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25021FD 1771 - Floppy Drive Controller TAX?

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  • joshbensadon
    Apr 3, 2012
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      In my continuing quest to get this Tarbell controller running.

      I found out that if 2 data pulses are recieved between the clock pulses, I then get a pattern of AA (or 55) read from the chip.
      It's almost like the Clock and Data pin inputs on the FD 1771 are reversed???? But the data sheet show them correct to the schematic which matches the board.

      Does anyone know if there was ever some ECO on this chip or the Tarbell 1011 Controller?

      I've got some NOS 1771's on their way to me, gonna try them.

      I've scoped the data and clock. They are both 250nSec pulses, a little ringing but otherwise reasonablly square. The ringing might be my cheap probes? They are between 0V and 3.9V, I changed the driver chip from 74LS08 to 74HCT08 and brought the voltage to 0V-4.75V but this had no effect. According to the datasheet, this is all good.

      Does anyone have some more details of what's happening just inside the Data/Clock inputs of the 1771?

      Cheers,
      Josh
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