[BRLTTY] BrlTty, NVDA, & BrailleNote Touch

[Dave Mielke]

[quoted lines by Jackie McBride on 2018/05/18 at 15:03 -0700]

>So might I ask--what does that really do, as it appears the device had 
>actually been discovered. Or am I misinterpreting the log?

No, you aren't missing anything in the log. You just lack a bit of knowledge 
about how Bluetooth works. I'd say that'd be a rather common weakness that most 
people suffer from. :-)

The BrailleNote Touch, just like lots of other braille devices, implements 
Bluetooth communication by emulating a serial port. Emulating a serial port 
over Bluetooth uses a protocol called RFCOMM, which, for the purposes of 
simplistic explanation, is kind of analogous to the TCP/IP protocol on a 
network. It supports several services (which Bluetooth calls profiles) running 
simultaneously, with each one using a different RFCOMM channel. Again, for the 
purposes of a simplistic explanation, you can think of RFCOMM channels as being 
kind of like TCP/IP ports.

So, yes, the device was detected. The problem was determining which RFCOMM 
channel to use. Some braille devices (mostly the older ones) always use the 
same channel, so brltty drivers have the capability to define which channel to 
use. Other braile devices (most of the newer ones) use a channel that's 
dynamically assigned when their Bluetooth interface is started. For those, it's 
necessary for the host to ask the device which channel to use for its Serial 
Port Profile. this is done via Bluetooth's Service Discovery Profile, so brltty 
drivers also have the capability to request that service discovery be 
performed.

The bug was that the HumanWare braille driver wasn't requesting that service 
discovery be performed. This has been fixed within the development code. The 
discovery=yes device parameter is the way to manually request service 
discovery.

>BTW. I'm just curious, you understand.

Of course! Curiosity like yours is encouraged on this list.

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