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I am builing a circuit, and i need to connect a few different IC's (obiosly). My circuit is based on 5V TTL logic and i found a part that is made only in AHC/AHCT variant.

So let's take it from the start:

  • HCT -> 5V TTL
  • HC -> CMOS technology
  • AHCT -> advanced 5V TTL ?
  • AHC -> advanced CMOS ?

I have read, that i should not mix HC with HCT together, so can i use HCT <-> AHCT and HC <-> AHC? Does it work like that? What if i want to use a CMOS chip in my TTL design?

Tanks for any help.

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    \$\begingroup\$ There are level translators ICs you can use. I would not assume you can connect a logic family together just because you haven't seen anyone say you can't. Check the logic thresholds in the datasheets. \$\endgroup\$
    – DKNguyen
    Commented Oct 28, 2019 at 21:59
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    \$\begingroup\$ HCT is not TTL logic. It's CMOS logic with TTL-compatible inputs. The outputs will likely be CMOS rather than TTL levels. \$\endgroup\$
    – The Photon
    Commented Oct 28, 2019 at 22:11
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    \$\begingroup\$ Can you share a datasheet to the specific part you're asking about? \$\endgroup\$
    – The Photon
    Commented Oct 28, 2019 at 22:12

3 Answers 3

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The families differ by speed AHC is faster than HC. HCT and AHCT are TTL compatible and HC and AHC are CMOS:

enter image description here
Source: http://www.ti.com/lit/sg/sdyu001ab/sdyu001ab.pdf

enter image description here Source: http://www.ti.com/lit/sg/sdyu001ab/sdyu001ab.pdf

Also, a useful compatibility chart, CMOS can go 'into' TTL, but TTL cannot go into CMOS (it can't switch it on)

enter image description here
Source: http://www.ti.com/lit/sg/sdyu001ab/sdyu001ab.pdf

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  • \$\begingroup\$ The HCT and AHCT families are still based on CMOS transistors, but have TTL compatible inputs. \$\endgroup\$
    – remcycles
    Commented Oct 28, 2019 at 22:37
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    \$\begingroup\$ "HCT and AHCT are TTL" Nope. Read the data sheets. HCT and AHCT have TTL compatible inputs but are HC or AHC inside. \$\endgroup\$
    – TimWescott
    Commented Oct 28, 2019 at 22:50
  • \$\begingroup\$ @Tim I'm not going to argue with TIs classification \$\endgroup\$
    – Voltage Spike ♦
    Commented Oct 28, 2019 at 23:36
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    \$\begingroup\$ You're looking the page that says what the voltage standards are, and which technologies adhere to those standards. That's no the same thing as what's inside the chip. The section in that same document titled "Advanced High-Speed CMOS" is about AHC/AHCT. The "C" in 74C, 74HC, 74HCT, 74AHC, 74AHCT, etc., stands for "CMOS". So -- I'm not going to argue with TI's classification either. \$\endgroup\$
    – TimWescott
    Commented Oct 28, 2019 at 23:44
  • \$\begingroup\$ "CMOS can go 'into' TTL, but TTL cannot go into CMOS (it can't switch it on)" However despite what the tables you have posted imply, "TTL compatible CMOS" can go into regular CMOS, because while it has TTL compatible input thresholds at has CMOS output voltages. \$\endgroup\$
    – Peter Green
    Commented Oct 29, 2019 at 13:49
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If they are all running on 5V, you can freely mix AC, ACT, HC and HCT.

The 'C' means CMOS, all these parts have a GND-Rail output swing.

The 'T' means TTL compatible inputs, that is 0-0.8V = lo, 2V to Rail = hi. This range means they can also be driven with a CMOS rail to rail swing as well.

The 'H' means high speed, and they are a lot faster than the 4000 series that was the previous popular CMOS family.

The 'A' means advanced, basically a few times faster than the H versions.

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    \$\begingroup\$ There might be a speed impact from using the 'T' versions, and probably a noise immunity impact. Unless the OP is aggressively pushing the speed envelop, though, this is the answer. \$\endgroup\$
    – TimWescott
    Commented Oct 28, 2019 at 22:51
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You can mix and match logic families, but it depends on the direction of the signals and the input/output logic thresholds that each family has. You want to make sure that the \$V_{OH}\$ of an output pin is greater than the \$V_{IH}\$ threshold of the input pin, and \$V_{OL}\$ of an output pin is less than the \$V_{IL}\$ threshold of the input pin.

This page graphs the threshold voltages for a few different families: http://www.interfacebus.com/voltage_threshold.html

See page 4 of TI's Logic Guide for another graph and a compatibility chart: https://www.ti.com/lit/sg/sdyu001ab/sdyu001ab.pdf

For example, a 5V CMOS output can drive a 5V TTL input, but a 5V TTL output can't drive a 5V CMOS input.

Since that particular incompatibility is a nuisance, there are some CMOS families that have TTL compatible inputs (they can be driven by TTL outputs). These families have "T" in the name, for example: ACT, HCT, etc. You can see this in the input thresholds graphs I linked to.

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    \$\begingroup\$ These are just the guaranteed maximum/minimum values. In practice, 5V CMOS and 5V TTL can work together in any combination. Just because a TTL chip has Voh of 2,4V doesn't mean it will always only drive so low. Under normal circumstances (i.e. load conditions), it will still pull the pin up to 5V. The more problematic issue is that CMOS devices can work on Vcc higher than 5V.. I've had people wonder why their stuff went up in smoke when they tried to drive a TTL input with a 12V output from a 4000 series chip. \$\endgroup\$
    – WooShell
    Commented Oct 29, 2019 at 12:42

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