Plc3 to plc5

A colleague of mine is in the process of trying to upgrade a large PLC3 system to a PLC5 processor.

Does anyone know of a good tutorial or instruction guide on what to watch for while doing this. In particular he is having trouble figuring out all of the RIO addressing as well as some of the instructions/data types (what the heck is a "D" data table). The documentation and drawings from the old system are very poor and he could use all the help he can get.

Thanks in advance

The "D" data table is for BCD encoded numbers.
Each data table section (B,N,T,C,D, etc.) could have 1000 files (0-999), each with 10,000 elements (0-9999), memory permitting.
The PLC3 could address a lot more I/O than a PLC5 - so when you say large, maybe he needs more than one PLC5.
At this point in time, I would be inclined to convert to ControlLogix rather than PLC5.