74 logic question

[Doomguy42]

Hi I'm working with various dip 74HC Ics when repairing battery damage. Is there any rules or theory with going up a level to a 74HCT? I saw a board recently running these in place... which could add more options on the obsolete types.

Also is there a list or PDF of all 74 logic Descriptions?

Thanks

 

[AnalogKid]

There is a difference, and it could be important to how the circuit functions or doesn't function. Note, this does not apply to Schmitt trigger input components.

The T in HCT stands for TTL. While an HC part has a normal CMOS input stage transition level of approx. Vcc/2, An HCT part has TTL transition levels: Below 0.8 V is a 0, above 2.4 V is a 1, and anything in-between is not allowed and has unpredictable effects on the output state. So while an HCT part would see 2.4 V as a 1, an HC part could see it as a 0. Not good.

In fact, old TTL parts have a single input transition level around 1.8 V, and CMOS ouptut stages go all the way up to 5 V (as opposed to TTL parts that swing up to only about 3.5 V), but I don't think you can bank on that without analyzing the full circuit schematic to see who is talking to what. I wouldn't.

AND ...

Some HC parts are rated for operation down to 2 or 3 volts, while HCT parts are limited to - wait for it - TTL power rails.

ak

 

[KeepItSimpleStupid]

Wikipedia should have some nice pages. At least they do for 4000 series IC's.

 

[sarahMCML]

Do a web search for the 'Texas Instruments Logic Guide' pamphlet, it has details of all the different TTL and CMOS versions. It's a 4.77Mb download, so I can't add it here!

 

[crutschow]

Here's a Wiki list of 7400 series ICs.