Safety relay for CAN toolboards

Basic Information:

Printer Model: Voron 2.4
MCU / Printerboard: 2x SKR 1.4 Turbo, Huvud 0.61, ERCF Easy

Fill out above information and in all cases attach your klippy.log file. Pasting your printer.cfg is not needed

Describe your issue:

Before switching to a CAN toolboard on my Voron 2.4, I’d been using a safety relay inline with the hotend heater. While thermal runaway events of the heated bed can be mitigated well with a thermal fuse, there are relatively few protections against runaway of a hotend heater should a MOSFET fail and an in-line relay was able to add a layer of redundancy.

Swapping to a CAN toolboard has complicated this somewhat though, as there’s only one power feed for all functions of the toolhead and its controller. Until Klipper has connected to all the MCU’s it will not enable the safety relay, but it cannot connect to the toolboard until the safety relay has been enabled.

Is there some way of having the Pi enable the safety relay via a GPIO before attempting to connect to any of the MCUs? Or perhaps specifying an order and timeout or delay for attempting to connect to MCUs? My thinking here is that the MCU connected to the relay could enable it, powering the toolboard, before Klipper attempts to connect to the toolboard.

This isn’t well supported in Klipper today. I think most people use Moonraker to control the RPi gpio pins for power relay purposes today. Configuration - Moonraker