Re: XTB-II TW523 Emulation

["Jeff Volp" <JeffVolp@xxxxxxx>]

Ah, I didn't make that clear.  As you know, each data bit is transmitted
with its compliment in adjacent half cycles.  I should have said it checks
the next 18 "half cycles", and exactly 9 of those should be "1". A collision
must add an extra "1" to be detected.  That causes uncertainty at that bit
position, and the message should be rejected.

Here is a case where a TW523 steps on a line message in such a way that its
own transmitted signal appears valid from the beginning of its own start
pattern:  (view in fixed font)

Msg 1:  1110101001101010010110 (line)
Msg 2:  ------------1110010110101001100101 (TW523)
Msg +:  1110101001101110010110101001100101 (collision)
Decod:  1110101001101110010110 -> Rejected (module)

Assuming the module can hear both transmitters, it should be decoding the
message on the line when the TW523 steps on it.  The extra 1 from the start
pattern causes uncertainty in that bit position, and the module should
reject the message.  The receiver in the TW523 should also have been
decoding the line message when its transmitter started, and should also
reject the corrupted message.  The XTB-II does the same.

Regardless of who steps on who, or how well data bits are aligned, I believe
the start pattern will almost always cause an extra "1", resulting in an
error.  The only case I can come up where this is not true is if the second
transmitter comes on during the last "1" of the first transmission.  In that
case the first transmission is valid even though it did not end in a gap.

Jeff

"Dave Houston" <nobody@xxxxxxxxxxxx> wrote in message
news:45284ee3.198752515@xxxxxxxxxxxxxxxx
> It wasn't clear from your wording that you were relaying all of the 512
bit
> patterns of the X-10 universe or just those that matched the code being
> transmitted. ADI originally did not report codes that differed from what
was
> being transmitted.
>
> From your wording, "It checks the next 18 bits, and rejects any message
> without 9 "1" bits." codes of 1110111111111000000000,
1110111000111000111000
> or even 11101111111111111111111111 could be valid - all with 9 "1" bits.
>
> I'm not familiar with how all the various controls deal with collisions.
> Initially, ADI would retransmit forever (as does ACT). I tried to persuade
> Dan Boone otherwise to no avail but I think they later did change it and
now
> retransmit some maximum number of times. The original LynX-10 could be set
> to retransmit (forever) or the auto-retransmit could be disabled. Some of
> the others talk about polite transmissions, checking for a clear line, and
> collision avoidance so it's obvious they never understood the TW523.
>
> The TW523 does report the first 22 bits of an extended code.
>
>
> http://www.davehouston.net
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