2 Sep
2014
2 Sep
'14
6:15 p.m.
Brent & All,
A big thanks to you, Brent.. you were a great help.
But yeah, the switch positions are essentially backwards from the
indications in the doc. I was able to sort that, but it's still kind of a
creepy feeling.. I sympathize for the (person like myself) 40 years ago
without any kind of experienced support & assistance on-tap. I've written
kit assembly & owners manuals for stuff that's more complex to build than
that board, and even I managed to get my ons and offs coherent and easily
understood!
I'm sure it's small potatoes for folks like yourself, but for me, this is a
big breakthrough.. some confirmatory evidence of my progress up the
learning curve. A couple more rounds like this, and I'll need some bigger
fish to fry.. then lookout, lol!
(Oh, and after reading about two dozen datasheets for 74XX I +do+ know how
TTL stuff is built, internally. Ain't nothing but xistors, diodes,
resistors and caps in there, for the most part. It could all be built from
basic discrete components - and more importantly, it once +was+. It's all
about the "I" in "IC", eh?)
On Tue, Sep 2, 2014 at 7:49 PM, Brent Hilpert <hilpert at cs.ubc.ca> wrote:
On 2014-Sep-02, at 3:12 PM, drlegendre . wrote:
So then, let's move on to the section on
configuring the switches on the
board. Frankly, it appears not only confusing and just plain wrong, it
also
appears to contradict itself. I think this is the
stuff that Brent was
mentioning earlier - if you set those switches the way the doc indicates,
the memory gets mapped to the top-end of the 64K range, as opposed to the
bottom end. That one I verified. Also, the switches on the board are in
reverse order vs. the switch tables drawn in the manual
It's confusing because:
- there's half-a-dozen levels of inversion going on in the circuitry,
- they installed some switches with the opposite open/closed orientation
to that of the design,
- and the off/on in the doc refers to the bit values, not the switch
positions.
I think the four address-group switch-sets for your board should be:
UUDD DUDD UDDD DDDD
U = up, towards top edge of board
D = down, towards edge connector
The other small switch-set WA/BO/BD looks opposite to the doc too.
Then there's the strange statement..
"closed=0 open=1", bottom of p.1-5.
Now that defies every convention known to man.. 0=false=open=off /
1=true=closed=on in my world.
Sometimes the convention used in some field of study (boolean logic)
doesn't match up with a physical implementation (TTL electronics).
Closing the switch pulls the line low, to GND. This is standard with TTL
because TTL is in an implementation class of logic called current-sinking
logic (look up a resource that shows how TTL works electrically).
It's much more power-efficient to do it this way than the reverse. Also,
sometimes it's preferable to have one side of a switch at GND rather than
+V.
Whether that closed switch is going to match or complement the '1' in an
address bit depends on how many inversions occur in the address comparison
circuitry.
Anyways .. Yay! - a functioning Altair.