Mark wrote: ↑Wed Apr 17, 2024 1:04 am
So I checked this on my KCS board.
When there is no activity on the serial line (idle state after reset), the 7476 (U64) inputs at pins 2,4,16 are low. Pin 15 is high, and the output on pin 11 is 2400hz - this makes the 2nd half of the 7476 a divide by 2.
After I poke 64512,241 on my C4P the input pins 2,4,16 are high, pin 15 is 2400Hz and the output at pin 11 is 1200Hz - pin 15 going low blocks toggling half of every clock pulse making the output at pin 11 a divide by 4.
For a 600 board, the poke would be 61440,241 which sets a serial BREAK condition.
After looking at the
datasheet for 7476, I thought the difference could be a 7476 vs 7476A since the clocks trigger slightly differently, but swapping a 74LS76A into my KCS board showed no difference.
Since pins 3,7,8 are inputs and are pulled high, I wouldn't expect any activity on those pins.
Output tone generation only depends on the 4800hz clock, serial data, and U64
CA3130 (U66) is only used for input, as are U69, U63 etc.
Why is U64 part 1 not responding to TxData input correctly? Bad IC? Is the signal at pin 4 OK?
Mark, I have to say thanks for this information, it's allowed me to get the cassette save side of things working!
So I went back and double checked and noticed something I'd missed earlier. Pins 2,4 & 16 of U64 were not starting out low, but actually at about 4v. Everything else was as it should be at U64.
So I checked the output of the 6850 on pin 6, that was high as was the input to U18, pin 13. However the output from U18, pin 12, which should have been low having gone through the inverter, was actually 4v and when I ran POKE 61440,241, it went up to 5v.
This was reflected at pins 2,4 & 16 of U64.
I swapped out U18 for a replacement, and U18 pin 12 now correctly reflected what was happening at the 6850, being low at idle, as were 2,4 &16 at U64.
I tested the 74LS04 that came from U18 and it was dead. So the low on the bad U18 was 4v and the high was 5v, meaning there was never a proper transition between low and high, so although the data pulses were seen at 2,4 & 16 of U64, they were always high.
Now if I hook up to pin 9 or 7 of J2 and feed them into my speakers, I get an actual output when I save
Next I need to test the cassette input and see if that works now that I can save.