@Simisteven, if this is on a running machine I do suggest you think about putting a project forward to replace it, this hardware is way beyond it's useful date & if fails will be a major problem, what I also suggest is if you have some drawings or at least some idea of the bits & pieces that give or require signals from the PLC you print out that listing & pencil in at the very least the I/O comments/symbols, this will give you two things, if there is a problem knowing what inputs may affect any output stopping it turning on will make it simpler to diagnose, if in the event it needs replacing due to failure, will be easier for someone to reproduce the code.