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I need a TTL chip 74 series, that takes in some amount of inputs and one enable bit. When the enable bit is high, the output is the same as the input. When the enable bit is low, the output is all low. Here is an unfinished truth table of what some of the inputs/outputs might look like.

S a b c out
0 0 0 0 000
0 1 1 1 000
0 0 0 1 000
1 1 1 1 111
1 0 0 1 001

I know this can be done easily with AND gates, however I am wondering if something like this or similar exists has a name and a chip. I need something that uses as few components and as few connections as possible.

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  • \$\begingroup\$ You may be able to find a buffer with an enable that will do this. Or a tri-statable buffer with pulldowns on the outputs. Worst case, a buffer with an AND mask on the outputs. \$\endgroup\$
    – mkeith
    Aug 13 at 21:31
  • \$\begingroup\$ A buffer chip will do this, but inside it's just going to be those same AND gates. \$\endgroup\$
    – gbarry
    Aug 14 at 8:52
  • \$\begingroup\$ I’m voting to close this question because homework needs an attempt at a solution \$\endgroup\$
    – Voltage Spike
    Aug 16 at 2:12

1 Answer 1

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This function can be done with three 2-input AND gates, as you note.

So one 74x08 Quad AND IC can be used or three individual AND gate ICs.

This uses one 14-pin IC or three 5-pin ICs.

There is no 74-series IC that would do it using fewer pins and using no other functional parts, such as pull-downs.

The 74x08 is the 14-pin single-chip solution that leaves a spare AND gate, unless your application prefers the three 5-pin ICs.

A note on component selection...

Component selection is nearly always a commercial, cost-led decision rather than being technically-led. There's almost always lots of options to do a function: 100s of different op-amps, resistors, capacitors etc that'll do the job fine. It's then aiming to choose one that's lowest price, always in stock, multi-sourced, easiest to buy, won't go obsolete, has fewest solder joints for reliability, uses least PCB space, most power efficient, is tried and tested by industry and your supply/manf/support chain etc. It's a big list.

Choosing a microcontroller's a good example. Most of the time, there's plenty that'll do the task to excess, leaving spare MIPs, spare pins, spare peripherals, spare memory. So it's cost-led by price, supply, development/support time/investment etc. Not often only one part will do it in all those millions of MCU applications all around us.

Here, there's several 74x possibilities, some needing extra components like pull-downs. The PCB tracking costs nothing unless the board is really tight there. So it's which of these circuit options will do it best for the list above. Which is the neatest technical fit is the low priority and nearly always is.

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    \$\begingroup\$ I think a 16-pin 74157 could do it all with one bit to spare. \$\endgroup\$
    – glen_geek
    Aug 13 at 21:46
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    \$\begingroup\$ @glen_geek The 74157 is in the MSI/LSI group of 7400 parts. We had to wait a while before we could see one of those. It's also 16 pins and more to worry about, pin-wise. And the LS version is about 50 mW, typ. The 7408, by comparison is in the SSI group and was available very early. It's also very easy to understand why it works in this case. And it's 14 pins. And the LS is 22 mW typ. The avg prop delay is close between them, but slightly favors the 74LS08, as well. I'd go that way \$\endgroup\$
    – jonk
    Aug 14 at 6:33
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    \$\begingroup\$ @glen_geek, why use 16-pin chip instead of 14-pin? And 74x08 leaves a completely free AND gate for something else, now or future. \$\endgroup\$
    – TonyM
    Aug 14 at 7:24
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    \$\begingroup\$ @jonk Agreed - based on KISS principle. And Tony, that free AND gate is likely far more useful than the 74157 extra MUX bit. \$\endgroup\$
    – glen_geek
    Aug 14 at 11:55

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