This was one unique custom design i made for a Mining Company for Ore Processing. It was Multi Zone Scanner controller.
It might have been for a rotary kiln of a large size. Four temperature sensors were connected to the system with brushes just like in a motor. Even the supply to the trasmitters was on the brushes.
The Instrument had a Analog Multiplexer with a Synchronous Sample and hold, as the analog value was coming via the brushes. It had four relays that would turn off or on every scan cycle, depending on the input values. The kiln system was slow it had inertia, so it did not require a fast acquisition for ‘Real time’ as you may find necessary in small Thermal Systems.
This was built with analog and digital chips, no uC in this design. This was because it was a front end controller, there was a risk of a uC system getting stuck. These days, that problem may not be so worrisome, but i am not so sure.
What if the watchdog goes for a short nap?
You can find part of the designs here – Industrial Process Control Circuits