23 Aug
2014
23 Aug
'14
9:57 p.m.
On 2014-Aug-23, at 8:32 PM, drlegendre . wrote:
On Sat, Aug 23, 2014 at 9:55 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.
Without any termination resistors, an undriven
bus line should nonetheless
be interpreted (by the 7404) as high, so the D2 LED should be on.
It would appear something else is pulling it low (or something is wrong
with the 7404/LED).
Not in this case.. and I mentioned why a few posts back, but I don't blame
you for missing it.
The non-gate side of the LEDs are connected to Vcc (5V), rather than Gnd.
So if the 7404 input is L, the output is H and the LED is +off+ (5V on both
LED terminals, no current). If the 7404 input goes H, the output is L and
current sinks from the Vcc line through the LED to the (low) gate of the
7404.