The Exatron MM+

written by Matthew Reed

Exatron flyer mentioning the MM+

Exatron flyer from 1981 mentioning the MM+

The Exatron MM+, also known as the Exatron Memory Plus Expansion, was an alternative to the Radio Shack Expansion Interface for the TRS‑80 Model I. It was designed and sold by Exatron of Sunnyvale, California for use with their floppy disk storage alternative, the Exatron Stringy Floppy. Despite the fact that it was barely advertised and seems to have been only sold briefly in 1981, the Exatron MM+ remains one of the best remembered Expansion Interface alternatives1.

The Exatron MM+ was aimed at users of alternative Model I storage options, such as the Exatron Stringy Floppy, the Meca BETA-80, or the JPC Products TC-8. Unlike the Radio Shack Expansion Interface, the Exatron MM+ didn’t include a floppy disk controller. But it did add additional memory, a parallel port, and a serial port. Unlike the Microtek MT-32, another lightweight Expansion Interface alternative, the Exatron MM+ also extended the Model I expansion bus. This meant that peripherals that connected to the bus, such as the Stringy Floppy or the BETA-80, could still be attached to the MM+.

The Exatron MM+ measured 17″ by 7″ by 3″ and cost $399.00. It was only sold by Exatron fully assembled, unlike most other Expansion Interface alternatives, such as the Micromint Disk 80 or the Microtek MT-32, which were also sold as kits.

The Exatron MM+ included its own power supply and offered these features:

  • 32K of memory
  • a real-time-clock
  • a RS-232 serial port (output only)
  • a parallel printer port
  • a light pen port

The light pen port would work with any TRS‑80-compatible light pen that connected to the cassette port. According to a review in Creative Computing, the port was designed to work with the $19.95 Photopoint light pen sold by Micro Matrix of Pacifica, California.

Exatron appears to have discontinued the MM+ by early 1982 when they introduced another product, the MM-800 Internal Memory. The MM-800 was a less expensive alternative that installed inside the Model I case and provided a full 64K of memory.


  1. Two very favorable reviews in 80 Microcomputing and Creative Computing, the latter of which was republished in the book The Creative TRS‑80, probably account for much of that recognition. 

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