Talk:Pull-up resistor

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After reviewing other site, I'm not sure if the picture included in this article is right. I suspect that it simply includes pull-up resistor, but there is also some irrelevant information on it.

The image shows a simple circuit using both pullup and pulldown resistors. Lunkwill 19:39, 15 March 2006 (UTC)

OK -- then what is a pull-down resister? What I mean is -- that the clarity of this article depends on another, currently nonexistent, article. (Wiki writer 17:58, 17 June 2006 (UTC))

They should both be in this article, though I don't know what best to call it. — Omegatron 01:02, 19 June 2006 (UTC)

I've been thinking about it, and I don't have any great suggestions for a title that would adequately identity both of these circuit functions. Instead -- it might must be easier to write a new article for "pull-down resister." (Wiki writer 20:49, 19 June 2006 (UTC))

[edit] Merge with Pull-down resistor?

Since a "Pull up Resistor" is very similar to a "Pull down resistor", and they are used for very similar reasons, I suggest that the two articles be merged, since they are both quite slim and repeat information between them. Can we have a quick "show of hands" as to the general opinion on this?--Wierdy1024 22:07, 7 June 2007 (UTC)

the link "See also : Pull-down resistor" is pretty much misleading since it goes back to the same page -- mike, july 3,2007

Thank you for your suggestion. When you feel an article needs improvement, please feel free to make those changes. Wikipedia is a wiki, so anyone can edit almost any article by simply following the Edit this page link at the top. The Wikipedia community encourages you to be bold in updating pages. Don't worry too much about making honest mistakes — they're likely to be found and corrected quickly. If you're not sure how editing works, check out how to edit a page, or use the sandbox to try out your editing skills. New contributors are always welcome. You don't even need to log in (although there are many reasons why you might want to). — Omegatron 16:05, 4 July 2007 (UTC)

[edit] Why use a resistor in the first place?

I'm looking for an expert on the topic of what damage/disadvantage say using a pull up or down resistor of say zero ohms is (ie. just connect an input straight to power supply or ground). I know it works, but that it's generally considered bad proctice but why? I just ask because on a project I'm working on I could dramaticly reduce the component count by wiring everything straight high or low.--Wierdy1024 22:07, 7 June 2007 (UTC)

What? That wouldn't work. You need a resistance so that the node can be pulled in the opposite direction by a smaller effective resistance in the transistor. Pull-up resistors are very cheap and small, so getting rid of them would not save much, even if it did somehow work. — Omegatron 16:10, 4 July 2007 (UTC)

I am curious why pullups and pulldowns are recommended on modern logic families. The high gate resistance at the input, in parts like SH74LVC families is specified for the input voltage level. Older TTL inputs needed current limitation on the input, so we used to use resistors. It seems acceptable to directly tie unused inputs to Vcc or GND. An exerpt from www.interfacebus.com

"The [old] TTL emitter input logic families required the resistor, while newer TTL families may not because they could accept a higher break down voltage on the input pin [protected by Schottky diodes]. The resistor value does not change based on the protection provided, but by what the input pin requires as a valid logic level. A resistor value of 1K ohm to 5k ohms is common and should work for all logic families as a pull-up. So determine if parts count or cost come into play with this design, are you building 3 prototypes or 10,000 units [to determine the cost and impact of the resistor]. Next determine if the resistor is really required, check the IC logic family being used, it may or may not require a pull-up resistor."

(jthargenrader Aug. 13, 2007)

An input that is pulled up to a logic 1 can be forced to logic 0 with a jumper - sometimes this is useful (for diagnostics, or setting options with jumper blocks or DIP switches), and not possible to do if the input is hard-wired to the power supply rail. --Wtshymanski 18:36, 13 August 2007 (UTC)