speed signal and DRAC

[Shannen Durphey]

Peter D. Hipson wrote:
> 
> Digital: a voltage that indicates a TRUE condition, no voltage indicates a
> FALSE condition. Some digital systems reverse that, but the result is the
> same--either TRUE or FALSE, but never between.

So you're saying a continous output of one voltage can be considered
digital?  If so, would that be a one bit word? (the entire signal
represents only one digit...)

> 
> Digital signals are either parallel, or serial. Most (not all) A/D
> converters convert an analog signal to a parallel signal (easier to
> interface with the A/D converter).

Wait.  What's this mean?  Serial or parallel refers to the way the
computer expects the information to arrive, yes? 
As it applies to auto ECMs, how is a parallel digital signal different
from a serial version at the source?   

 
<snip>
> Finally the DRAC buffers the output from the re-sampler to drive the
> speedometer, cruise control, external speed limiter or alarm (some foriegn
> markets) and the TCU/ECU. Now, to further confuse *everyone* some TCU/ECUs
> are not calibrated at 2000 pulses per mile! Instead they are driven from a
> second speed sensor. This is a common situation where there is a non-turbo
> diesel, 4L80E automatic, electrical speedometer, and 4WD.

I'd like to compare notes about DRAC pinouts if you have them. 
Service manuals are full of conflicting data about the various
outputs.  The RWAL signal is 128,000 ppm.  Some speedo signals and
some ecm signals are 2kppm, some are 4kppm.  Some ECM's use a 40 pulse
per revolution speed signal. Cruise module is listed as 4k ppm in some
apps.
> 
> (Note: most speedometers would work if attached to the speed sender, but
> would (probably) be very inaccurate!)

This sounds suspicious but I'll keep it in mind next time I have an
instrument panel out.
Shannen