Output coil instructions, help!

Hey, I'm having a little trouble answering this question, and was wondering if someone could help:
1- Why should duplicate output coil instructions be avoided?
2- How can this problem be resolved.

We don't provide homework answers here, but we will help you work through it. Output coil instructions are not devices, they are computer instructions that change the data stored in a location in memory. With that in mind I think you can work through it - namely if you have an instruction addressed to a location in memory that modifies the value in memory, and later another instruction modifies the value in that same memory location what is the result?

Would the result be that there would be no output, as both address are trying to access the same memory space? Or would the second the address access the memory space as that was the last seen address scanned by the cpu?

Here is a hint. Normally, Ladder Logic is scanned one rung at a time, from left to right, then the next rung is scanned.

Alright I think understand the answer to it now, thank you !

Part 2 of your question could have just a sarcastic answer - "Don't do that" - but someone developing a control program would have to analyse why the code seems to demand control of an output from different areas.

One could be that the output should be on if any of the conditions in the sections of code enable it.

Another would be if the sections of code are mutually exclusive, either one controls the output or the other does.

There may be other logical considerations for the sections (if this one or that one but not if that one is off ...)

These situations can demand different methods of solving the problem, many methods you discover after much time scratching your head (not that many older programmers have balding heads.)

Now that you think you have the answer, understanding what is going on in memory is important because different PLCs will respond differently depending on whether the IO is synchronous or asynchronous. If the IO is synchronous to the scan then whatever value is in memory at the end of the scan is the value that is transferred to the physical IO. (This is most common and is probably the answer your advisor is looking for.) However, some of the more modern and most popular PLCs have asynchronous IO updates, the physical IO updates based on memory contents on a predetermined rate, even if it is in the middle of a scan. If you don't understand what is happening in memory on one of these machines then a double coil can really cause some perplexing, and potentially dangerous, process behavior.

Ron Beaufort, a forum member, has some videos on Youtube that explain and demonstrate this. Search for his name at Youtube.com to find them.

BTW, welcome to the forum, hope to see more of you. Controls and Automation is a fun and challenging career path so hang in there.

in hardware you can use a coil only once so do it.