[Gmecm] a question of timing

[Mike V]

Thanks David,
the reason I ask, is I have a 12v tach signal available in a swap I'm doing.
Using a transistor, I was hoping to create the needed 5 volt square-wave
for a 7749 to manage only the fuel.  Thanks for the detail on the low side
of the signal.    I'm correct in the"high" being +5 (ish), yes?
Thanks
Mike V

At 07:45 AM 3/13/2006 -0800, you wrote:

>   Ok- The ref low signal is a ground reference inside the HEI module.  The
>Ref and Ref Low inputs to the ECM and differential and the input circuitry
>in the ECM looks for voltage difference between the 2 wires.
>   When the module is preparing to fire, it provides a high-current ground to
>the ignition coil.  This may create an elevated voltage on the module's
>ground.  Under this condition, when the module creates a grounded reference
>pulse it is pulled to the slightly elevated ground voltage, instead of 0V.
>This may not be below the threshold voltage where the ECM sees "a ground"
>and therefore the ECM may miss a reference pulse.  SO to prevent this the
>ECM compares the reference voltage not to it's internal ground, but to the
>module's ground.  It does this using the Ref Low circuit. Neither of these
>circuits should ever go below 0V relative to chassis ground.  If the
>module's ground is elevated to 1.0V, the ECM detects this on the Ref Low
>circuit and will then consider the Ref circuit to be "ground" at 1.0V
>instead of 0V. It uses a special IC designed for this purpose - this is in
>hardware not in the code.
>   This same scheme is used in lots of electronics, really.
>Hope this fully declarifies things!
>David