Subject:
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Re: RCX Communications.
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Newsgroups:
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lugnet.robotics
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Date:
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Tue, 11 Jan 2000 18:29:52 GMT
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Original-From:
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Russell Nelson <nelson@crynwr.!NoSpam!com>
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Viewed:
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460 times
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Paul Frost writes:
> I am considering building a PIC based remote control for the RCX, & I am
> slightly confused by your communications details.
>
> Can you confirm that a '0' bit is transmitted as 417uS 38kHz IR & '1' as
> 417uS of nothing. This seems back to front to me, unless the 38kHz IR is
> continuously transmitted. In the abscence of any IR, the RCX would appear to
> receive a continuous stream of 0xff characters. With '1' transmitted as no
> IR, it would seem difficult to reliably detect the start bit.
Can someone help Paul? Maybe the following message from R.J.Hamlett
helps? Is the web page indeed wrong?
> R.J.Hamlett writes:
> > Hi, while browsing around, following my own attempts to 'decipher' the
> > RCX, I came across your page.
> > Good stuff, but unless I am missing something, there seems to be a fault
> > in the description of the communication packets. I have 'summarised' it
> > as follows:
> > The description gives the format as:
> > 0x55 0xff 0x00 D1 ~D1 D2 ~D2 ... Dn ~Dn C ~C
> > In fact the checksum appears to be reversed. If you calculate the sum of
> > D1...Dn, this gives the logical not of the penultimate byte, and so the
> > format should be:
> > 0x55 0xff 0x00 D1 ~D1 D2 ~D2 ... Dn ~Dn ~C C
> > Also the 'example' packet given for the message transmission has the
> > wrong checksum. The correct packet is:
> > 55 ff 00 f7 08 12 ed 01 FE
> > I was trying to program a PIC to generate the correct format to operate
> > as a substitute for the remote control, and it was only when I found
> > these faults, that it all started to work :-)
> > Interestingly, because of the way that they transmit the complement of
> > each character sent, the 'message' blocak will allways have the same
> > checksum, since the bytes in the rest of the message will allways sum to
> > the same value.
> > This makes generating a 'message' block rather simple.
> >
> > Many thanks for posting the information, it helps provide a good
> > 'leader' to the right data.
--
-russ nelson <sig@russnelson.com> http://russnelson.com
Crynwr sells support for free software | PGPok | "Ask not what your country
521 Pleasant Valley Rd. | +1 315 268 1925 voice | can force other people to
Potsdam, NY 13676-3213 | +1 315 268 9201 FAX | do for you..." -Perry M.
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Message has 1 Reply:
 | | Re: RCX Communications.
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| (...) to (...) It helps (indeed is critical), to remember that the serial you see over a RS232 'wire', is electrically the inverse of the 'logic', so the wire goes high, when at logic '0', and low when at logic '1'. The description of the actual (...) (24 years ago, 11-Jan-00, to lugnet.robotics)
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