Entering Programs - Programmable Logic controllers

Each horizontal rung on the ladder represents an instruction in the program to be used by the PLC. The entire ladder gives the complete program. There are several methods that can be used for keying the program into a programming terminal. Whatever method is used to enter the program into a programming terminal or computer, the output to the memory of the PLC has to be in a form that can be handled by the microprocessor. This is termed machine language and is just binary code, such as 0010100001110001.

Ladder Symbols

One method of entering the program into the programming terminal involves using a keypad with keys with symbols depicting the various elements of the ladder diagram and keying them in so that the ladder diagram appears on the screen of the programming terminal. The terminal then translates the program drawn on the screen into machine language.

Computers can be used to draw up ladder programs. This involves loading the computer with the relevant software, such as RSLogix from Rockwell Automation Inc. for Allen-Bradley PLCs, MELSOFTÀGX Developer for Mitsubishi PLCs, and STEP 7À Micro/WIN V4 for Siemens PLCs. The software operates on the Windows operating system and involves selecting items, in the usual Windows manner, from pull-down menus on the screen.

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