I don't know how far to go with this, as it may be of little interest here. I started off on this journey by being interested in computer control of my layout. A journey which led me through using NCE Mini-panels in myown DIY switch panel to finally understanding(?) JMRI and getting a "glass panel" One Function Switch style UK set up. I am very pleased with the resuts.
It dawned on me that I would need Block Occupancy detection to interlock points, signals and routes. The cost appaled me but I carried on. To get BOD for my small station cost over £350 using an NCE Auxilliary Input Unit and BOD20s, not counting the kilograms of copper wire! It was fraught with problems cutting into laid track to get the block splits I needed. Plan first, then lay track! Easier said than done!
I really wanted to move on to my fiddle yard - but I kept delaying every time I costed it out. Scots blood you know! The BOD20s are £18-£20 if you can find them plus another AIU at £55.
Now what follows isn't original, nor is anything (except my implementation of it) my own work. By serendipity I fell accross "Little Wicket Railway" on YouTube. The bloke is a genius level teacher and demonstrator. Clearly he was wanting to do automation but was horrified at the costs. There are a series of videos of his development of his solutions, the key one for me was "Arduino, CMRI and JRMI - Part 3 - Multi Node CMRI Networks"
https://www.youtube.com/watch?v=WsfczIPbhsU&t=1234s
What this is all about is using an RS485 network from a PC using Computer Model Railway Interface (CMRI) protocol to link several "nodes" to JMRI software on a PC. My nodes are are Arduino Nanos (I use clones at £30 for 5!) with an interface board to link them to the RS485 BUS (5 for £5) and a USB to RS485 converter (£5) a few 400 pin "breadboards" and some jumper wires. The BODs are from Megaapoints Controllers (£6 each or £42 for 8) and they are accurate and sense a single 10k ohm wheelset. The whole thing for less than a third of the station cost!
I have a test set up with 2 nodes working with JMRI but only one BOD, I am awaiting the full number in the next day or so.
IF anybody is interessted I could post more details and photographs of my testbed here, or answer PM questions. Or just forget it.
Let me know......
Pinout for Fiddle Yard BOD system.pdf