Subject:
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RE: Serial Port Protocol
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Newsgroups:
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lugnet.robotics
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Date:
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Tue, 17 Apr 2001 01:46:11 GMT
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Original-From:
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James Matthews <generation5@btinternet+ihatespam+.com>
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Reply-To:
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<jmatthews@NOSPAMgeneration5.org>
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Viewed:
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815 times
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Hi there,
I've just gone through this myself, only a few days ago. Check
http://www.generation5.org/aisolutions/rob08.shtml
...and it should answer your question. But to cut-and-paste what I wrote:
===========================================
Sending the Same Message
You cannot send the same message to the RCX, since the RCX never executes
the same opcode twice in a row. Therefore, every opcode comes with an
alternate opcode with the 3rd bit set (1000 in binary, 0x08 in
decimal/hexadecimal). Therefore, if I wanted to check for the battery level
again, I'd have to send another opcode set, with the 0x08 bit set and all
the complements and checksums updated. Therefore, my second battery request
would look like:
char buffer2[7] = { 0x55, 0xff, 0x00, 0x38, 0xc7, 0x38, 0xc7 };
Want to check the battery again? Then change back to the normal code and
alternate.
===========================================
FYI, my first battery request looked like:
char buffer[7] = { 0x55, 0xff, 0x00, 0x30, 0xcf, 0x30, 0xcf };
HTH,
James.
> -----Original Message-----
> From: news-gateway@lugnet.com [mailto:news-gateway@lugnet.com]On Behalf
> Of douglas edric stanley
> Sent: 17 April 2001 01:23
> To: lego-robotics@crynwr.com
> Subject: Serial Port Protocol
>
>
> Hi all. I'm new to the list.
>
> I'm using Robolab and NQC on a Mac, interfacing with Macromedia Director 8
> through the serialXtra. So far so good, thanks in no small part
> to the info
> posted by Dave Baum and Kekoa Proudfoot. I can make the Lego
> "beep" and send it
> messages which control the RCX. All from Director. When I get it working
> perfectly I can put it open-source somewhere, as I saw on the list that
> periodically people ask about interfacing Director with
> Mindstorms. I've done a
> lot of robotics interfacing with Director, and I've always found
> it a perfect
> match.
>
> But it's not perfect yet. I've run into the odd repeat-serial
> communications
> protocol issues. When I clear out the message buffer in my
> Robolab program,
> message sending works fine, i.e. I can send over and over the
> same message.
>
> But when I do a "beep", as Dave Baum explained how to do, I run into the
> messaging problems Kekoa outlined at:
>
> http://graphics.stanford.edu/~kekoa/rcx/#Protocol
>
> Notably: "Messages sent by PC seem to alternate between having
> 0x08 set and not
> set. This 0x08 bit is a sequence bit only in one special case. RCX never
> executes same exact opcode twice in a row. Second and beyond are
> dropped. But
> the same reply that was sent the first time is sent again. Toggle
> 0x08 bit to
> make sure same opcode twice in a row is accepted."
>
> I'm not totally sure I understand this. I don't know what Opcodes are. I
> undstood the following comment, "Sometimes packets start with aa
> ff 00 instead
> of 55 ff 00", so I made a flip-flop header routine that switches
> between "55"
> and "AA" (I guess this is 0x55 and 0x100-0x55 ?) in Lingo and I
> stop getting
> parity errors from the Serial Xtra. But I still haven't
> understood exactly what
> the 0x08 bit is. Where is this 0x08 bit? For me, the 0x08 bit is
> 2^7. So do I
> add 128? Or is this some sort of signed/unsigned issue? (I'm not
> an expert at
> all this, I get by as well as I can).
>
> When Dave wrote:
>
> "At the packet level, all packets look like this: '0x55 0xff 0x00
> D1 ~D1 D2 ~D2
> ... Dn ~Dn C ~C' where D1...Dn are the message body, and C = D1 +
> D2 + ... Dn.
> The data for sending an IR message is F7 followed by the 8 bit
> "message". For
> example: '55 ff 00 f7 08 12 ed 09 f6' "
>
> I got it, and it got messaging working, but this was incomplete.
> When you try
> this with sending a beep (D1 = 0x51, D2 = 0x03) you can only send
> it once. You
> have to send another message before sending a second beep.
>
> So I'm kind of trapped between Dave's explanation which is, for me, half
> complete (because he doesn't explain how to deal with toggling
> issues at the
> packet level); and Kekoa's response which was just slightly over
> my head. I've
> done bitmasking and all that fun stuff, but again I'm not a pro.
> I only get by
> with tinkering...
>
> Douglas Edric Stanley
> destanley@mac.com
>
> http://www.abstractmachine.net
>
> Professor of Digital Arts, School of Fine Arts, Aix-en-provence
> Artist-in-residence, Laboratoire SCAN, Villa Arson, Nice
> Artist/Researcher, Interactive Aesthetics Lab (LEI), University of Paris 8
>
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Message is in Reply To:
 | | Serial Port Protocol
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| Hi all. I'm new to the list. I'm using Robolab and NQC on a Mac, interfacing with Macromedia Director 8 through the serialXtra. So far so good, thanks in no small part to the info posted by Dave Baum and Kekoa Proudfoot. I can make the Lego "beep" (...) (24 years ago, 17-Apr-01, to lugnet.robotics)
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