I probably would have posted this in the class a form. The multiplex system is one of the reasons I would not replace my DP with one much newer if I had to. When it works it's great but when it doesn't it can be hard to find the problem. You can download the following and start learning:
https://www.parker.com/literature/Electronic%20Controls%20Division/Literature%20files/VMM1210_Instruction_book_HY33-5001-IB.pdf
The red leds (12 in & 12 out) indicates what inputs there are and if that item is active...on.
Before buying I would make sure everything works that it controls and if not I would not buy until the problems are fixed at the sellers expense.