Making a given modification may be simple. Remembering what you have changed, and how to restore the PLC to its original state may not be so simple.
This is a trap that many programmers fall into.
Always Know How To Return A Program To Its Original State.
I like to keep a handwritten log of the changes I make anytime I modify an existing program. That way, if things don’t work out, I can simply retrace my steps and make it just like it was.
Additionally, the program that is running in the PLC should be uploaded from the PLC to the laptop and stored in a safe place; perhaps on a thumb drive that can physically be removed from the computer.
However, downloading a program from your laptop to the PLC to restore the PLC to its original condition is not always ideal. First, you have to make sure you have a fresh version of the program. A program that was uploaded and saved a month ago might have old data. You might inadvertently erase any data that had been collected in the last month.
Also, downloading requires you to take the PLC out of “Run” mode and put it in “Program” mode. That means the PLC cannot run the equipment during this time. That might be a problem.
When modifying programs on the factory floor, I believe the best method is to simply write down the changes I have made. In most cases, it is much easier to go back and manually undo the mods you have made, as opposed to downloading a copy of the original program.
It is true that some program modifications are relatively simple. Picture this, though; as you are going through the rungs and changing the logic, you get a call regarding another machine that has gone down. Suddenly, your priorities have shifted. You may have to disconnect from this PLC and take your laptop over to the other machine.
When you return to your original task a few hours later, you might not remember where you left off. Simply referring to your notes is the easiest way to get back on track.
Besides, when the job is done and someone asks you what you did, you can easily pull out your notes and show them.