--- Tothwolf <tothwolf_at_concentric.net> wrote:
> On Wed, 2 Apr 2003, Patrick Rigney wrote:
>
> > Does anyone recognize these three boards?
> >
> > http://www.rigneyfamily.com/patrick/
>
> I'd imagine the larger header connector on the processor board is the
> memory bus...
Probably not with all those traces going to the 6821 chips.
> ...which is similar to what you see in older SGI and VAX systems.
That much is true. DEC (don't know about SGI) went to "PMI" - Private
Memory Interconnect - to get around problems with address bus width
and memory speeds with processors above the MicroVAX-I (the Qbus is
limited to 22 address bits and about 3MB/sec transfer rate).
> (Can anyone verify if the 6821/6822 are some sort of bus interface
> chips?)
They are not. The 6821 is a common "Parallel Interface Adapter". It
has two 8-bit parallel ports, several handshaking lines and a couple
of internal timers. It is functionally equivalent to the MOS 6520,
and is somewhat ancestral to chips like the 6522 and 6526. They are
commonly used in 1970s and 1980s gear to strobe keyboard matrices,
to drive external parallel devices, and to control LED and LCD
displays (there's a 6520 on the display board on a Rockwell AIM-65,
for example - I've put a 6821 in mine for testing).
-ethan
Received on Thu Apr 03 2003 - 09:43:00 BST