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Compatible replacements for "F" version (74F244N)?

Discussion in 'Datasheets, Manuals and Component Identification' started by TTL, Jul 5, 2017.

  1. TTL

    TTL

    180
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    Oct 24, 2013
    A vintage computer I'm working on has a 74F244N (20 pin DIP package) which I want to desolder, place in a socket and possibly replace (just in case something goes wrong), but my supplier (Farnell) no longer sells it.

    According to Wikipedia's 7400-series article the "F" designation appears to indicate "fast" (well, that's my interpretation anyway) as it explains:
    I found 12 currently available types of the '244 (DIP-20 package) at Farnell excluding the slower 74, 74LS and 74S series mentioned above):

    SN74HC244N
    CD74HCT244E
    SN74ABT244AN
    CD74AC244E
    SN74HCT244N
    CD74ACT244E
    SN74ACT244N
    SN74AC244N
    CD74HC244E
    SN74AHCT244N
    SN74LVC244AN
    SN74AS244AN


    Can I replace the 74F244N with any of the above?
    I read something about CMOS variants not always being compatible with TTL types. Those are the ones with "HC" somewhere in the name, aren't they?
     
  2. kellys_eye

    kellys_eye

    4,284
    1,145
    Jun 25, 2010
    The 'AS' series is generally widely available and a better (i.e. faster) series than the 'F'.

    Unless you're mixing CMOS with TTL then there's usually no problem with swapping out for a 'faster' version (of course you can't do the reverse!).
     
  3. TTL

    TTL

    180
    5
    Oct 24, 2013
    You mean the SN74AS244AN ?
    It's considerably more expensive than the rest of the '244 variants I found there, and being for the same Atari ST I mentioned in my thread about recapping its SMPS it's probably overkill.
    Will any of the other ones than the SN74AS244N work equally well (or better) than the 74F244N?

    About CMOS and TTL variants of the same chip: should I keep to the same technology (TTL or CMOS) as the original chip on a PCB when replacing it as they aren't compatible with each other?
    And if so, is looking at the name for an "HC" designation what tells a CMOS device apart from a TTL variant?
     
  4. TTL

    TTL

    180
    5
    Oct 24, 2013
    I still have a lot to learn on the subject and found "Logic thresholds for TTL, CMOS, LVCMOS and GTLP IC families" which, if I've understood things correctly, says that there are different logic voltage level standards used in the various families.

    So with the 74F244N being within the TTL family it's not directly plug-in compatible with those other ones (74AC, 74HC, 74AHC, 74C) which are CMOS because their signals are different, so they won't work with the rest of the computer. Neither the "Low voltage" devices (74LV, 74LVC and 74ALVC) will do I guess, so that's probably why you suggested the SN74AS244AN, being an "AS" according to that diagram shows it being a TTL, and from the looks of it the only TTL family DIL '244 Farnell has to offer. And the one they have just happens to be faster than the "F" :)

    Have I understood it properly?
     
  5. kellys_eye

    kellys_eye

    4,284
    1,145
    Jun 25, 2010
    The logic level transitions only become a particular issue if you're using the two difference devices on different supplies - if they are both run from a 5V rail and the logic levels are simply 0V and +5V then, for most circumstances, the use of CMOS in place of TTL won't become a problem. Unless you tell us exactly where/how the original device is being used (schematic) we can't make an accurate statement.

    Farnell aren't exactly the cheapest place to get parts - have you looked elsewhere?

    But if it's an Atari ST then I'd be more than surprised to find that the bog-standard 74LS series wouldn't work.
     
  6. TTL

    TTL

    180
    5
    Oct 24, 2013
    I see. I do have a basic understanding of reading schematics, but I don't know what goes on inside the ICs and how they interact with each other.
    So I can possibly replace a TTL type IC with a CMOS (and vice versa I would think) if I do have an understanding of what the part does in the circuit, but if I'm not up to that level and just want to play it safe I should get the same technology type chip (same speed or faster)?

    Yes, Farnell aren't that cheap, but being in Norway everything's expensive anyway. Digikey, Mouser, Jameco and other US dealers are usually cheaper, but with their rather high shipping costs to outside the US together with import duties/taxes I'll probably end up paying even more than with Farnell (no additional taxes/import duties to pay). There's also RS Components which I can order from locally, but I haven't tried them yet and have found their prices to be slightly higher even than Farnell.

    Ebay isn't an option for buying semiconductors, capacitors or whatever component that can be faked/of sub-par quality though connectors, wires and such are OK, but someone suggested Arrow electronics. I just checked and they have the 74F244 and as far as I've heard shipping isn't too bad outside the US either. Perhaps I should give them a go. Tayda has also been suggested, but I've read mixed reports of the quality of their parts and there are also reports of them selling fakes but I'm not sure if that's true.
     
  7. kellys_eye

    kellys_eye

    4,284
    1,145
    Jun 25, 2010
    If you socket the device then popping in whatever version you can get is easy - no harm will come to the PC by using non-F versions but if you can get the 'proper' device then go for it.
     
  8. TTL

    TTL

    180
    5
    Oct 24, 2013
    Good point about trying them out when having a socket.
    I'm going to try to get the "F" version though.
     
  9. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,178
    2,690
    Jan 21, 2010
    The CMOS parts with T In their name have TTL compatible inputs. The AHT version may be the closest to a CMOS replacement for the F series TTL if you don't want to use the AS series.

    If it's being driven by other logic then (nose immunity aside) the outputs are generally more of an issue than the inputs.
     
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