23 Oct
2011
23 Oct
'11
1:11 p.m.
I still don't understand the reason for the rev.
polarity, but it
It's similar ot the difference between NPN and PNP transistors.
Everythign flips upside-down :-)
seems that high would then be -9v?
Probably. You cna actualyl take the logic levels whichever way you want,
and provided you're totally consistent (and consistent in how you name
the gates), it'll all work.
There is nothing wrong with the statement that 'The 7400 is a quad
2-input NOR gate with a +5V power supply and logic levels such that '1'
is between 0 and 0.8V and '0' is between 2.4V and 5V'.
[Before I get an long string of messages telling me the databook says
that the 7400 is a NAND gate chip, I know that!.But notice I've also
swapped the logic levels round...]
I guess its because of the way the PMOS gates are
made.
Why are the LED drivers so difficult to make, is it because of the
'high' (a few mA) current ?
Baiscally, yes. The logic transistors work at a much lower current. Often
the separate LED driver chip was a bipolar (as opposed to MOS) device.
-tony