Quote Originally Posted by metric_taper View Post
The Modbus between the Nanodac, and Thyristor pack, they must have an interface standard to write coils and read registers between them. I see what I believe is the Nanodac on a DIN rail mount. Nice safety backup. worked on autopilots, and all the safety things, back before they trusted a silicon switch for positive disengage of the servo motors, the switches in our cockpit panel, had 3 DPDT switches all ganged together and wired in series to ensure the +28VDC was removed from the engage clutch, one path for the elevator, and one for the aileron, the yaw damper had it's own separate system if needed. I learned you can't trust relays for anything, their failure rate is too small for aircraft safety. Preventing house fires is very important, I like seeing that.
Thanks @metric_taper .

The Nanodac is the thing that's mounted in the door; the thyristor pack on the DIN rail mount is an EPACK. Both come from Eurotherm, so they're well suited to talking to one another over the modbus TCP link. All that link does at the moment is write a set-point to the EPACK Thyristor pack and read the current back (for display).

I also don't trust relays, but I don't trust thyristors or solid-state relays either! At least with the contactor (which is just a big relay really) it isn't switching as often as the thyristors are so there's a reasonable chance the contacts won't have welded. I do like having the physical switch in line with the heater coils as a belt-and-braces thing though. At least that means I can guarantee that the coils are electrical isolated even if something goes wrong with the contactor. For an overheat event to happen, there needs to have been at least two failures: something going wrong with the thyristor pack and something going wrong with either the over-temperature detection or the contactor. I'm sure that wouldn't be enough redundancy for an aeroplane application, but hopefully it's good enough for a heat treatment oven!