🚀 go-pugleaf

In <26118@life.ai.mit.edu> elwin@ai.mit.edu (Lee Campbell) writes:

>Howdy pardners! we're using T805's to control a 2-legged running robot
>and we're sending attitude and dynamic state data out over differential
>links to be saved for later analysis. We are using B415 trams and Belden
>9804 shielded twisted pair cable (awg 28, capacitance 15.5 pF/foot) to
>connect the differential links.

>My question is: what is the specification for maximum length cable that
>can be used for each data rate? We are currently running at 20Megabits/
>sec over a 50 foot (17 meter) cable. From what I can see in the manuals
>this is outside the spec, but the manuals are not all that clear. In 
>the future we may want to run longer cables (40m or more) and we want
>to know what data rates we must use.

In the tech note on links the only hard specification (apart from
data levels - from transputer databook) is timing skew. I beleive
that the maximum allowable edge timing skew on a 20MBit data stream
is 5ns, that is a pulse (lo-hi-lo or hi-lo-hi - whatever way you
want to look at it) should not vary in length by more than 5ns due
to the effects of your transmission/reception kit.

The only effects you have to watch therefore seem to be dispersion
in your twisted pair. I think the 15pF per foot spec is misleading
-- we are talking transmission line effects here. As long as your
drivers can drive it, then consider it driven. If you are getting
significant reflection effects in the cable, this too may cause
problems - solved by well matched termination. The only other
constraint I can think of wrt the twisted pair is the attenuation
factor. Hopefully this should be fairly low over the distances
you want. 

Final consideration - timing skew due to drivers/receivers. This
may be where the RS422 spec falls down. If drivers are designed
to this spec, then it could be that there timing parameters will
not be as good as you require. The easiest way around such problems
is probably to design your drivers/receivers using either known
tight-spec MSI components or discretes.


This is not a complete answer -- I am just attempting to can my
experience into a short tome. If you have any queries on what
I have expounded above - drop us some email and I will see if
I can help further.

Cheers
	Don

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|Don Goodeve		email: don@uk.ac.york.ohm |
|Adaptive Systems       tel : (+44) 904 432381    |
|Electronics Dept.      fax : (+44) 904 432335    |
|University of York  -----------------------------    
|UK                 | Give it to the Softboys....
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If only life were in an open D tuning...

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| Don --- Well why not? Someone has got to be!|
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