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It only takes a minute to sign up. I'm trying to cut power to the chip to "disable" all outputs without needing to know the input values or changing them. Would that work? I'm trying to use a npn because my circuit has one unused channel in an ULN and I'm trying to reduce my component count. The inputs to the chip will remain active and more than likely any "low" inputs will "power" the ground of the 74HC86 via protection diodes on the inputs.
I don't think it will work. The reason that's a bad idea is that looking at the ON Semiconductor 74HC86 Datasheet for example you'll notice that all the maximum and recommended voltages are referenced to ground. When you remove the power ground you have the situation where an input can be at ground, while Vcc is still being applied and that can cause current to leak through the chip along paths not intended.
I'd recommend adding some additional logic to handle the situation properly even though it will increase the part count. You could for example look at a buffer that has an output enable pin, maybe something like a 74HC Also as Wouter has pointed out in a comment even without those limits on the input voltage the datasheet has a 'recommended operational conditions' section.
Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Asked 7 years, 2 months ago. Active 7 years, 2 months ago. Viewed times. Active Oldest Votes. Andy aka Andy aka k 15 15 gold badges silver badges bronze badges. I have seen a "slightly unreliable" TTL circuit where one chip had no ground connection whatsoever. It only malfunctioned when all its inputs were high at the same time!
Otherwise the logic levels were none too stable, but good enough for that application. PeterJ PeterJ Be aware if you did cut both GND and Vcc you still may have the same problem anyway, in that case one input could be low and the other high and cause the same problem. There are also limits about an input not being above Vcc.
You can of course try it and see what the chip does, but that is not exactly sound engineering. My problem is that if I use the 86 to only need two instead of four digital pins, there is no real "Off" for the motor because some windings are always energized.
I'd use the p-ch mosfet to cut power from the motor, not the 74hc86 or the Uln Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown. The Overflow Blog.
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74HC/HCT86 Quad 2-input EXCLUSIVE-OR Gate
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74HC86D - Quad 2-input EXCLUSIVE-OR gate 输入异或 Quad 2-input EXCLUSIVE-OR gate(2输入逻辑或门) 输入异或门（输入逻辑或门
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