[BRLTTY] Braille protocols for new device
Dave Mielke
dave at mielke.cc
Wed Jul 11 19:26:00 EDT 2012
[quoted lines by timothyhobbs at seznam.cz on 2012/07/12 at 00:59 +0200]
>I am sitting in Geneva with Samuel Thibault discussing the proper protocol
>for a new open source Braille device I am making. It is an opportunity to
>define some sort of open protocol for Braille devices.
I'm sure you're probably aware that there is (or, was) an OpenBraille project.
Have you had a look at it?
>We have some questions about the best practices the frame details.
>
>I was thinking that we would use something like PPP as an example at level 2
>and start with a frame:
>
>[FLAG(1 octet)|TYPE(1 octet)|SUBTYPE(1 octet)|INFORMATION|CRC(2 octets)|FLAG
>(1 octet)]
>
>I am wondering if we should DROP, AK, or NAK the frames when the checksum is
>bad,
NAK is only ultimately useful if it can be treated asynchronously and if
there's a way to relate it to a particular request. This, of course, would
introduce the need to add some sort of a request number to the packet (and, of
course, to the NAK).
Waiting for ACK/NAK just slows things down unnecessarily. Not waiting makes ACK
essentially useless. NAK, however, could be useful in order to detect, for
example, that some feature isn't supported.
>and whether CRC is overkill.
Probably. We just don't see byte errors in real life. Some old drivers contain
code (and accompanying comments) for supposedly poor quality serial
communiation, but, while I can't prove it, I rather suspect the real cause to
have been UART buffer overflows.
>Samuel has suggested that we diverge from PPP at least in the FLAGs as when
>the same character is used for START and END flags it can cause
>synchronization problems.
Yes, using the same character for both is a bad idea.
>Also, should we include a LENGTH specifier? Or should we simply end our
>INFORMATION section with the reception of the END FLAG?
I believe there should be a length.
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