| Electronics > Beginners |
| Replacing a 74ls14 with a 74ls04 |
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| andybarrett1:
Hi Pic enclosed There is a 1Ok pull up on the output and no diode across the 100R resistor Thanks for looking Andy |
| capt bullshot:
That circuit should work with any kind of 'HC14 or 'HC04 (CMOS) chip. A "normal" LS or Standard TTL will pull up its input (on its own), even though there's a 10k resistor to ground. So the output will be stuck low. An LS14 (or Standard '14) might be somewhat different regarding its input structure, so then the 10k is enough to pull the input down. A Standard or LS TTL input does source current from +5V, so the "open" state is "high". You'd need a rather low resistor (< 1k) to pull such an input low. The Schmitt-Trigger inputs of the '14 apparently are different, one would have to look up the datasheet. Modern chips ('HC basically all CMOS logic) behave different, they don't sink or source current from their inputs, so an open CMOS input is "undefined" and you must use a pull-up or pull-down (10k ... 47k ballpark). |
| David Hess:
If you are going to use that configuration with TTL, then the switch should pull the input to ground instead of Vcc because of the TTL gate's asymmetrical input bias current. |
| andybarrett1:
Very Helpful this is what I was missing :-) |
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