PPS echo signals are not detectable on the parallel port. The irq handler in ppbus/pps.c creates the echo pulse by setting AUTOFEED, handling the pulse and immediately unsetting AUTOFEED. The "handling the pulse" phase is not long enough to create a usable echo pulse. Adding a DELAY of 100ms before disabling the bit creates good pulses, however this is not a feasible solution in an interrupt handler.
Add a delay (maybe 100ms) before disabling the AUTOFEED bit.
Connect a PPS source (e.g. GPS receiver, signal generator) via the parallel port (pin 10). Set PPS_ECHOASSERT (with pps-api utility in ntp). Try to read the echo on pin 14 (e.g. with oscilloscope, LED, GPS receiver...).