Where you got very lucky was that the plc or other weird industrial system was controlled over rs232, and not from a manufacturer proprietary 8 or 16-bit ISA interface card that comes with device drivers to load in config.sys, and requires direct memory access. Can only run on bare metal. Yes there's vendors for industrial motherboards you can buy brand new with ISA buses on them...
And that's before you deal with the rarity of an interface card that's literally impossible to purchase at any price today, so you better hope someone has saved spares.
Yup, Home-Brew ISA Cards is a thing. There are people out there who have reverse engineered some to create clones, not only for ISA but for more modern buses too.
And that's before you deal with the rarity of an interface card that's literally impossible to purchase at any price today, so you better hope someone has saved spares.