The hardware

For monitoring and control of most hard-wired sensor/actuator devices I use Siemens Simatic Series 1200 PLCs.

These are actually used in industrial environments (so they meet my requirements regarding robustness) and were priced within my budget.

The picture shows a sub-distributor during the construction phase. Right beside the multimeter you can see a 1214C PLC with 3x DI/DO expansion modules.

The PLCs are programmed with SCL (Structured Control Language), which has a Pascal-like syntax.

In the following I describe the user program in excerpts.