Commodore 64/128 Diagnostic Harness

$5.00$49.00 USD

This diagnostic harness will help make sure that your Commodore 64 or 128 is performing up to spec. To perform a diagnostic, the kit must be used along with a diagnostic cartridge. (The BackBit Cartridge can be used for this purpose).

The Base Harness Kit can be used to diagnose a C64 or C64C. You can add options to support additional systems, like the SX64, C128, or C128D.

SKU: HARN Category:


Here’s what’s included in the base kit:

  • User port loopback plug (in easy-to-insert angled case);
  • Cassette port loopback plug (in easy-to-insert angled case);
  • Joystick switching adapter (w/ case);
  • 3.5mm wind-up cable to go between cassette plug and joystick adapter;
  • Serial port loopback plug;
  • Keyboard loopback plug for C64.

If you additionally want to run diagnostics on more machines than just the Commodore 64, you’ll need:

  • SX64: Add SX64 keyboard dongle; also, 3.3K resistor must be added as shown here to compensate for having no cassette port;
  • Commodore 128: Add C128/D keyboard dongle (has switch for boot-up while inserted);
  • Commodore 128D: Add C128/D keyboard dongle, and Cassxtender™ cassette port extender.


  • No messy wires or multitudes of cables;
  • Fully supported by Commodore 586220/785260 diagnostic software;
  • Cases made of sturdy 3D-printed ABS.

Does not include BackBit cartridge which supports diagnostic software (available for purchase separately).

Additional information

Weight 6 oz
Dimensions 1 × 1 × 1 in
Kit Options

Base Harness Kit for C64, C128/D Keyboard Dongle, Cassxtender™ Cassette Port Extender (for C128D), SX64 Keyboard Dongle


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