PLC Ladder Logic Outputs

Question

On a single ladder rung how many outputs can you have. If you have more than one. Would it be AND Logic, or OR Logic. Series, or parallel. I'm trying to make six lights flash using timer on delay instructions with a closed input instruction. I will using an Allen Bradley SLC 500 series PLC.

Answer 1

In a ControlLogix or CompactLogix PLC a ladder logic rung may have as many outputs (OTE) as you like, both at the right hand end of logic rung and even in the middle of a logic rung.

Each output is controlled only by the logic leading up to it. If you have multiple outputs at the same point in the rung, they will all have the output reflecting the logic condition from the rung start up to that point. This is a common method used to drive several outputs with the same signal at once.

If you have multiple outputs at different points in the rung, each will have outputs that correspond to the logic leading to that output. Logic downstream from an OTE acts as if the OTE wasn't present.

Now, you may have complex devices (e.g., Timer) controlled by logic within a rung. Obviouosly, further logic that depends on the output of the complex device (e.., Timer Done) will not be independent of the behaviour of complex device. But just like OTEs, you may have lots of complex devices in a rung.

Answer 2

If you are programming an SLC500 then you cannot have an OTE in the middle of a rung. It must be at the very right hand side of the rung. You may however (and is a common practice) create a branch around the OTE and have another OTE (or OTU, or OTL, or any other output) on its own branch (again at the very right of the branch).

So using this method you can have as many OTEs on any given rung. However a best practice is to limit the number (to say 10 or 20 per rung) for readability and split them onto several rungs as necessary.