Charge Pump

If you have an unused pulse output on your break out board you can use a stepgen in hal to generate the required frequency.

If you want a circuit board search for ' 555 Pulse generator'

It'll give you things like this: www.ebay.com/itm/1Pcs-Chip-Unstable-NE-5...7-3-F1R/123972633505

 

I think what he is wanting it a circuit that sets an output high only if it sees a valid cyclic charge-pump signal.

Here is one, but it is not exactly inexpensive: www.cnc4pc.com/c4-safety-charge-pump.html

There are some examples here: wiki.linuxcnc.org/cgi-bin/wiki.pl?About_Charge_Pumps

But I think I would probably use an Arduino Nano like this example here.
www.cnczone.com/forums/gecko-drives/4093...uit.html#post2254806