speed signal and DRAC

[Peter D. Hipson]

At 10:50 PM 6/25/99 -0400, you wrote:
>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...)

For example, TTL (transistor-transistor logic) expects a 5v level for a
TRUE, and a 0v level for a FALSE (actually these voltages are slightly less
than 5 and greater than 0...). Voltages between either turn on (full output
of 5v) or turn off (zero output, 0v) the device receiving the signal. A TTL
digital device cannot have an output that is between the two digital
values. As to a continious voltage representing a digital signal, yes you
are correct, but the signal in that case probably wouldn't tell you much. <g>

>
>> 
>> 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?   
>

Serial transfer of information uses one line to transmit each bit of
information serially. There are many clocking systems so that the receiving
end knows which bit is which. Requires only one line, but slower since
eight times the information must be sent, and the protocols are difficult
to implement (at the hardware level) Your modem, keyboard, and mouse are
all examples of serial communications.

parallel transfer transfers (typically) eight bits at one time, on eight
lines. Protocols are usually easier to implement, and can be much faster!
Your printer, hard disk, and some other items are typically parallely
interfaced. 

> 
><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

Don't forget the CYA, where I said 'most'! <bg> 

>
>
>
Thanks, 
        Peter Hipson (founder, NEHOG)
        1995 White NA Hummer Wagon