I am now designing a single-axis prototype for interfacing +/-10V servo drive and RS422 encoder for Colorlight i5 devboard (basically to verify hardware building blocks before making a modular multi-axis one).
One feature I noticed is lacking is some kind of watchdog that expects an update (patting) from linuxcnc and disables an output if not updated every 10-100ms. Something similar to this is often used in controllers that are commanded by PC, as a safeguard for PC/OS/linuxcnc crash. Hardware would then drive the outputs to a safe state (i.e. disabled motors and spindle).
Another possible implementation to safeguard against FPGA malfunction (be it gateware issue, a poor contact in clock signal or just user deciding to reflash FPGA with motors running) is to have FPGA generate a low frequency PWM when update is received, and hardware monitoring that there is an activity on the pin (IIRC there are watchdog ICs for this purpose), instead of just low or high signal coming from FPGA.
I'll probably take a shot at implementing this once I'm happy with the single axis prototype.
Edit: I realized such a feature can be implemented in a higher level (and sure, it already is implemented) without special support in driver/hardware controller.
