Hi C688...

No. The clock speed. I made the 'scope with a 6 bit CA3306 A-D converter that was capable of 10MHz or better sampling rate. The data lines were dumped straight into the AMIGA parallel port via a 0.5 metre parallel cable. On a real 'scope this was easily visible...

The capacitance between the cable's data conductors and clock were _shunted_ to ground due to the capacitance above 250KHz giving erroneous readings, and, direct connection was not a goer due to the size of the end unit. The unit was hard wired on perforated board using terminal pins - no printed circuit at all.

Also there was a deliberate roll off at around 75KHz to help eliminate aliasing as the cut off freq approached clock speed.

It was ugly and big but was mainly for kids to practice soldering and basic engineering both electronic and mechanical. It required 2 off PP9 batteries too. OUCH!

It is still connected to my A1200 in my old computer room, but not switched on.

I also built a simple 10MHz frequency counter accurate to 100Hz to for the old tub too.
That is also on AMINET, uses stripboard instead though.

Done loads for the PC SPP and RS232 ports too, sooner or later they might come on here. ;o)

EDIT:-
The archive is in LHA format so I will try and post some piccies here of the CCT and construction.

Here you go; aspect ratio a little off but you get the idea...