Hey there! I totally get the struggle - inheriting undocumented PLC code from a retired engineer is like trying to solve a puzzle without the picture on the box. Veterans in the field have developed some solid systematic approaches over the years. Here's what they typically do:

First, they start with a full backup of the existing system - this is non-negotiable. You need a complete snapshot of everything as it currently works before you touch anything.

Next comes the "observe and document" phase. They'll run the system in production mode and carefully watch what happens, taking detailed notes about inputs, outputs, and timing. Many veterans create a simple spreadsheet or diagram mapping I/O points to physical devices.

Then they dive into the code systematically, starting with the main routine and working outward. They look for patterns - how alarms are handled, how sequences are structured, and where safety logic lives. The key is to add comments and documentation as they go, not all at once.

Veterans often use a "sandbox" approach - they'll create a test environment where they can safely experiment with code changes without affecting production. This lets them test their understanding of the logic before making any real modifications.

They also recommend breaking the system down into functional blocks - motors, valves, sensors, etc. - and documenting each piece separately before trying to understand how they all work together.

The most important veteran wisdom? Go slow, document everything as you learn it, and never make changes to the production system until you're absolutely certain you understand the consequences. It's a marathon, not a sprint!