C64/C128 Diag Harness

C64/C128 Diagnostic Harness
C64/C128 Diag Harness

The C64/C128 Diag Harness harness is a full test harness to be used with a 586220 (c64) or 785260 (c128) diagnostic cartridge.

This test harness is based on Sven Peterson's design.


Our full harness kit comes with:

  • User Port Module
  • Cassette Port Module
  • Serial Loopback
  • 2 Controller Port Connectors
  • C64 Keyboard Dongle *
  • All Needed Cables

Every component comes in a 3D printed case to protect against ESD and comes with vinyl labels for proper identification.

* If you want to use this with a C128 diagnostic cartridge and want to test the C128's keyboard then you will require a keyboard dongle that is specific to the C128 line of machines (C128, C128D and C128DCR).


C128 Keyboard Dongle
C128 Keyboard Dongle

Our harness will work with the C128 line of computer but due to the keyboard connector being different on the C128 and C128D, you will require a different keyboard dongle. You can pick one up here.


When paired with the C64 Diag or the C128 Diag cartridge, the test harness provides the needed feedback to accurately test all of the I/O connections on your C64 or C128.

Connect all modules to their appropriate ports and then turn your Commodore 64/128 on with the appropriate cartridge plugged in.

Please refer to the original support manual below on how to diagnose any failures that might be reported by the diagnostic software.