Thursday, February 25, 2010

Introduction to PLC Ladder Logic

One method of the PLC programming is using ladder diagram method. Ladder diagram consists of a descending line on the left, with lines branching to the right. This line is the line branching instructions. Throughout this instruction line consists a combination of logic that states when and how the existing instructions on the right side are done.

PLC Ladder Diagram Example
PLC Ladder Diagram Example

The logic combination of ladder diagram as following:
A. Instruction LOAD (LD) and LOAD NOT (LD NOT)
The first condition that starts any logic block in the ladder diagram associated with LOAD instruction (LD) or LOAD NOT. (LD NOT). Each of these instructions requires one line of mnemonic code.

B. Instruction AND and AND NOT
If two or more conditions that are connected in series on the same instruction line, then the first condition using LD or LD instruction and the remainder NOT use the instructions AND or AND NOT.
AND instruction can be imagined to produce ON if both conditions are linked with this instruction in all ON conditions, if any one in the OFF state, let alone both OFF, the instruction will always result AND OFF too.

C. Instruction OR and OR NOT
If two or more conditions connected in parallel, meaning in a different line of instructions and then joined again in the same instruction line, then the first condition associated with LD or LD instruction and the rest is NOT related to the instructions OR or OR NOT.
In this case imaginable OR instruction will always result in ON execution condition when any one of two or more conditions connected with this instruction in the ON condition.

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