I wrote a program to check the computer
interaction with the RPM counter.
Tricky. In the old days this would take
days if not weeks to get it right.
The ZBasic is a great diagnostic tool.
I found a hardware mistake so it is a good thing
I did not order the pc boards yet :)

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Public x as long
Public y as long

Sub Main()
Register.TIMSK2 = 0 'turn off real time clock

for x = 1 to 20000000
'output is high Q
'both set and reset are high

if getpin(43) =  0  then 'enable line went low
                         'reset line went went low Q
                         'Q went low
for y = 1 to 50  'simulate reading  RPM delay
next y           'do not use Delay() as it
                 'uses the real time clock.

Call PutPin(13, zxOutputLow) ' pin 13
'reset the latch and put the enable line back high.

Call PutPin(13, zxOutputHigh) '

  end if

'from here to the next 555 timer pulse Q stays high.
'both set and reset are high.
 'Q stays high until the next time.
'RPM counter is counting up.
for y = 1 to 600 ' delay simulate the count
                 'time. About .6 seconds.
                 ' lots of comp. work can be done
                 'in that period of time.
next y

next x

End Sub
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This thing is going to be super accurate on RPM.
The system stops counting revs within a nano second
or so of when it is supposed to. Far faster than
any computer can respond to a hardware interrupt.

Paul Lamar