[ajsnigrutin]

What are you going to do with just the transmitted data? That's just a tiny part of operations, when the machine is working correctly. What if something breaks? Do you know what all the possible error states are? What does the machine have to do if it gets an eg. "machine on fire" alert? Does your software even recognize the data sent by the machine, if it hasn't been sent before and correctly recognized?

Sniffing a protocol to reverse engineer stuff, is like trying to learn a new language by listening to conversations of a ground-level monitor and a crane operator.

[raxxorrax]

You can download the program from a PLC that supplies the logic aside from labels and comments, if that isn't available anymore. Some PLC even supply the complete project, but that is unlikely on such an old device.