Deus Ex Machina Redux

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This rather grubby looking piece of kit (I deliberately didn't clean it up to show the state I found it in) is a Mitsubishi Programmable Logic Controller and its handheld programmer. The PLC was part of the AV installation we did for The National Grid at the museum in Llanberis, North Wales: this particular one was woefully over-spec for the rather trivial task it had to perform; being a delay after power-up, a timed switch-closure and then to just sit and wait for the next power-up. I could have used a timer-relay or some such to do the job, but this was to hand and the budget afforded us the luxury of such extravagant over-specification.

This pair of objects, along with the PLC that handled the rather more complex fail-safe routines for the show, have languished in the back of sheds, cupboards and in various damp, rodent-infested corners since I pulled them out of the rig when it was decommissioned. I decided today on a whim to power up one of the units to see if it and /or the programmer would work at all. The PLC above was the quickest to do as it's a 24V unit and I'd still got its power supply to hand. Not only did everything fire up OK without sparks and blue smoke, but my original program was still sitting happily in the brick's non-volatile memory: the screen on the programmer shows the first four lines of ladder logic code of the routine. And the code still works - brick and hand-held are still in perfect, if soiled, nick.

This would be remarkable in itself if the kit had just been sitting on a shelf for ten years in a controlled environment: but I programmed this thing in early 1994, and it has been lying around in that multitude of unsavoury and unsuitable environments since 1997. Truly a ghost in the machine. My son was just two and a half when I coded this routine: he's twenty-nine now. Practically nothing I coded or oversaw the coding of in the same era, for desktop machines, is retrievable; let alone usable, now.

The next step is to see if I can extract the code to a modern computer for storage and archiving (I'm sentimental like that): I suspect that this will be a relatively easy task even at this distance in time from its origination: the Mitsubishi PLC ecosystem is still there and everything seems backwards-compatible - it's pretty simple stuff and I guess basic serial comms and terminal emulation will suffice. Amazing stuff, and I will be definitely be re-using the PLC's when I've rescued my code from them for posterity. That's got to represent good value for money in anybody's estimation: and there was me thinking the kit was over-priced when we specified it.

Postscript: it would seem I can buy a USB to RS422 cable/convertor from EBay for about eight quid that would do the job, if the necessary drivers are available [obviously not class-compliant, then] - I think I'll keep looking...

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