Jeff wrote:
-I am correct that these systems use a 3000 Hz TTL square wave?
> -Is there a published (or defacto) standard?

Jeff,
    I happened to be looking into this for other reasons.   The EI display
is not very picky about the freq.  get details at www.buy-ei.com.
Also - see attached.

Colyn, Jeff:

 

Interesting.  The VM system uses the same type of probes although, if I remember correctly, the VM computer could handle freqs at least up to 20K.  Higher freqs are associated with shorter sensing tubes.  My header tank probe stands straight up and is 9 inches long (VM minium length was about 14 inches) -- Oops, my short probe produced freqs up to about 33K.  VM said their computer couldn't handle that rate so......... I built a CMOS frequency divider (cut freq in half) out of Radio Shack parts and by consulting Mr. Mims little circuit handbooks.  It is just a bulge in the connecting cable since there is 5 VDC available, along with the signal, at the probe.   I see accurate fuel indications because I was very patient in getting data points in gallon increments and has been working since 1996 (wish I could have said that about everything else).