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Thread: ANNOUNCE: VCF East XIII 8085 Single Board Computer

  1. #41

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    Just finished building mine. This is a really nice kit. The board well designed and is clearly marked so you can almost build it without the directions.

    I did run into one odd problem with the serial clock. The oscillator is putting a clean signal into the 74HCT4040 counter, but the output has spikes that are making the baud rate 3x what it should be. See the attached traces. This is Q0, which should be clk/2. The bottom (yellow) trace is the clock input and the blue is the Q0 output.
    Q0-NewFile9.png
    At the Q1 output, it should be at clk/4, so there should be 6 edges out for every 24 in, but instead there are 18 out. Even with the asymmetric clock, it is reliable at lower rates, so if the board is jumpered for 1200 and the terminal set for 3600 it works flawlessly.
    Q1-NewFile7.png
    Does anyone who is better at hardware than me (which is everyone), have an idea of what might make it behave like this? I resoldered all of the connections to the 4040 and its bypass cap with no change. Bad chip or something else to check?

  2. #42

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    Replying to my own post with the solution from Glitch. Replacing the original 74HCT04 with a 74LS04 fixes the clock problem. The newer boards already ship with the LS part so they do not have an issue.

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