Diagnostics
11.  Miscellaneous (Sub routines)

A Ladder Logic Network

The standard relay logic type are of two types which are:

1.  Normally Open Contact: This instruction examines its memory address location for an ON condition. If ON instruction is ON. If OFF instruction is OFF.
2.  Normally Closed Contact: This instruction examines its memory address location for an OFF condition. If ON instruction is OFF. If OFF instruction is ON.

Timers are of three types which are:

1.   Timer ON Delay
2.   Timer OFF Delay
3.   Retentive Timer

The three parameters associated with each timer are:

1.   Preset Value: This is the constant number of units of time that the timer 'times to' before being energized or de energized.
2.   Accumulated Value: This is the number of units of time recording how long the timer has been actively timing.
3.   Time Base: This indicates the unit of time in which the timer operates .

There are basically two types of counters which are:

1.   Count Up Counters: The counter increments the accumulator value by 1 for every transition of the input contact from false to true.
2.   Count Down Counters: The counter decrements the accumulator value by 1 for every transistion of the input contact from false to true.

The move instruction moves the source value at the defined address to the destination address every time it is executed.
A sub routine file is a stand alone module of ladder logic code which is used repeatedly by the main program.
There are two ways of transferring control of laddrr logic program from the standard sequential path in which it is normally executed are:

1.   Jump to a part of the program when a rung condition becomes true
2.   Jump to a separate block of ladder logic called a sub routine.

Restrictions in the use of Ladder Logic Diagrams

1.   Number of coils and contacts per rung (or network)
2.   Vertical Contacts
3.   Nesting of Contact
4.   Preset Value Ranges
5.   Direction of Power Flow