Siemens Simatic S5
SIMATIC S5 is a card-based PLC you tailor on a rack: slot in the PSU, CPU and the I/O, function or comm modules you need, wire the terminals and load the STEP 5 program; the family ranges from palm-sized 90U up to 155H with redundant processors, lets you cable remote I/O kilometres away, and its metal frame and forced-air option have kept it alive in harsh plants for decades.