Dumb question: CMOS vs TTL

Discussion in 'General Electronics Chat' started by NateG, Jun 20, 2008.

  1. NateG

    Thread Starter Member

    Jun 20, 2008
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    I'm currently working on a synthesizer using 74153 4 to 1 line data selectors. I had been using CMOS 74153s, but I fried a couple of them, and all I am left with is TTL. The TTL chips dont seem to work at all, I plug them in and they just give me 0 on the output with all 4 input terminals set to 5v. I figure since this happens with every TTL chip that I have, I must be doing something wrong.

    Is there a special way I should hook up the TTL chips in order to get them to work correctly? I was thinking maybe capacitors across the Vcc and Gnd to reduce noise from the source, but I really have no idea.
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,649
    2,348
    Hello,

    There is a difference in FANIN and FANOUT (input and output current) between TTL and CMOS.
    CMOS has a much lower output current and can drive 1 TTL input.
    Levels can also give problems.
    More information on CMOS can be found here.
    http://www.educypedia.be/electronics/digitaltechnologycmos.htm
    and more topics here.
    Digital electronics: [​IMG] Arithmetic circuits D/A-A/D converters Number systems Codes and decoders Flip flops Technology Counters and registers General overview Technology-CMOS Digital logic Memories Technology-TTL Timers and oscillators
    Greetings,
    Bertus
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Do you have the strobe inputs low? You won't get a logic high level output if the strobe is high.

    It would help a great deal if you posted your circuit diagram/schematic.
     
  4. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    Welcome to the AAC forum! There is no such thing as a dumb question. You are free to ask, and feel free to ask any time so we can help you.

    TTL and CMOS inputs can be directly connected to 0V and 5V with no problem. Now, as for the 74153s, they are TTL chips too (any chip started with 74, 74L, 74S or 74LS is TTL, but 74HC and 74HCT are high speed CMOS). Did you checked if the enable pins were low? According to the datasheet, the outputs will be forced to logic low when those pins are logic high. You also must check if your select inputs are set to the respective should be values.
     
    Last edited: Jun 20, 2008
  5. NateG

    Thread Starter Member

    Jun 20, 2008
    22
    0
    Actually, I had my strobe inputs floating. Once I hooked them both to ground it seems to be working =)

    However, I seem to be getting a volt lower than what I would get with the CMOS chips.
     
  6. NateG

    Thread Starter Member

    Jun 20, 2008
    22
    0
    Yeah, the CMOS ones I had were actually 74AC153Es from TI.

    Im now using 74153P's from Hitachi.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    The 74153's are original TTL. They will only output about 3.8 volts, and will not play well with 74HCxx or 74ACxx logic. They should work with 74LSxx logic.
     
  8. NateG

    Thread Starter Member

    Jun 20, 2008
    22
    0
    Ah, ok. Thats what Ive been getting. This is going to be input into a basic stamp 2. So as long as I can get it to read high (when its supposed to) on the BS2 I should be ok.

    Thanks for all the responses guys!
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The TTL IC's can sink more current than they can source. You can use 470 Ohm resistors connected between the outputs and Vcc in order to raise the high-level output voltage; one per signal. Your low-level output will still be under around 0.9v. Resistor networks (either DIPs or SIPs) are very handy for this kind of thing.
     
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