I’m building some automation equipment for my shop that will be used in extreme noise environments (plasma cutting and TIG welding nearby, for example) and I’m trying to come up with the best signaling for connecting modules that is still reasonably cost-effective. The modules will live in a couple of rack cabinets, and have dual-shielded 8P8C modular cabling connecting units within each rack and between cabinets to carry both moderate-frequency (200kHz-4MHz) and near-DC signals.
My initial direction was to use AM26C31/32 RS-422 ICs on these links, for cheap and easy differential communications.
The ICs are rated for +/-7V common mode, and the power supplies are isolated. Therefore, the racks require some mechanism to limit ground potential differences. The RS-422/485 standard recommends terminating the cable shields at both ends through a few hundred ohms to tie the systems together, but if the building ground is continuous there is now a room-sized loop and it's connected to ground through a low impedance. Some of that might be mitigated by inductors on the shield-ground tie but I’m not sure to what extent, and high-frequency high-energy noise could radiate through the stub.
Second, in the case that the line is undriven, such as when a cable is unplugged, the received voltage has an indeterminate state. Therefore, they need fail-safe biasing, which potentially connects uneven power supplies. I am now considering using a single-ended driver for one wire of each twisted pair, the other wire being a dedicated return tied to ground at the driver but not receiver. Receiving would be done by optocoupler, resulting in full isolation. This has the advantage of ignoring the GPD, removing the need to tie the cabinets strongly to ground and so while the ground loop through the building still exists, it should be better isolated from the ground plane. The other advantage is that logic zero in this system is <5mA or so, and the undriven state of the line is 0mA for a decent noise margin without biasing. Sadly, high-speed optos are expensive. It’s still technically differential, but the swing is half that of the RS-422 drivers and I’m not sure how much unbalanced drive will affect noise emission at the wavefront vs the opposing magnetic fields of balanced drive. Four pairs share each cable so that noise would be of concern. CAN uses unbalanced drive in voltage mode and is generally considered pretty robust, but it’s not clear how much of that is its fault-tolerant protocol and lower data rates at short range vs RS-485.
I will have to use at least one of each, since off-the-shelf stepper drives use optoisolator inputs, and there is a required RS-485 bus to serve, but RS-485 transceivers are available with features to mitigate the need for biasing networks (though not GPD). So, given the possibility of a 200A welding arc going off a few feet away, what is the most noise-immune thing I can do? Can I solve the problems with RS-422 without going to expensive fully isolated receivers? Can anyone shoot some new holes in my non-EMC-engineer-approved plan? Am I overthinking it and both systems are good enough?
Thanks for any input anyone can provide.