This may be already documented, but I haven't run across it. I have an
SIO-A card and the downloaded docs with the errata sheets showing a "new"
Baud Rate Selection Chart (for the jumpers). This is the one using "4100"
as a constant in the calculations, not "4096".

Baud rate 9600 and 19200 are in error:

Chart says:

9600: 1111 1111 1000 (FF8)
19200: 1111 1111 1110 (FFE)

Should be:

9600: 1111 1111 0111 (FF7)
19200: 1111 1111 1101 (FFD)

I came across this the hard way, when I couldn't get the Altair to talk to a
laptop @ 9600 baud. I was getting gibberish and it looked as though the
baud rate was off. As part of my double-checking everything, I programmed
the equation for the preset count frequency into an HP-65 and rebuilt the
entire table. Only 9600 and 19,200 baud were in error. I reset my jumpers
to the new 9600 baud preset count (FF7), and it worked.

Maybe there is an errata sheet for the errata sheet out there somewhere; it
wouldn't surprise me. For a bunch of guys that designed something that
helped change our world, they sure were scatterbrained sometimes.