Introduction of Relay Ladder Logic

Relay ladder logic (RLL) is the most common programming language used to communicate with PLCs and was developed to mimic relay logic.

It uses graphical representations of contacts, coils and special instruction blocks and was originally designed to facilitate simplicity and ease-of-use. It is still the industry standard today.

The language can be viewed as a set of connections between logical checkers (contacts) and actuators (coils). If a path can be followed from the left side of the rung through asserted (true) contacts to the output, then the rung is true and the output is asserted (turned “on”). If this path cannot be followed, then the output is false.The actuator would not be energized.

Each coil or contact correlates to the status of a single bit in the PLC’s memory. A ladder program can refer an unlimited number of times to the status of a single bit.

This makes the ladder program equivalent to an electromechanical relay with an unlimited number of contacts. This is one reason why the relay ladder logic language is so widely used.

Contacts may refer to:

• pushbuttons

• limit switches

• internal storage bits generated in the program


Some of the symbols used in RLL are:

—( )— A regular coil, energized whenever its rung is closed.

—(\)— A “not” coil, energized whenever its rung is open.

—[ ]— A regular contact, closed whenever its corresponding coil or input which controls it is energized.

—[\]— A “not” contact, closed whenever its corresponding coil or input which controls it is not energized.

Conversion of Line Diagrams to Relay Ladder Logic

Line diagrams refer the schematic diagrams used in relay logic circuits. In these diagrams, the inputs and outputs are drawn in a series of lines.

Relay ladder logic was designed to mimic relay logic (line diagrams) so it is fairly easy to convert one form of communication to the other.

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  1. The explanation of the RLL information is simplified by the way its shared on this sight. Thank you