24 Aug
2014
24 Aug
'14
1:06 p.m.
I began asking this, as when I powered-up the boards
for first smoke test,
I noticed that all of the Data LEDs are on.. +except+ for D2. Investigation
found that all data lines (except D2) are sitting at 1/2 Vcc causing the
connected 7404 inverter outputs to go low, which turns on the LEDs (as the
other leg of the LEDs is tied to Vcc). So when the inverter output remains
high, both sides of the LED see Vcc and hence no current - inverters go
low, and they sink current and turn-on the LEDs.
This is quite a nroaml way to do it. A TTL output can sink a larger
current tahn it can source, so when driving LEDs it is noraml to lin kthe
LED from the otuput to Vcc so that the output has to sink current to turn
it on.
It was that D2 being off that drew my attention. And upon further reading,
I don't think +any+ of those data LEDs should be on, when no other boards
are installed.
I am not so sure. Since a TTL output can sink better than it can source
it is noraml to have termination which pulls the input to a logic 1
state. In which case all the LEDs should be on if there are not other
boards in the machine (and thus the only thing 'driving' the data lines
are the termiation resistors.
-tony