>Michael Rigby-Jones wrote:
>>j galt (Dave) wrote:
>>>Olin Lathrop wrote:
>>>
>>>The speed of light is about 1 foot/nanosecond in vacuum.  However,
>>>it will be significantly less on a transmission line.  It depends
>>>on the impedence of course, but figure about half as a rough rule
>>>of thumb. Still, making reaonable distance measurments will required
>>>speeds many times faster than what a PIC can handle.
>>>
>>
>>Actually it depends on the "velocity factor" of the line which is
>>another characteristic.
>>
>>What I have pondered is a simple circuit that would turn a
>>storage/digital scope into a simple reflectometer.
>>
>>
>Something like this?
>
>http://www.web-ee.com/Schematics/TDR/tdr.htm
>

Yes, that looks interesting. Basically a circuit to periodically generate a
sharp pulse. If the pulse repeats at a few hundred Hz you don't even need a
storage scope as the screen will be refreshed. As others have mentioned
there are also many other newer faster logic families available.

If you can get a reflection back that is large enough to cross a logic level
you could have it stop a high speed counter. You would have to experiment to
see what sort of reflection amplitudes you get with different lengths of
cable. The scope has the advantage of displaying smaller reflections and
multiple reflections. Also the reflection you get from an open will be
different from the reflection from a short.

>
>As an ultra cheap and nasty method, how about making the cable
>the C part of a CR relaxation oscillator.  The PIC measures the frequency
>and by having a set of fudge factors for different cable types, the length
>could be calculated.  Granted it will never be as accurate as a commercial
>instrucment but I think the idea is possibly workable.
>

There might be some way the pulse transit time on the cable might be made
into a sort of oscillator circuit.


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