Dos 6.x (highly customized to run a custom machine)
Arcnet network for machine control. (2 of them)
Windows 95 (last version of Windows that had arcnet drivers) Hardware was mostly Pentium 90 level or similar.
All of the above came delivered as part of a system from the manufacturer, and my predecessor claimed to have had no say in the matter
Several times over the years I was asked "Why not upgrade?"
Answer: New software/hardware did *exactly* what the old did -- no better, no faster and certainly not cheaper.
All of these old systems controlled machinery, and the machinery itself was the rate-determining step.
In the 15 years I ran the department, we lost less than an hour of production time due to computer failure -- and that was when our up-to-date SAN crashed.
So what happened in January?
They closed the plant permanently.