C64/C128 Diag Harness

Fig. 1: Full Diagnostic 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).*


Fig. 2: C128 Diag 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.