The ideas behind using a Raspberry Pi for process control are published in raspberry4distributedControl.pdf (dir) at a-weinert.de.

Process control features

RasPi4 with RS485 and other GPIO

By certain configuration and utilising constraints we get 24/7 base for control software with

cycles (1ms, 10ms, 20ms, 100ms, 1s)
support for Modbus, MQTT, 1-wire and more
HMI via Web-interface
watchdog

Compared with industrial PLCs and weAut01 there is

no supervising of power supply, and, hence
no (cycle for) programmable reaction on power outages

Devices supported by the library include

modbus power meters
PV inverter
bar code / QR code reader
DCF77 receivers and else

The preferred Raspberry test and target platform for control applications is a Pi3 or Pi4 with a non graphical OS. But other (Pi 0) and older platforms (Pi1) may be sufficient sometimes. To handle differences (GPIO pinning e.g.) in a uniform way, the include files in this directory describe those platforms. On the other (dark) side you won't find Pi5 due to its breaking all GPIO handling compatibilities of Pi0..4.

A note on Copyright and License

For software and documentation developed for this project:

The software is open source and available by a download link (to be asked for at the moment.
It uses open source libraries by other authors. Those parts keep their original license and author's copyright.

Raspberries' architecture differences

The different Raspberry types, like Pi1, 3, 4, Zero (0) etc. but no Pi5 can be handled transparently to the control programs. This is done by a make variable TARGET identifying a concrete target machine and, hence, the architecture, via the PLATFORM variable. The latter can also be set directly as PLATFORM = raspberry_03 e.g.. raspberry_04 is the default by the way.

By the variable TARGET we do also switch between a 32bit and a 64 bit cross platform tool chain. Obviously, the installed OS is a "fixed" property of the concrete Pi. The mechanism used is calculated make includes and calculated C includes. See config.h and config_raspberry_04.h for the mechanism and the 40 pin IO. (For 26 pin see config_raspberry_01.h.)