🚀 go-pugleaf

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. Manual citations follow.

In "THE TRANSPUTER DEVELOPMENT AND iQ SYSTEMS DATABOOK" 2nd edition
1991 INMOS doc # 72 TRN 219 01 in chapter 28 (IMS B415 Differential
link buffer TRAM) on page 229 it says "With cable lengths up to 10m,
20Mbit/s link speed is possible. Longer cables up to 100m support
lower link speeds." Unfortunately, it never says what lengths at
what link speeds.

Similarly, in "TRANSPUTER TECHNICAL NOTES" copyright 1989 INMOS doc #
72 TRN 200 in chapter 2.3.3 (Differential line drivers/receivers) page
35-36 it says "This system has been use by INMOS for reliable link
transmission over 160 metres of twisted pair at 5Mbits/s. It should be
noted that the RS 422 specification limits the maximum bit rate to
10Mbits/s at a maximum distance of 15 metres using 24 awg twisted pair.
At 5Mbits/s the maximum length is about 25 metres. The RS 422 specifi-
cation is, however, deliberately conservative." This seems to imply
that *any* communication at 20Mbits/sec is outside the spec but maybe
we can do it anyway?

Thank you for your help.
				- Lee Campbell <elwin@ai.mit.edu>