[BRLTTY] Braille protocols for new device

[Dave Mielke]

[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.

-- 
Dave Mielke           | 2213 Fox Crescent | The Bible is the very Word of God.
Phone: 1-613-726-0014 | Ottawa, Ontario   | 2011 May 21 is the End of Salvation.
EMail: dave at mielke.cc | Canada  K2A 1H7   | http://Mielke.cc/now.html
http://FamilyRadio.com/                   | http://Mielke.cc/bible/