3B1 motherboard variances re: 26LS31 usage

[Thad P Floryan]

Recently, someone (Little David? :-) commented about building the ICUS two-HD
P5.1 modification directly on the 3B1's motherboard and wanting to use the
existing 26LS31 to drive the 2nd HD, but discovered the "uncomitted" pins
were, in fact, being used for "something."

Then Gil asked (paraphrased) "Any idea where they're being used?"

After I posted my "new boot ROM code idea" earlier today, I looked again at
the schematics, page 14, and said "WTF?!"

The first set of schematics I examined were using pairs of Schottky inverters
from a 74LS14 to send the "serial" signals (TxD and *RST) to the keyboard; the
set of schematics I glanced at THIS time showed the other pins of the 26LS31
being used for that purpose.  So I checked the 3rd set of schematics, and find:

	motherboard	serial drivers to keyboard
	-------------	--------------------------
	D-08-00222-00	74LS14
	D-08-00225-00	26LS31
	D-08-00230-00	74LS14

So that explains why there ARE some other pins being used on the 26LS31 on
SOME 3B1 systems.

Thad

P.S.  And one person brought a "D-08-00216-00" motherboard to the last Users'
Group meeting and, whew, was THAT board different.  Even the pinouts of the
34-pin connector to the floppy drive were COMPLETELY REVERSED from all other
3B1 systems I've seen; at least the pinnings were silkscreened on the board.

Makes me wonder just how MANY "something new and completely different" 3B1
systems there ARE out there?  C0WABUNGA!!!

Thad Floryan [ thad at cup.portal.com (OR) ..!sun!portal!cup.portal.com!thad ]