Re: Bad Input to Hardware TTS

[Bill Haneman <Bill Haneman Sun COM>]

Hi:

Beth CC'd the list one side of a number of private correspondances which 
we had.  I won't copy them here, except to say that I am quite sensitive 
to the seriousness of losing access to the TTS.

There does seem to be some misunderstanding of the technical issues 
behind this problem, however.  If we had nice plug-and-play capabilities 
for all these devices, and they all participated in a standard interface 
which we could use to identify them, then we could avoid sending 
inappropriate data to a device.    Unfortunately things aren't so simple 
at the moment.

However, this isn't the case for all current output devices.  The USB 
protocol offers a little help for devices that implement USB properly, 
but USB actually makes things worse for old-style serial devices that 
connect via a USB converter.  Such devices usually don't identify 
themselves via the usual USB mechanism, and because of limitations of 
"hot-swapping" of USB devices, the associated serial ports can actually 
get renumbered on reboot.  The linux kernel folks are working on fixing 
this, but it means that even if things are 'properly' configured to use 
the right serial ports, there is a possibility that the output could be 
directed inappropriately after a reboot.

For specific cases like the DECTalk case, I would be optimistic that we 
can determine a 'probe' sequence which we can use to prevent this 
serious problem from happening in the future, possibly by detecting the 
presence of a DECTalk, or at least failing to send serial output to a 
braille device without attempting to confirm that the expected braille 
device is responding as expected.  (This may be possible only for 
certain braille devices).  But there is no simple way to prevent any 
other such situation from arising again - for instance if a serial port 
gets renumbered, or even if a user plugs a TTS into a different USB port 
by accident.  Unfortunately we can't foresee all the possible failure 
scenarios involving all possible devices, some will have to happen at 
least once before we can fix them.

- Bill