Are PLCs being replaced more due to failures or lack of spare parts?

15 years or so and the only PLC I replaced never actually worked and was faulted when starting commissioning. Usually it's obsolescence that drives it.

I have a customer with some 20+ year old Toshiba T1's (18 of them)

Outputs are failing like billyoh. When I told them it would be £20K to replace with modern PLC's they nearly cried.

Solution: Ordered 100 internal relays from Japan (only place I could source them) and replace and repair as we go along.

20k for 18 PLC's, that is not too bad. Or is it 20k each?


I don't know how it compares to cost for unscheduled downtime due to hardware failure. That might help tip the balance in favor of updating.

Ronnie has just jogged my memory. some 15 years ago I was called to a machine that had a failed output, this was an IMO, but 12 of the relay outputs were pulsing on average every 1.5 seconds, one relay had failed (note: this was only 8 months old), the relays were obtainable from RS so I replaced all 12 and kept some spares, over a period of 6 months we replaced some of them again a number of times. We decided to replace it with a Mitsubishi with transistor outputs which meant we would have to do some re-wiring & make a patch plate to replace the HMI. As this was always in production we were not given the downtime to replace it, however it was a silly thing as the total downtime in replacing relays was considerable over a time period but that's production for you. Eventually I was called to a breakdown & by lying lol, told them it had completely failed, we proceeded to replace & re-wire, however, to my horror realised the stores had ordered the wrong PLC it had relay outputs, so a quick re-think, just replaced it & the HMI in two hours. For the next 14 or so years it never failed on a relay, just goes to show you that some components are better than others.

The 20k was an on the spot guess.
The replacement mitsi plcs are about £700 each.
A rewrite, conversion of one - then a straight swap for all the rest.

I have replaced ... 2 .. PLCs and maybe 12 or 15 IO cards in various PLCs (mostly Rockwell) over the last 30 years for unknown reasons. Just random electronics failure. At least, we weren't able to figure out the problem and replacement worked.

Some upgrades for 'we don't remember how it works any more' (someone retired or left) so lets upgrade it. During the upgrade they get new drawings, documented program, and modern HMI.

More upgrades for 'it's old and we can't get parts'. The side benefits were the drawings, wiring labels, new panels, documented program and modern HMI

Very few upgrades for 'we need these features and our old system won't do that'. Not many. In my experience, most customers use very little of the available features in what they have. They use fewer of the available features in what they buy next.

I have been in the game for over 35 years and have not replaced many plc cards/processors in reality. It is a rare occurrence that a card fails electronically only had a couple of input cards & a number of output cards (mainly due to relay failure), a few years ago I had an analogue card fail, this was a Mitsubishi Q series, when I opened it up the whole card was green with fungus on it water had continually dripped into the card from top entry cable glands and had probably been doing this for years. I could not believe that only one channel had failed and how it had managed to work for so long, when I tried to brush off the green slimy fungus one of the chips came away. This is what I call extreme environmental protection.