Buying ic's

Discussion in 'General Electronics Chat' started by arthur92710, Jan 26, 2008.

  1. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    whats better
    7400: Quad 2-input NAND Gate
    7401: Quad 2-input NAND Gate with Open Collector Outputs

    What would be the best. A normal or one with open collector outputs?
     
  2. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
    10
    It depends on the technology you use. If you are driving a TTL gate with a NAND gate, it doesn't make a difference if the output of the gate is open collector or not, since TTL inputs are driven in sink mode only. If you are driving a CMOS chip, then the story is different but with the same end result. CMOS inputs need to be both sinked and sourced. An open collector won't drive a CMOS gate per se. In that case you should use a collector resistor at each output of the 7401. 10K should be enough. Curiously, the 7400 also needs a resistor, but for a different reason, since TTL output levels are incompatible with the required CMOS input levels. So a TTL chip won't drive reliably a CMOS chip without a pull-up resistor at each output. Again, 10K should be enough.

    P.S.: You only need resistors at the outputs that are effectively driving a CMOS chip.
     
  3. hgmjr

    Moderator

    Jan 28, 2005
    9,030
    214
    To summarize what cumesoftware stated, it depends on how you plan to use the logic gates in your design. For example, with open-collector gates you can tie the output of two or more of them together and provide a common pull-up resistor. This would allow you to hard-wire "OR" the gate outputs at least in the negative-logic sense.

    hgmjr
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It really depends upon what you're planning on doing with them.

    Most TTL gates have what is known as "totem pole" outputs; that is they can both source and sink current (not much, though).

    Those that are listed as "open collector" means that they require a "pull up" resistor, or load connected between the output and Vcc, in order to give an output.

    In other words, "open collector" outputs can SINK current (act as a ground), but they cannot directly SOURCE current (act as a voltage/current supply).

    Open-collector outputs are very useful if you are attempting to interface with different technology IC's; for example CMOS. You could have a portion of your circuit in TTL, and use open-collector output TTL devices (in conjunction with pull-up resistors) to translate to the CMOS circuit.

    What to use for pull-up resistors will depend upon the individual application, but 470 Ohms is quite popular.

    There are resistor networks available in several forms, including SIP (single-inline pins), DIP (dual inline pins) and SMT/SMD (surface-mount technology/devices)

    (ETA: I should know better than to post a reply that took me an extra 20 minutes because my spousal unit wanted me to do something :rolleyes:)
     
  5. arthur92710

    Thread Starter Active Member

    Jun 25, 2007
    307
    1
    Well to be future-proof what would be the best. Which one will work both ways? The open collector right?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The "open collector" will work both ways. But using those makes it mandatory to have extra components to make them work; namely the pullup resistors.

    It is better to have a few of each on hand - if you feel you must.

    However, you may wind up with a garage that looks like mine does. :rolleyes:
     
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