Nevertheless, the S-Tec autopilot is not much different from an industrial PLC.

I suspect you're wrong there. I once interviewed an engineer who
worked for that particular manufacturer for a possible position with
our company. If he's representative of even the bottom quartile of
their engineers, I would say there is a huge difference - the engineers
who design industrial PLC's are dramatically more competent.

My guess is that the S-TEC is to PLC's what the Concorde or Gill
battery is to lawn tractor batteries - lower quality than even the
cheapest commercial offering, at a price slightly higher than the most
expensive commercial offering.

Michael