PLC look-up table instruction ladder diagram

A look-up table is commonly found in the low level programming language - assembly. Likewise, in the high level programming language such as C, it's called an array.

In the PLC ladder diagram instruction, a look-up table is combined in only one instruction.

Look-up table instruction

• A dest (destination) is any variable to copy the content to.
• LUT[i] is a table with its index i.

The look-up table must have a name, a specific size i with its index i-1. When the input to this instruction true, it sends an element with a specific index to the destination. The overall parameters list below.

Parameters of look-up table instruction

In a simple ladder diagram below, I use this instruction to store a set of ASCII character. It's five in size. The element content of initial index 0 is 65 denotes the ASCII character 'A' and vice versa.

Look-up table ladder diagram example

Every one second the PLC send the letter from A to E to the host PC over UART repeatedly. 

Schematic Diagram

Click here to download this ladder diagram programming example.