Working on a PLC system that you did not originally write certainly has its own unique set of challenges, regardless of if you are troubleshooting a system that is down or if you are adding a new system or functionality. I am guessing this is something that many others here have had to do from time to time, either for a client or your employer. I wrote an article/blog on the subject, partially from the point of view of things that an employer or client should keep in mind, so it doesn't go into great detail, but there might even be a few things that a green PLC programmer could gain from it. Keep in mind the target audience, but I would be interested in receiving constructive feedback (please be nice;-) ). Thanks in advance, and here is the article: PLC Troubleshooting