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The Ladder Logic programs look like this:

They are made up of “RUNGS” that form what looks like a ladder. Two bars down each side and Lines between them.  They even use what look like relay symbols.  The ladder is separated into two sides.  The conditional side, where we have the instructions that determine the sequence of conditions necessary to perform a function.

And the Action or Do Something side.  These instructions are actual actions the program must perform, when there is at least one set of TRUE conditions leading to it.

The top two rungs are TRUE as there is one set of continuous GREEN instructions in front of the Action instructions.  The third rung’s action is not executed, because there is no complete GREEN path to it.  This is called a false rung.  It is false because T1.DN is not TRUE (Green).

Programming and trouble shooting are based on the TRUE/FALSE theory.  NOT THE OPEN AND CLOSED CURRENT FLOW THEORY!!!  If you want an action to happen, you have to program a TRUE path in front of it.  As soon as there is even one false instruction in the path, the action will stop or not be done.

This TRUE/FALSE concept makes trouble shooting a basic relay ladder program very simple.  So simple that most people don’t believe it is that easy. If something should happen, look for at least one complete green path.  Whatever is NOT green, but should be to make the action happen, is the problem.  Find out why it is not green.  The inverse is also correct.  If something will not turn off, go to the rung, and check and see what is GREEN that should not be.  Whatever should NOT be GREEN is the problem.  Find out why it is still Green.

Note that the symbols on either side of the program are referred to as INSTRUCTIONS NOT contacts!!  There is really nothing on the screen that actually has any contacts, much less, anything that opens or closes.  I called these INSTRUCTIONS.  Because that is exactly what they are.  Just like everything else in a “Ladder Logic” PROGRAM, each symbol is an instruction.  These instructions tell the PLC what to do, and where to do it.

Instructions are made up of at least 2 parts.  The action, symbolized by the type of instruction, and the address of where to get the information.



In the case of Do something instructions, we again have what action to do, and where to perform the action.



Instructions come in basically 2 sizes.  BIT or BOOL size.  Meaning something that is ON or OFF or is only one bit of memory. 

The next size is either a Word or an INT, which is 16 bits long. 

The last size is a Double Word (Dword) or Double Integer (DINT). 

When working with instructions you have to be aware of the size of the data used by the instruction.  A bit instruction cannot use a Word address.  An Integer instruction cannot handle a Dint sized address or data type.   Most PLC’s will give you an error if you do not have instructions with matching data types.