SX 64 I/O board

Commodore SX 64 I/O Board

  • only two versions known (to me) 251106 Rev A and 251106 Rev 1, difference seems to be capacitor c9 that is missing in Rev 1.
  • contains the logic and two CIA’s that the SX uses to communicate to external equipment like drives, printers and modems (and internal drive)

Integrated Circuits

PCB Identifier Chip NumberRemarks Memory Location/bit
UC1 4066


Commodore SX 64 I/O Board Schematics 251107
Commodore SX 64 I/O Board Schematics 251107

Board Image

251106 REV A
251106 REV A
251106 REV 1
251106 REV 1

10 thoughts to “SX 64 I/O board”

  1. interesting about the C9 on the rev 1
    I just ran into that issue installing a switchless jiffydos board
    I think c= directly soldered it to the 556 on that version
    if you look at the 2 picks of UE3 v1 has a resistor soldered on as a afterthought and the other its on the board properly marked as C9

  2. Schematic lists:
    UC1 should be 4066
    UE2 should be 74LS74
    UB2 should be 6526A CIA
    UB3 should be 6526A CIA
    UD2 should be 7406
    UE2 should be 74LS74
    UE3 should be 556

    1. Dear Michael,

      Thank you for telling me,
      I listed the parts list of the floppy disk controller card instead of the I/O card.

      I fixed the error.



  3. HI,

    a CIA in my SX 64 is giving me modem problems.
    On the site of Ray Carlsen I found out I should replace CIA 2
    So I looked for the right location of this CIA on this site.

    Below “Integrated Circuits” it says:
    UB2 = CIA 1
    UB3 = CIA 2

    But on the Commodore SX 64 I/O Board Schematics 251107 on this page it says:
    UB2=CIA 2
    UB3=CIA 1

    One of them must be wrong?

  4. Hi Jeroen,

    Thanks for the remark about the userport problem.
    The userport was already “fixed” in the last century.


  5. I’ve been having trouble with my sx 64 not sending 5v to the user port at the back any ideas psu is fine no image display

    1. Dear Ross,

      the 5v of the user port comes direct from the connection of the I/O board to the CPU board.
      so if the 5v line is ok, then check (refit) the connection between those boards.

      it could also be a bad wire between the I/O board and connector Board, but that wouldn’t explain the black screen

      also check SX repair notes from Ray Carlsen for more help

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