Discussion:
[N8VEM: 17267] Unused gates?
joe.herdler-/
2014-01-30 19:45:00 UTC
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Hi,

Here is a newbee question. What should I do with the unused inverters in a
7400? Both inputs and outputs.

Thanks in advance!

Best,

Joe H
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Joachim Gaßler
2014-01-30 20:00:32 UTC
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Joe,
Post by joe.herdler-/
Hi,
Here is a newbee question. What should I do with the unused inverters
in a 7400? Both inputs and outputs.
Ground the inputs, leave the outputs open.

Joe "the other one"
John Coffman
2014-01-31 07:37:37 UTC
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In a 7400, leave both inputs and outputs open.&nbsp; If you ground the
inputs, you waste power by drawing 1.6ma on each grounded input.&nbsp;
The current draw is less, but NOT grounding inputs to TTL logic
applies to all families:&nbsp; 74xx, 74LSxx, 74S, 74ALS, 74H, 74L, 74F
and any others I have omitted.&nbsp; REASON:&nbsp; TTL logic is current
operated, so opening an input sets the current to zero.&nbsp; All outputs
must be open.<br>
<br>
The advice to ground the inputs is relevant to CMOS (i.e., 74HC00).&nbsp;
CMOS inputs should be tied either high or low.&nbsp; There is no current
issue.&nbsp; CMOS logic is voltage operated, so leaving an input open
leaves the device sensitive to static.&nbsp; CMOS families of logic
include:&nbsp; CD4000, 74C, 74HC, 74HCT, 74ACT, 74AHCT, etc.<br>
<br>
--John<br>
<br>
<br>
<br>
<br>
On 01/30/2014 12:00 PM, Joachim Ga&szlig;ler wrote:
<blockquote cite="mid:52EAAF60.608-UPO/6gOIxNZppp2RfTnSCRvVK+***@public.gmane.org" type="cite">Joe,
<br>
<br>
Am 30.01.2014 20:45, schrieb <a class="moz-txt-link-abbreviated" href="mailto:joe.herdler-/***@public.gmane.org:">joe.herdler-/***@public.gmane.org:</a>
<br>
<blockquote type="cite">Hi,
<br>
Here is a newbee question. What should I do with the unused
inverters in a 7400? Both inputs and outputs.
<br>
<br>
</blockquote>
Ground the inputs, leave the outputs open.
<br>
<br>
Joe "the other one"
<br>
<br>
</blockquote>
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Sergey
2014-01-31 20:04:49 UTC
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It is not a good idea to leave inputs open. Especially not with CMOS
parts...
So you can connect inputs either to ground or to VCC via a pull-up or
pull-down resistor (say 4.7k-10k). I like the latest option, since it
allows you to use the gate later for patching, and reduces current
consumption (a very little current will flow through that resistor and the
gate).
Post by joe.herdler-/
Hi,
Here is a newbee question. What should I do with the unused inverters in a
7400? Both inputs and outputs.
Thanks in advance!
Best,
Joe H
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