Some Altair questions, if I may..

Well, they were /functionally missing/. So many little parts all crammed together, all alike, and no clear pin numbering in sight - I couldn't tell whether or not the 330 was present until I pulled the board back out, flipped it over and started counting. Then I found the bunk joint where #41 should be soldered.. As things moved along, I was able to measure the 470 to gnd, tho, so I knew that much was there. Now back to your original point, the one you felt I was missing.. Pin #41 had the 470 to gnd, but no 330 to VT+, so it was at a true logic low, pulled down by the 470. This causes the 7404 output to go high, turning off the LED. When I connected the 330 part, Pin 41 came up to 2.8VDC (about 1/2 of VT+) which I guess the 7404 sees as a high (?) switching off the 7404 output and turning on the LED. All good so far?

if so, then I don't understand why you were stressing that a typical TTL chip would see a floating input ("undetermined") as a logic hi. Since all of the bus pins in the D0-D7 series connected to the 7404s had either 2.8V or 470 to gnd, none were just floating, were they? Or were you speaking of some other lines left floating that I should be concerned about? See what I mean?

>> - A number of lines are biased at +roughly+ 1/2 VT+ by being placed at the >> junction of a 330R and 470R -> Gnd resistor. >> >> And for some reason, all of the data lines - with the exception of D2 - are >> in the latter category. They're biased at the 330 / 470 junctions. So my >> current questions are.. >> >> Why did they skip D2 of all things?

On Sun, Aug 24, 2014 at 3:52 AM, Brent Hilpert <hilpert at cs.ubc.ca> wrote: > On 2014-Aug-23, at 11:12 PM, drlegendre . wrote: >> On Sat, Aug 23, 2014 at 11:57 PM, Brent Hilpert <hilpert at cs.ubc.ca> > wrote: >> >>> No, you're missing the point. >>> >>> You said the D2 LED is OFF, which by your logic implies the 7404 input > is >>> LOW. >>> >>> But, as I've stated twice now, an undriven TTL input (which should be > the >>> state of the line without termination resistors) should be interpreted > by >>> the inverter as HIGH. Thence (as per your desc) the output of the > inverter >>> would be LOW and the D2 LED ON. >>> >>> This input behaviour is a characteristic of TTL logic at the electrical >>> level. It may seem counter-intuitive but an open or unconnected input > in a >>> physical logic implementation is not inherently a logical low. Pulling > TTL >>> inputs high with resistors is not done for the sake of setting the > input to >>> a logic high, it's done for noise margin and speed issues. >>> >>> There is other stuff in the front panel connected to the DIn lines, but > I >>> don't think any of it would be driving the lines in the idle state, so > all >>> the data LEDs should be on with no boards plugged in. You might confirm > the >>> data switches are all in the same position and toggling them has no >>> immediate effect on the LEDs. >>> >>> >> Then I'm not sure what to say, because when I fixed the bunk connection > in >> the term. network (for pin 41 / D2) the LED came on just like the rest of >> them.. shrug. > > > Well, that's good. If the 470-ohm R to ground was intact and the 330 R to > +5 had a bad connection that would explain the symptoms. Your earlier > messages left the impression the D2 Rs were missing.