[rescue] Vesa Local Bus video cards

[Jonathan C. Patschke]

On Mon, 31 Mar 2003, Joshua D. Boyd wrote:

> I've heard that EISA was 32bit, but how did they do that while using the
> same pin count as ISA16?  Or am I wrong in remebering that the two were
> the same form factor?

EISA is 32-bit, but has four rows of pins, as opposed to ISA's two.
EISA's pins are interleaved, taking advantage of the pessimistic pitch
of the traces on the ISA connector.  So, while each side of an ISA
card-edge connector looks like this:

XXXX   XXXX   XXXX   XXXX   XXXX   XXXX   XXXX   XXXX   XXXX
XXXX   XXXX   XXXX   XXXX   XXXX   XXXX   XXXX   XXXX   XXXX
XXXX   XXXX   XXXX   XXXX   XXXX   XXXX   XXXX   XXXX   XXXX

EISA's looked like this:

XXXX Y XXXX Y XXXX Y XXXX Y XXXX Y XXXX Y XXXX Y XXXX Y XXXX
XXXX Y XXXX Y XXXX Y XXXX Y XXXX Y XXXX Y XXXX Y XXXX Y XXXX
XXXX Y XXXX Y XXXX Y XXXX Y XXXX Y XXXX Y XXXX Y XXXX Y XXXX
     Y      Y      Y      Y      Y      Y      Y      Y
   YYYYY  YYYYY  YYYYY  YYYYY  YYYYY  YYYYY  YYYYY  YYYYY
   YYYYY  YYYYY  YYYYY  YYYYY  YYYYY  YYYYY  YYYYY  YYYYY
   YYYYY  YYYYY  YYYYY  YYYYY  YYYYY  YYYYY  YYYYY  YYYYY

Of course, EISA had to have a much longer card-edge and different
gapping for the "extended" pins so that an ISA card would plug into an
EISA slot unmodified.

-- 
Jonathan Patschke  *)  Q:  What do you call the hum of a rack of Apple
Thorndale, TX      (*      XServes?
                   *)  A:  The Al Gore Rhythm.