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Topic: Trying to understand pull-up resistors (Read 2 times) previous topic - next topic

pwillard

#15
Nov 08, 2012, 10:07 pm Last Edit: Nov 08, 2012, 10:12 pm by pwillard Reason: 1
My 2 Cents...  markT gave an excellent description of how 1-wire works...

There are basically 3 states in TTL logic.  "High", "Low", and a word I like "Uncommitted".  We commonly call that "floating", and in some cases,  it is used on purpose with TTL devices that claim they are "tri-state", like a 74125 for example.

When you have devices that can only create a connection to GND (switches, transistors, whatever) , a 5V "high" logic level signal does not magically appear in the absence of GND or "low" state.  You can assume it will be low... but a nearby finger can make it be otherwise.

So, back to the transistor idea... if you have a ttl input attached to TP, without R2 to pull the input signal pin HIGH, no amount of turning "on" or "off" either transistor will guarantee that signal at TP will go to logic "1" or "HIGH".  Adding R2 will PULL UP the voltage on TP to hold the signal "HIGH" or at "1" unless either or both transistors turn on, pulling the signal... in a stronger  manner.. closer to ground... changing the logic state at TP from Logic 1 "high" to Logic 0 "low".

The PULLUP will eliminate an undesired "3rd" state.


vdavidoff

I'll need to sit down with this all later, but MarkT, your advice/explanation to ignore current for this discussion has been extremely helpful. I didn't realize that in this context the current wasn't really important, and thinking about where current was flowing was definitely confusing the hell out of me.

So in general then (at least in situations as simple as the one we're examining), is it true that if part of a circuit is connected to more than 1, what I'll call terminator (ex: +5v, GND), measuring voltage at that part will always reflect the connection to the lowest impedance terminator?

Andy

AWOL

Something that often surprises is the case of a 1 kohm resistor with one end connected to, say, 5V and the other end to one side of an open switch whose other side is connected to ground.
What is the voltage across the open switch?
And if the resistor is 1 Mohm?
"Pete, it's a fool looks for logic in the chambers of the human heart." Ulysses Everett McGill.
Do not send technical questions via personal messaging - they will be ignored.

retrolefty


Something that often surprises is the case of a 1 kohm resistor with one end connected to, say, 5V and the other end to one side of an open switch whose other side is connected to ground.
What is the voltage across the open switch?
5V
And if the resistor is 1 Mohm?
Still 5V
Lefty


dhenry

Quote
Still 5V


It depends on what this switch is.

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