Electronics > Beginners
74LS14 testing circuit pls
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rstofer:
There is nothing wrong with 74LSxx devices.  It is up to you to read and understand the capabilities and limitations as given in the datasheet.  Every single number printed in the datasheet means something.  Sometimes it is a factor in your design, sometimes it isn't.

All TTL logic is asymmetric in terms of pull-up and pull-down current.  That's why loads are always pulled down.  This isn't true for CMOS...

For the 74LS family, IOL (pull down capability) is 8 mA - hardly enough to light an LED but it will work.  OTOH, IOH (pull up capability) is only 400 uA and is incapable of driving an LED. 

If you look at IOL and IIL you will see where the device output can drive 20 inputs.  Same for IOH driving IIH. I wouldn't push the 20 gate fanout but I would certainly do 10 or 15 gates.  The point is, the 74LSxx devices are perfectly capable of doing performing their intended function in their intended environment.  It's up to you to design around the capabilities and limitations.

http://ee-classes.usc.edu/ee459/library/datasheets/DM74LS14.pdf
Chriss:
ebastler:

--- Quote ---Have you tested it with (a) clean power supply, decoupled with suitabe 100nF capacitor, (b) strong, well-defined input test signal from pullup resistor and switch to GND, and (c) checking the output on an oscilloscope, without a load connected? It is not clear to me from the thread if you have done tests where all three conditions were met. If the anser is "yes", ad the chip still did not behave, then it is probably bad.
--- End quote ---

Yes, I done these tests several times gate by gate and only one gate had a good signal of the two IC's.
That's made me crazy because I bought 2 IC's and I tried them and they are looking bad.

I tested before too in the past this type of IC's with my scope without loads and all they was ok, with a nice inverted signals,
but these two ic's didnt do anything. Or non stop low, or non-stop high, or non-stop nothing. Just one gate is acting well.

Here is two circuits what I was used also before for testing and button debouncing.
When I say "before" I mean years back...



Is the 1. circuit good enough to just test the gates on this 74LS14?
I ask because I done this type of connections in my projects and they still working even after 7 years.

Also the 2. circuit, is that good enough because that is also working years back till now...
This circuit I used for button hardware debouncing for uC's...

When I tested the 74LS14 in the past, I never pulled up the not used inputs, and I never faced this kind of problem like now.
I should maybe avoid this type of wiring, to let the inputs floating...
Ian.M:
For the second circuit, it works with a switch, but if you attempt to drive it from a transistor, unless you reduce the 1K resistor, you may not meet the guaranteed logic '0' threshold.   

e.g. the OnSemi 74LS14 datasheet says it can take up to 0.4mA to pull the input down.  Therefore there can be up to 0.4V across the 1K resistor and to get the input under the 0.6V guaranteed logic '0' threshold (parameter: 'Negative-Going Threshold Voltage') the left end of the resistor has to be below 0.2V.  However the typical output '0' voltage is 0.25V so you are already S.O.L. if you want to reliably drive a low pass filter like that from another gate on the same chip.   Keep that resistor under 500R and you'll get away with it.

RC debouncing for 74HC/HCT is much easier as their input current is negligible so you can use much lower value capacitors and higher series resistors.
ebastler:
Chriss - when you say „this type of IC“ has worked for you in the past, do you mean the exact batch of Hungarian ICs you are using now, or some orher brand? If you were successfully using something else before, it seems pretty clear that the Hungarian chips are bad - toss them!
amyk:
Not fake, but certainly very old and possibly defective.

Does the bottom look similar to this?
http://www.silirium.ru/pictures/ussr-Electronpribor-KR531IP3-brown-back.jpg

A quick Google suggests these parts were found in Commodore C64s

http://cbm-hackers.2304266.n4.nabble.com/CBM-s-CP-M-Z-80-carts-for-the-64-td4656117.html

--- Quote ---I have seen lots Commodore systems that used
EL7406, EL74LS14 and others. Those have no makers logo, come in a brown
casing instead of the usual black and are, as far as I know, russian in
origin.
--- End quote ---
https://www.forum64.de/index.php?thread/45601-west-trifft-ost-russ-ramchips-im-c64/

Not sure what the EL manufacturer prefix is either, but Elektronpribor is a good guess.
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