24 Aug
2014
24 Aug
'14
1:52 a.m.
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:
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.
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.