CNC4PC has a stand alone detector along with a detector on many of their breakout boards. They are intended to work with Mach's 12.5kHz pump. Information that could help someone understand how these detectors work and how to use these with LinuxCNC could be inserted here.
CNC4PC has a <html><a href="http://www.cnc4pc.com/Store/osc/product_info.php?products_id=51">stand alone detector</a></html> along with detectors on many of their breakout boards. They are specified to work with Mach's 12.5kHz pump. The detector is microcontroller based and uses a 15ms period to count pulses. Specified valid input frequencies are 3 kHz - 15.5 kHz, which corresponds to valid base periods of 32258 nanoseconds to 166666 nanoseconds.