### Author Topic: Suspect resistor value  (Read 1623 times)

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#### @rt

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• Posts: 1000
##### Suspect resistor value
« on: August 01, 2016, 12:28:17 pm »
Hi Guys
I’ve reproduced this circuit schematic:

At first I assumed the schematic had an error, because the physical hardware has
a 220R resistor (rightmost in the photo), rather than the 2K2 resistor at the bottom of the schematic.
The buffer around it are tri-state buffers, and the 2K2 ? resistor holds the outputs
normally low if the buffers are switched high impedance between their inputs & outputs.

Now it seems to me the 220R value is harsh for a pulldown resistor.
If the outputs became high, the 220R seems a bit close to short to be in the logic circuit.
Am I correct in my thinking, and it should be 2K2 ?

#### tatus1969

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• Resistance is futile - We Are The Watt.
##### Re: Suspect resistor value
« Reply #1 on: August 01, 2016, 01:55:26 pm »
the 74LS125 has a max output high current of 2.6mA, which is by far exceeded by the 220R. You could measure the high-level voltage and check if it is above 2.0V.

2k2 seems by far more reasonable. Maybe they had timing problems, as they serially clock out data from the 74LS165. But I would rather solve that by software then...

Did you reverse-engineer the circuit? The schematic seems to be incorrect.
We Are The Watt - Resistance Is Futile!

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#### @rt

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##### Re: Suspect resistor value
« Reply #2 on: August 01, 2016, 02:14:10 pm »

Hi It wasn’t me that drew the schematic, but I did a prototype from it, except I used a 220R resistor, and it does work.

the 74LS125 has a max output high current of 2.6mA, which is by far exceeded by the 220R. You could measure the high-level voltage and check if it is above 2.0V.

2k2 seems by far more reasonable. Maybe they had timing problems, as they serially clock out data from the 74LS165. But I would rather solve that by software then...

Did you reverse-engineer the circuit? The schematic seems to be incorrect.

Smf