Tools for PLC ladder 'decode'

Hello all.

I have am dealing with a Mitsubishi MELSEC Q&A.
Currently the ladder has about 19000 lines of code in it.

I need to go through and basically find out how it works so that I may make modifications and improvements to the code especially in the PID section.

Is there a mind-mapping or organization tool that can help me keep track of some thousands of devices and latches and function calls so that I can make some semblence of organized flow chart for the ladder?


Any sort of advice will be appreciated.
Thanks.

Are there any device names and rung comments?

If not then thats the place to start - presumably you know what the physical I/O is, so the first thing I would do is give these lables. From the I/O names you should be able to work out what sets some of the internal bits/registers and then give these labels, you should also describe the function of the rungs you have worked out with rung comments (regardless of how simple the rung function may seem to be - believe me it will help you). With these bits/registers described/labled you should be able to go through the program again and work out the function of more bits/registers . It's then a process of iteration - a bit like solving a giant sudoko puzzle.

That's how I've done it before - not easy - sometimes damn near impossible. If anyone has a better way then I'd be glad to hear of it too.

I couldn't agree more with burnerman. If you have to do it by yourself start with labeling the inputs and outputs and try to follow the code to give all the bits, words, latchs, etc a comment

Don't forget to learn how the machine works, what it does, what is not allowed to do, the settings, what the operator do, how does the operator work, if something is not longer in use...

Recap all the info you can before start digging the code because it will help to understand things easily

tools?

Yes, there are comments for most of the devices used.
I am currently in the middle of what you have been saying, tracing all the devices back.

I was asking if there are any techniques to mapping though. For example, one particular device will be used more than 300 times in the code. And keeping track of when those devices are ON/OFF via timers, latches, counters can be mindbogging to keep track of in one instant.

A simple flow chart doesn't seem to cut it, so thought maybe advanced users had a way of organising devices and timers etc.

and never forget what the machine is suppoused to do and what the machine is NOT suppoused to do, the intelocks, ...