Subject:
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Re: IR Tower
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Newsgroups:
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lugnet.robotics
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Date:
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Thu, 24 Dec 1998 06:36:50 GMT
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Viewed:
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1175 times
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In article <6yb3qzAWkJg2Ew0N@phesk.demon.co.uk>, lego-robotics@crynwr.com
(Peter Hesketh) wrote:
> In article <dbaum-2212981930170001@pm3-6-12.chi-focal.enteract.com>,
> Dave Baum <lugnet.robotics@lugnet.com> writes
> > I think there's also some sort of AGC on the
> > pre-amp which is why tranmissions are always balanced (same number of 0's
> > and 1's).
>
> I have come across this statement (balanced 0's and 1's) several times
> but it is not quite true. It would be true if the RS232 frame was 8
> data and no parity, but AIUI the RCX uses odd parity. This means that
> there will be times when a sequence of bytes is sent which all contain
> the parity bit set to a 1. Admittedly this only unbalances the signal
> by one part in eleven*, but it is still an imbalance.
>
> (* 8 data + 1 start bit + 1 stop bit + 1 parity bit = 11)
Right you are - but EE's often ignore effects less than 10% in calculations :-)
I was referring to the practice of formatting the data such that a byte is
always followed by its complement (except for the first part of a sync
which is 0x55). This keeps the reference level from drifting too far
since the average over any 22 consecutive bits will always contain between
10 and 12 0's. If you didn't do this, then it would be very difficult to
establish a bias for the AGC, and without that you'd have a difficult time
getting any sort of dynamic range in terms of distance between transmitter
and receiver.
Dave
--
reply to: dbaum at enteract dot com
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Message is in Reply To:
 | | Re: IR Tower
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| In article <dbaum-2212981930170...ract.com>, Dave Baum <lugnet.robotics@lugnet.com> writes (...) I have come across this statement (balanced 0's and 1's) several times but it is not quite true. It would be true if the RS232 frame was 8 data and no (...) (26 years ago, 23-Dec-98, to lugnet.robotics)
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