You can disable the heatbed:

M140 H-1

Hello David,
Thank you for the clarifications. The auto detection of the stepper drivers to toggle the functionality of the EN/CS pin is a nice touch.
I suppose a solution would be to remove the TMC drivers, then use the signals from those headers to drive external stepper drivers, but this is not very elegant.
Perhaps I will edit the firmware to output a drive enable signal to E6_EN to control all four brakes together, to match the activity on the Z axis.

I know this topic has been previously discussed, but I was hoping to implement this a little differently.
We have a platform driven by four screws. These will be driven using stepper drivers usually meant for E1, E0, X, and Y (so the first four connectors on the board), to take advantage of the bed levelling feature. Each of the stepper motors is fitted with a normally closed 24v brake. I noticed the Duet has some unpopulated headers (TP5, TP4, TP3, TP2) with en/step/dir pins. My plan was to use the signal from the enable pins to release the brake when the drive was enabled.

Unfortunately, those pins, despite being labeled X_EN etc are in fact used as the chip select for the TMC drivers' SPI comms. I also noticed that the TMC drivers have an ENN pin which is connected to PC6 on the microprocessor, and which the manual indicates would allow us to switch power on/off to the drives. For drives E2 and upwards, the enable pins are broken out to the expansion header and operate as I expected.

I wonder whether the EN/CS and ENN pins for the first four axes are actually wired as intended?

Basically, I need a signal from the Duet to indicate when the vertical axis drivers are not powered so that I can put the brakes on. I was hoping to use standard firmware and pins, but as I cannot find a suitable feature I will most likely allocate a spare pin to give the desired signal.

Unless someone can point me in a better direction?

Hello,
This new functionality was developed and tested on a Duet 0.6 running 1.18-b2 (ch fork). It should work with all recent versions of Duet/RepRapFirmware.