23.1 SCRIPTS

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Another method that can be used to explore a state based program is a script. Another way to describe this is "what is happening, what started it and what will stop it?"

The general hierarchy for a script is below,

After we get a process description it must be rewritten as a script. Care is needed to check for consistency.

This can be put into ladder logic. (clearly this logic is not complete and extra work is needed make it ready for the PLC)

Implemented for an Allen Bradley PLC it might look like.

General rules for scripting are,

1. At least one sentence is needed for at least each output.

2. The list of conditions should refer to other conditions.

3. The script should refer to actual inputs/outputs, memory locations or state names.

This method is not rigorous and can be inadequate for many designs.

Use the method to design a parking gate controller.