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74LS688 vs 74LS521 [message #9789]

Sun, 27 February 2022 05:49

lynchaj
Messages: 1080
Registered: June 2016

Senior Member

Hi

Are the 74LS521 and 74LS688 equivalent? They sure seem similar when reading and comparing the datasheets. My chip tester detects 74LS688s as 74LS521s. They are both 8 bit comparators and appear to have identical pin outs. What is the difference if any?

I have a bunch of 74LS521s that never seem to get used so if these are close enough to 74LS688s I may start substituting them just to use them up.

Thoughts? Discussion? Thanks, Andrew Lynch

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Re: 74LS688 vs 74LS521 [message #9790 is a reply to message #9789]

Sun, 27 February 2022 10:08

monahan
Messages: 25
Registered: November 2015

Junior Member

Andrew I may be wrong but I seem to remember the 521's need a resistor pull-down on the second row of inputs while the 688's have that built in
John

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Re: 74LS688 vs 74LS521 [message #9791 is a reply to message #9790]

Sun, 27 February 2022 13:26

lynchaj
Messages: 1080
Registered: June 2016

Senior Member

Hi John
OK, Thanks! Not quite drop-in replacement but close.

Andrew Lynch

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Re: 74LS688 vs 74LS521 [message #9798 is a reply to message #9791]

Mon, 28 February 2022 16:45

etchedpixels
Messages: 333
Registered: October 2015

Senior Member

Not sure they can be very different. The tube of 74HC ones I have are stamped with both 74HCT688 and 74HCT521 on each chip.

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Re: 74LS688 vs 74LS521 [message #9799 is a reply to message #9798]

Mon, 28 February 2022 19:29

mrgcms
Messages: 7
Registered: August 2020

Junior Member

Yeah - I looked at the datasheets and they seem to be equivalent. I was going to just drop one into one of my boards to see if it worked then remembered I hadn't used sockets for any of these boards!

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Re: 74LS688 vs 74LS521 [message #9810 is a reply to message #9799]

Wed, 09 March 2022 11:31

Sergey
Messages: 236
Registered: October 2015
Location: Portland, OR

Senior Member

I haven't seen 74LS521, there are some 74F521, 74ALS521, 74HC521 and 74HCT521
The pinout of these parts is the same as that of 74LS688
AMD AM25LS2521PC is also compatible with 74LS688

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Re: 74LS688 vs 74LS521 [message #9811 is a reply to message #9810]

Wed, 09 March 2022 11:33

lynchaj
Messages: 1080
Registered: June 2016

Senior Member

Hi Sergey
Yes, I re-checked my parts and they're 74F521s. I had them confused with something else but the question is still valid. It looks like I can substitute the 74F521s for 74LS688s. I'll try it with a known working board and check if it causes any issues.

Thanks, Andrew Lynch

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