Pull-down resistors

[matthewwren]

When i am feeding an IC input value from the output of another IC, am I correct in assuming that the input value I am feeding will require no pull-down resistor?

 

[Pommie]

It depends on the output stage of the IC. If it is open collector then you will need a pullup.

Mike.

 

[Dean Huster]

Pull-up and/or pull-down resistors are also sometimes required if you're changing from one logic family to another, not normal unless you need a special function (e.g., you're working with 74LSxxx TTL and need an analog multiplexer from the CMOS family) and need reliable switching.

By the way, in TTL it's one heck of a lot harder to pull an input down than it is to pull it up, so design your circuits to eliminate inputs from switches and such that require pull-down -- pull up and invert instead.

Dean

 

[dknguyen]

Yeah, if it's the same logic family and the output is "normal" then you don't need pull-ups or pull-downs. If the output is a open-collector/open-drain you need pull-up resistors. If it is open-emitter/source, then you need pull down resistors.

http://www.interfacebus.com/Design_Translation.html

Wth regards to using a pull-up so that a TTL output can drive a CMOS input, how does that work exactly? I don't quite understand how when the TTL output goes high the pull-up resistor would boost the output just a little bit to allow it to drive the CMOS. It would seem to me that the TTL output would be fighting the pull-up resistor and whichever one could sink more current would flat out win, or there would be a precarious balance.