The VIC-20 MIDI Cartridge is the result of one of our customers asking “could we?”.   A group of VIC-20 enthusiasts approached RETRO Innovations about producing a VIC-20 MIDI design found long ago in a MAPLIN magazine article.  The original design centered about a 6850 UART, a no longer-produced Motorola 68XX support IC, and was minimal in functionality.  To update the design for manufacturability, the 6850 was discarded in favor of a 8250-based UART, and an on-board ROM was added to the design to support serious musicians who preferred to run the VIC-20 headless and not bother with a disk drive and/or monitor.

Along the way, the design matured into a more general-purpose communications and memory expansion cartridge that includes the following features:

  • MIDI In, Out, and Through capability
  • RS-232 communications support
  • 512kB user programmable flash ROM
  • 128kB (expandable to 512kB) RAM

The increase in capability was further enhanced by supporting a flexible memory allocation mechanism denoted “Ultimem”.  This capability greatly increases the utility and configuration options of the on board memory.


  • Commodore VIC-20
  • A MIDI output device (the VIC-MIDI can output MIDI, but it’s primarily designed as a MIDI instrument)
  • A 5 pin DIN MIDI cable
  • VIC-MIDI software (download files vicmidi and vicmidi.prg. Load and run vicmidi from drive #8)
  • a MIDI file


VIC-MIDI can be purchased in our store.

Technical Details

VIC-MIDI supports the Ultimem registers at $9ff0 (40944), while the ST16c450 UART registers appear at $9800 (38912).  Ultimem registers and usage are described on the Ultimem product page, while the ST16C450 utilizes the industry-standard 8250 register set.  The UART implementation is straightforward, with only 2 specific notes:

  • The OPT1 bit (bit 2 of $9804/38916) should be set to 1 to enable the MIDI drivers and set to 0 to enable the RS232 drivers.
  • The crystal frequency of the UART is 18.432MHz.



Project Files

EAGLE v6.0 schematic and PCB design files, as well as a full bill of materials are available in the downloads folder.

Frequently Asked Questions