> weird bit of a drain on the 3.3v Yeah, 3.3v isn't really viable as a signalling voltage for anything that leaves an enclosure unless you take steps to mitiga

> weird bit of a drain on the 3.3v Yeah, 3.3v isn't really viable as a signalling voltage for anything that leaves an enclosure unless you take steps to mitigate interference such as differential signalling, shielded cable or some sort of protocol rather than presence/absence of voltage. It's just too easy to induce that voltage - albeit very temporarily - through capacitive or inductive coupling on even 10m of wire; as you experienced it's also too easy to have that voltage mysteriously leak out into what a multimeter will tell you is a definitively open circuit. If you're thinking of doing something like this again, I'd suggest: 1. Definitely use PLCs, their inputs are inherently hardened and you can use 24/48/240v signalling voltages - and by extension a whole universe of very robust industrial indicators and switches. 2. Don't use data cable for non-data transmission, the twisted pair coupling will give you issues you don't want and the stuff is fragile as anything. If you want a good cable which supports multiple non-data channels the term 'pilot cable' or 'control cable' will see you right, that's a multi-core cable which usually has 4 thick cores and a bunch of thin pairs, it's traditionally used for carrying 4-20ma signalling loops in factories and oil rigs etc, it's well shielded. 3. Do use RS485 to loop your PLCs together. 4. Don't use weak voltages on stuff that's being handmade and screwed together onsite. You run into all sorts of transient issues, 3.3v will balk at a loose terminal whereas 48v will romp home if there's even a hint of contact.