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For a project, I am connecting the output ports of a 74AC04E hex inverter to bnc ports.

Something could be (but not necessarily) be plugged in at one or more ports at a given time.

Should the ports have pull-up:s / pull-down:s or could they be left floating?

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Am I understanding correctly that the inputs will not be floating? – Kevin Vermeer Nov 8 '10 at 16:12
@reemrevnivek - Yes, thats correct, the input is properly defined, and will not be floating. I should probably have noted that. – Theodor Nov 9 '10 at 8:50

4 Answers

up vote 2 down vote accepted

There is no problem in left the ports floating. If you want to insert a resistor (avoiding some high frequency noise problems) its better a pull down, as the IC has greater capacity to load a low level (8mA) than a high level (-0.4mA).

For a 5v a 47k ohms resistor would be ok (from datasheet min output resistance is 2k).

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up vote 3 down vote

Logic gates either have open drain outputs or push-pull. If it's open drain you need a pull-up to get a defined level when the output MOSFET is off. If it's on it will pull the output level low.
The 74AC04 has push-pull outputs. This isn't mentioned explicitely, but you can tell from the driving capabilities: "Outputs source/sink 24 mA". Push-pull outputs always have a defined output level, either 0V or \$V_{DD}\$, so you can leave them open.

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up vote 2 down vote

The outputs will never be 'floating', they'll always be driven by your IC, so you don't need to add pull-ups/downs.

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Perhaps they are trying to ask what state the outputs will be in if the inputs are left floating? – mjh2007 Nov 8 '10 at 21:56
@mjh - Nope, the poster said the input is properly defined, and will not be floating. in a comment to the question. – Kevin Vermeer Nov 9 '10 at 16:56
up vote 1 down vote

If you just connect an output of your 7404 chip to a BNC connector you don't have to worry about pull-ups or pull-downs on the output side. It looks like outputs of this chip cannot be tristated so you should always have a known output of the BNC connector. The output will be determined by the input of the NOT gate. You may need to put a pull-up or a pull-down on the input, but that depends on that portion of the circuit.

These mating BNC connectors (that would be inputs) however should probably have a pull-up or a pull-down. Which direction you pull depends on your circuit that mates to this circuit. Just think of the situation where you would have your BNC unconnected, resulting in an input that is not actively driven, do you want it to be GND or VCC?

I hope I'm helping and not confusing the situation more.

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