WildFireDefenceSystems (Customer) asked a question.
The PLC will be placed in a cabinet, I would like to Start and Stop the program with two push button switches located outside of the cabinet. Power always supplied to the PLC.
There is a "Stop Program" instruction so that one is easy but what about starting the program?
This is not the way PLCs are generally programmed. Using the STP command is probably not a good idea in general but if used should only be used for a very specific reason, not as a general matter of course. Logic dictates how the program behaves in response to inputs. Stopping the CPU from running is not what you're trying to do.
The machine I'm building would require sometimes to be stopped by the operator lets say to clear a product jam. So I have a start and stop switches for that. It really would no matter anywhere in the program it stops and starting at the beginning at any time is ok too.
If there is any chance your machine could injure an operator in any way, you should employ someone experienced in programming and machine safety.
Once the Stop Program instruction is executed you can only put the CPU back into run via the GUI or doing a stop -> run via the switch on the CPU. There is no way programmatically to put the CPU back into run.
In food processing settings the electronics are never exposed, so somehow the machine needs to be stopped and started without access to the CPU. I have start and stop switches that will be wired to the input card I just don't see how to write this part of the program.
What is the PLC running? Motors? Air cylinders?
What you're not understanding is the PLC CPU is not stopped when you wish to stop the operation; you program "stops" by breaks in a circuit of your PLC code. An example is a 3-wire Start/Stop circuit controlling a motor starter.
Thanks, after speaking with AD Tech I understand generally LL can't start once stopped. So I have a mechanical work around so I won't need to stop then start it again.
I think you are misunderstanding how the industrial PLC controls function and the flow of the program within them. Machines start and stop all the time but you do not need to start and stop the program from running. Often people who have some knowledge of "computer coding" try to go into PLCs and they have a fundamental misunderstanding of the functionality because they are not really similar in how the program is designed and executed. Not a ding on your or anyone else with that background, it is simply differences in the design that need to be learned.
I'd encourage you to watch these videos and consider a training course put on somewhere for PLCs such as from Interconnecting Automation:
https://interconnectingautomation.com/PLC-Training-Course-Intro
https://www.youtube.com/watch?v=ReTtgzN-Dmc&list=PLln3BHg93SQ85ymy4VvtmRGxo2Stps2Iv&index=2
https://www.youtube.com/watch?v=qaI48NCUvkA
https://www.youtube.com/watch?v=4xF9-a47Ziw
And this video showing how a program is working in a simulator with a stop button. Just ignore the specific things about a Seimens PLC in it, the functionality is the same though..
https://youtu.be/nYr8Q21nG0k?t=457
It's pretty common to keep your PLC in a sealed cabinet. Depending on how your program is written, if you're using a step sequence, you can just have a seperate routine to monitor the start and stop buttons and reset the step number to 0. When the start button is pressed, it would reset everything that needs to be reset and then load the first step number into the step counter.
Sometimes people use interlocks on each rung that would be cleared during a 'stop'. Both of these times the program is still 'running'.
Thanks, after speaking with AD Tech I understand generally LL can't start once stopped. So I have a mechanical work around so I won't need to stop then start it again.