25 Apr
2009
25 Apr
'09
9:44 p.m.
On 24 Apr 2009 at 17:56, Steven Hirsch wrote:
I have some schematics from the web, but they are
extremely fuzzy and
hard to read. Can anyone help with theory of operation for an 820
keyboard? I gather that it's a parallel data interface, but past that
I'm a bit in the dark.
The 820 series is strongly related to the Jim Ferguson "Big Board"
kits sold in the early 80's. Bitsavers has both the schematics for
the 820 and the Big Boards.
The keyboard interface is as simple as it gets--8 bit postive logic
ASCII character code and a negative-going strobe to reflect a
keypress. ISTR that BIg Board users tended to use George Risk
keyboards, but just about every keyboard maker had a parallel ASCII
model (e.g. Cherry, Keytronics, etc.). Other than looking for bad
connections or discrete components on your keyboard, there's not much
to be done, as all of the "smarts" are usually located in a single
mask-programmed keyboard interface chip.
To see if your 820 mainboard is at fault, try driving the keyboard
lines with the output of a PC's parallel port (not possible with a
"real" computer like a Mac).
--Chuck