speed signal and DRAC

[Peter D. Hipson]

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.

Analog: Like real numbers, there are an infiniate number of values between
the lowest and highest.

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).

The DRAC takes an analog signal that looks much like a sine wave. The
signal is generated by using a coil that is wound around a pernament
magent, with a gap in the end. When a toothed gear closes the gap, there is
a voltage induced into the winding of the coil. That voltage is in the
order of 3 to 20 volts. The waveform is a basic sine, though at very low
speeds the waveform may be distorted. 

In the DRAC are three components. First, a CMOS buffer converts the 3 to 20
volt signal to a digital signal, of the same frequency. That is, the buffer
clips the tops of the sine wave, if it is less than 5 volts, it may
amplifiy it as well. The input is sine wave, the output is basically a
square wave. 

Next, the DRAC re-samples the frequency. This is done by dividing and
multiplying the signal based on teh jumpers that you see in the typical
DRAC. In fact, any DRAC can be re-programmed for any rear-gear/tire size by
changing the jumpers! (Why buy a new one? <g>) The output signal from the
re-sampler is basically about 2000 pulses per mile, which is what the
speedometer expects.

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.

(Note: most speedometers would work if attached to the speed sender, but
would (probably) be very inaccurate!)



At 10:16 PM 6/24/99 -0400, you wrote:
>Peter D. Hipson wrote:
>> 
>> Actually it is not an A/D converter, it converts an analog input to a
>> digital signal, however the output of the DRAC is not standard TTL, the
>> input is frequency, not voltage sensitive. Most electronic speedometers
>> could be driven directly (but inaccurately) from a magentic sender.
>> 
>> (FWIW, and A/D converter converts varying voltage to a digital value, if
>> the DRAC did that then the output would not change unless your speed
>> changed! <g>)
>> 
>
>Well, I think I've understood what you meant, but DRAC output doesn't
>change unless speed changes.  I hope not, anyway.... ; )  
>
>Is a signal of only one voltage level considered digital?
>Shannen
>
>> At 08:18 AM 6/24/99 -0400, you wrote:
>> >> >>
>> >diagram for the Syclone.  Oh yeah, the DRAC is an A/D converter as
>> >well as a frequency converter.
>> >Shannen
>> >
>> 
>> Thanks,
>>         Peter Hipson (founder, NEHOG)
>>         1995 White NA Hummer Wagon
>
>
>
Thanks, 
        Peter Hipson (founder, NEHOG)
        1995 White NA Hummer Wagon