RS-232 Tx / Rx monitoring LEDs?
mouse at Rodents-Montreal.ORG
Thu Aug 20 12:11:55 CDT 2015
>>> Totem pole outputs have comparable drive strength in both directions, that$
>> That's true for CMOS outputs. TTL outputs pull down much more strongly than$
> Ok, but when you refer to "drive strength" I assumed you were talking about $
Maybe _rated_ current, but, even there, I don't think so (my TTL doc
hasn't been unpacked yet, or I'd go check, but I'm fairly sure they are
generally specced to sink more current to GND than source from Vcc).
Certainly, look at the equivalent circuit for a TTL totem-pole output:
there's a resistor in series with the top half, to Vcc, but to GND
there's nothing but the E-C of a transistor.
Or just take a chip you can afford to sacrifice :-), get it to drive an
output low, connect it to Vcc, and observe that it fries the output;
ground a driven-high output and notice that it does nothing much. But
make sure it's real TTL first; as others have pointed out, this is not,
in general, true of "TTL-compatible" CMOS circuits - the TTL
compatability refers to input thresholds and Vcc and the like.
This does have the arguable advantage that, if you accidentally wire
two outputs together, it doesn't usually fry anything if they fight.
I'm inclined to doubt that's the reason, though; it seems more likely
to me that the resistor was put there to give warm fuzzies about
ensuring nothing fries if the output transistors happen to both turn on
for a split nanosecond as signals propagate through the circuit. But
even that is pure speculation on my part.
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