Pull up vs pulldown

For logic inputs, we can either use pull up resistors to 5V for pull down to ground. What are the pros and cons for boths and which is better to use and why? Thanks

Reply to
wt
Loading thread data ...

Usually pull-ups are used. This is mainly because TTL devices or open collector outputs are able to handle that kind of inputs. TTL devices are able to sink more current (to Low) than to source current (to Hi) As a rule of thumb: use pull up instead of using pull down. Just my one Euro cent Cheers Klaus

wt skrev:

Reply to
Klaus (DK)

Pullups fit in better with the characteristics of the older logic families such as TTL, because TTL inputs and outputs were very assymetrical in their '1' and '0' input and output voltage margins, input currents and output currents. The result of this is that pulldowns for TTL needed to be no more than a few hundred ohms, and a TTL output couldn't drive such a low-valued pulldown high, and a switch or other device which could pull the pullup high would have to pass (waste) many tens of mA. However, pullups could be many kohms, and a TTL output could easily drive such a pullup low.

If the characteristics of the input and output circuitries you're using are symmetrical in terms of input voltage margins, input currents and output currents, then the question pullups or pulldowns comes down to your own choice. If they're not symmetrical, then that assymmetry will dictate which is to be preferred (which is pullups in ever case I've ever seen).

Reply to
Mike Silva

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.