This was one unique custom design made. It was Multi Zone Scanner controller. The Instrument had a Analog Multiplexer with a Synchronous Sample and hold, as the analog value was transient.
It had four relays that would turn off or on every scan cycle, depending on the input values. The 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