Seems like if I rapid my machine (<100IPM), MACH miss counts my 1 micrometer glass scale? It's acurite senc 150 , 1UM res. Go slow and the counts make sense.
Any thoughts?

Doing some rough math, I come up with a requirement for about 50,000 samples per second needed to avoid aliasing if you are trying to read this scale moving at 100 inches per minute using software.

Are you connecting your scale to a parallel port supported in software, or is this configuration using an external hardware device? Does the Darwin parallel port driver even support encoders yet?

I too realized I might be exceeding the samples per min. reading. Increased the kernel speed some and slowed the rapid.