Subject:
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RE: RCX transmitting on the IR over 2400
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Newsgroups:
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lugnet.robotics
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Date:
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Fri, 10 Aug 2007 19:37:59 GMT
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Reply-To:
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<dickswan@sbcglobal.net%saynotospam%>
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Viewed:
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4622 times
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> Rodrigo Santos wrote on Thursday, August 09, 2007 9:00 AM
>
> I am trying to use the lego RCX 2.0 over 2400 bps, actually at 4800
> bps. I have no problems transmitting but Very rarely receives
> correctly. Any idea about this? Have you been in this road before?
4800 baud is not always easy to achieve. Success is very dependent on
what RCX vintage and which tower type you're using.
Yo9u haven't mentioned what RCX programming system you're using and
what is the message format you're using. If it is one that allows you
to customize the firmware, then there is amply opportunity for
improvement over the "standard" firmware IR implementation that can
quadruple your effective bandwidth for small messages while still
working at 2400 baud. Hints: (1) Eliminate the 55 FF 00 header bytes,
(2) eliminate the "double byte" transmission of every byte with
original byte followed by its inverse and (3) rewrite the ROM
interrupt handler and execute from RAM so that you can fix the bug in
firmware that forces 30 msec delay between sending and receiving a
message.
For 4800 baud you ideally want to use a "matched" set working at the
76 KHz IR carrier frequency. This means you want transmission at 76
KHz carrier and you want to IR receiver h/w in both the RCX and tower
to be using the chips that are tuned for a 76 KHz filter and not 38
KHz.
As I recall, the IR receiver chips need to successfully receive
several consecutive cycles of carrier frequency before recognizing the
signal. There was very little "margin" if you want to operate at 4800
baud with a 38 KHz carrier. I think there was a post from John Barnes
that explained this in more detail.
John's post also covered the characteristics of the IR carrier
frequency h/w on the various devices. From memory, a quick summary is:
o RCX 1.0 IR receiver is 38 KHz. Transmit carrier can be either 38
or 76 Khz.
o RCX 2.0 IR receiver is 76 KHz. Transmit carrier can be either 38
or 76 Khz.
o Serial tower IR receiver is 38 KHz. Transmit is 38 Khz only.
o USB tower IR receiver is 76 KHz. Transmit carrier can be either 38
or 76 Khz.
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Message is in Reply To:
 | | RCX transmitting on the IR over 2400
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| Dear friends, I am trying to use the lego RCX 2.0 over 2400 bps, actually at 4800 bps. I have no problems transmitting but Very rarely receives correctly. Any idea about this? Have you been in this road before? Thanks Rodrigo (17 years ago, 9-Aug-07, to lugnet.robotics)
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