Suspect resistor value

[@rt]

Hi Guys
I’ve reproduced this circuit schematic:
http://gerdkautzmann.de/cd32gamepad/a100schematic.png

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]

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.

[@rt]

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.