Expected output voltage of XNOR gate

Hi,

I have a TI SN74HC266 XNOR gate. If I pull two inputs high (4.9v) then the output should be high - what I'm seeing is 2.6V. Is this right? I may not be asking this the right way: basically, I'm trying to compare two inputs are either both high or both low and in either case, I want a high output to feed into a SN74HC08 AND gate. I've checked the voltages on the XNOR pins and they are indeed high, but the corresponding output for that gate is 2.6v - I don't think that's enough to pull my AND gate high (at least it's output given 2 inputs at 2.6V is 0V.) I've also checked the XNOR where the two input pins are 0V and the output pin is also 2.6V.

I can't tell from the data sheets if this is right, although I struggle to understand these in detail at the moment.

Thanks

Andrew

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DAB over 7 years ago

You may have trouble with the HC chip driving the input to the next stage.
Add a 1k ohm pull up resistor to the HC output pin. That should help pull the output to 5v.

DAB
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Andrew J over 7 years ago in reply to DAB

Thanks Dab, that's all it needed. I'm using a number of HC chips in the overall circuit and not needed pull up resistors to drive inputs down the chain (until I reached the 266) but I'm now measuring 4.9v on the output pins. Something to file in my brain for the future.

Thanks again, I really appreciate it.

Andrew
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gdstew over 7 years ago in reply to DAB

The 74HC266 should have no problem driving a single 74HC08 input. The spec sheet says it is capable of driving up to 10 LSTTL loads. However it does use open drain
outputs and so it does as you suggest need a pull up resistor on the output pin. Since it is driving single HC input, a 4.7K Ohm resistor would be better.
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Andrew J over 7 years ago in reply to gdstew

That's what I thought originally - however, the pull up resistor does seem to be needed. Would I have been better off with a 747266?
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gdstew over 7 years ago in reply to Andrew J

A open drain/open collector (the bipolar transistor equivalent of open drain) output will always need a pull up resistor. Most gate outputs use an active (transistor)
internal pull up connected to the supply voltage. Open drain outputs do not have a connection to the supply voltage and require an external pull up resistor to supply
this connection. This can be useful as it allows multiple outputs to be connected together using a single pull up resistor to perform a "wired-or" function (any of the
outputs going low will drive the output low) without damaging the other outputs.

I assume you mean a 74266 (google did not turn up any useful results for 747266), it uses open collector outputs.
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Andrew J over 7 years ago in reply to gdstew

Thanks, I think I follow that.

The 747266 is a similar XNOR gate: https://uk.farnell.com/texas-instruments/cd74hc7266e/logic-quad-2-in-ex-nor-gate-14dip/dp/1739947
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gdstew over 7 years ago in reply to Andrew J

Not sure why my Google search did not come up with that but looking at the link you gave says that the part uses active pull ups and so it does not need the resistor. For
a standard quad gate logic part it is an expensive replacement. If the cost is not a problem you might want to use it instead however the 74HC266 with the resistor should
cost a lot less.
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Andrew J over 7 years ago in reply to gdstew

I don’t need it right now, but for future reference I will pay attention to the active pull ups. Thanks.
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