Re: baum: NVDA does not detect usb-braille

[Alexander Epaneshnikov <aarnaarn2@gmail.com>]

Hello Samuel,

On Sun, Oct 18, 2026 at 09:10:39PM +0200, Samuel Thibault wrote:
> Does the device sends the cell count first or the device identity
> first?

I don't know for sure, as I don't have a real Baum display; mine is a
HumanWare Brailliant, which uses its own protocol. The comment in
NVDA's driver lists them as "device id, serial number and number of
cells", so the identity seems to come first, but that is the only hint
I have. BRLTTY's Baum driver doesn't use 0x15 at all, it asks for each
of them explicitly.

NVDA itself doesn't depend on the order: it stores the cell count and
the identity separately as they arrive and continues once it has both.
I tested sending the identity first and then the cell count, and NVDA
detected the display.

On Sun, Oct 18, 2026 at 09:10:39PM +0200, Samuel Thibault wrote:
> (splitting the actually-unrelated questions)
>
> Alexander Epaneshnikov, le dim. 18 oct. 2026 21:52:44 +0300, a ecrit:
> > On Sun, Oct 18, 2026 at 06:26:53PM +0200, Samuel Thibault wrote:
> > > Ah, that is a requirement of the protocol? Then we need to do that
> > > indeed. What is the value of the uint8_t protocol field when it
> > > expects to receive 0x01 and 0x84?
> >
> > It is 1 (on). For serial displays, NVDA's Baum driver
> > (source/brailleDisplayDrivers/baum.py) first sends 0x15 with 0 to make
> > sure the protocol is off, then sends 0x15 with 1 twice, "in case the
> > display misses the first one". After that it waits for the cell count
> > and the device identity. According to the comment in that driver, a
> > real display answers only when the protocol goes from off to on. On
> > that transition it sends its device identity, its serial number and
> > its cell count, and it sends nothing if the protocol was already on.
> > NVDA itself only waits for the cell count and the identity.
>
> Does the device sends the cell count first or the device identity first?
>
> Samuel

--
Sincerely, Alexander