Re: [CAD_CAM_EDM_DRO] TTL questions

In a message dated 22-Mar-01 10:17:54 Central Standard Time,
implmex@... writes:

> What on earth is a "pull-up" resistor???
>

Marcus: I am gonna stick my big ugly foot in it before I read all the e-mail
to follow, which will surely contain a PROPER answer to your good question:

In "logic" (that's the electronic kind, where "gates" in IC's are connected
up to make a functional "logic circuit") which performs a job such as
counting or adding, etc., and very, very complex strings of gates make
microprocessors that make computers, etc., etc.

In most "logic families" (there are several such families: RTL, TTL, CMOS,
ECL, etc.) if an INPUT to a gate is not connected to anything, evil
imminations can exude from the IC, not just out the "output-pins". (That is,
outputs that are "undefined" and sometimes damaging to the IC's, specifically
the CMOS kind). Therefore, an input that, say, has only a switch-contact
that is either OPEN or connected to GROUND (minus) when CLOSED, must be
connected to positive through a "pull-up resistor" which will then "pull-up"
that input to full-positive, when not connected to negative (ground) via that
closed switch-contact. The switch, then, when CLOSED, "shorts" this resistor
(and the gate's input) to ground, "wasting" some current. This "waste" is a
trivial disadvantage of some "quick-and-dirty" logic lash-ups.

Have I confused you totally? I did not try to do so! Jan Rowland, Troll


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