EEVblog Electronics Community Forum

Electronics => Projects, Designs, and Technical Stuff => Topic started by: samsmock on December 03, 2018, 06:45:50 pm

Title: Need help with an alternate part!
Post by: samsmock on December 03, 2018, 06:45:50 pm
I've got PCB's that I've built for years.  Parts went obsolete on them years ago.

I bought a bunch from Hong Kong and have had enough to keep me going for quite a while.   Now I'm looking again.  One of the suppliers pointed me to a possible substitute for the flip flop I've been using.  I've never come across it before, and would like to get someone elses opinion of whether or not it's a match. 

Things look like they'd work to me, but I want to make sure I'm not missing something.

So the part I've been using is here.


http://pdf1.alldatasheet.com/datasheet-pdf/view/53719/FAIRCHILD/MM74C74.html (http://pdf1.alldatasheet.com/datasheet-pdf/view/53719/FAIRCHILD/MM74C74.html)



The alternate they are suggesting is here.


http://www.alldatasheet.com/datasheet-pdf/pdf/15662/PHILIPS/74HC74D.html (http://www.alldatasheet.com/datasheet-pdf/pdf/15662/PHILIPS/74HC74D.html)


Can someone double check me on this?

Title: Re: Need help with an alternate part!
Post by: tsman on December 03, 2018, 07:25:32 pm
There are several differences between those two parts. Things like the timings + propagation delays are completely different. The supply voltage range is far bigger on your old MM74C74. Only you can tell if it is compatible by looking at the rest of your circuit.
Title: Re: Need help with an alternate part!
Post by: Benta on December 03, 2018, 07:37:26 pm
If your circuit is running a 5 V supply, the 74HC74 is a drop-in replacement, although the faster signal edges/delays may cause issues.

If your circuit is operating at higher voltages, eg, 12 V, the functional replacement is the MC14013/CD4013, which will require a PCB layout change or an adapter board.
Title: Re: Need help with an alternate part!
Post by: samsmock on December 03, 2018, 08:18:55 pm
Yeah, it's running at 5v.  I may order some samples.   Unfortunately, I do the assembly part and am out of touch with the guy who designed it for me in 2007!
Title: Re: Need help with an alternate part!
Post by: t1d on December 05, 2018, 06:26:21 am
Don't forget about checking directly with the OEM for what they suggest as a sub. They have technical assistance, just for this purpose. And, the big component distributors also have personnel to help you with this very type of issue. Mouser, Digikey, etc.

In doing your research, if you discover a new method to track down parts, please post it here.
Title: Re: Need help with an alternate part!
Post by: aju11 on December 06, 2018, 10:37:10 am
1. Check the supply on the board.
2. Check the noise margins. Voltage levels of clock, data and reset lines
3. Check compare timing diagrams