Industrial process controls require the use of many types of input signals, from switch closures, relay contacts, or voltage levels which can be either DC or AC levels. The Raspberry Pi is being used more and more in these applications. Here is the Pi-SPi-8DI Eight Channel Isolated Input module:
This module uses the MCP23S08 GPIO expander to allow up to 4 addresses from the same chip select line. The Pi-SPi-8DI has two available (via jumper) Chip Select lines. With eight modules connected together in a daisy chain, the RPi now has 64 isolated signal inputs "right out of the box". (The first module connects to the RPi, the second module connects to the first module, and so on)
Here is a partial schematic showing the front end of the Pi-SPi-8DI:
If the inputs required are simple switch closures such as panel switches, push buttons, or relay contacts, then the J4 jumpers are installed - this provides the +V power out to the field (to wet the contacts) and the signal is brought back into the board to the Dx input. Each input circuit has a series LED to show activity and the opto-coupler used is the Toshiba TLP292-4 that has 3,750 Vrms isolation.
If the inputs have to be isolated, the J4 jumpers are removed, and now all the inputs are fully isolated. The input circuits are rated for 24 VDC or VAC. The TLP292 series of opto-couplers are rated for AC inputs. Most commercial HVAC type controls (building systems, etc.) use 24 VAC as a main source of power and control signals.
Here is the PCB layout of the Pi-SPi-8DI.
More information, schematics, layouts and sample code written in 'C' can be found at this link:
Does/can field power also power the pi?