The Micromint Disk-80 Expansion Interface

written by Matthew Reed

Disk-80 advertisement

Micromint advertisement from the August 1981 issue of 80 Microcomputing

The Disk-80 was an alternative to the Radio Shack Expansion Interface that was sold by Micromint Inc. It was designed by Steve Ciarcia and was featured in his “Ciarcia’s Circuit Cellar” column in the March 1981 issue of BYTE.

The Disk-80 was sold in several configurations:

  • a bare printed circuit board for $48.00
  • a complete kit (with printer port) for $275.00
  • assembled and tested (also with printer port) for $380.00.

Although completely compatible with the Radio Shack Expansion Interface, the Disk-80 was smaller than other Expansion Interface replacements. It used a “low profile case” that was only 2½″ by 9¼″ by 8½″ and weighed only 3 lbs.

The Disk-80 offered the same features as most other Expansion Interface alternatives:

  • sockets for 32K of extra memory (for a total of 48K)
  • a floppy disk controller with support for up to four floppy drives
  • a printer port (originally optional but later standard)
  • a heartbeat counter for measuring time

Unlike most other Expansion Interfaces, the Disk-80 didn’t offer a port for a second cassette recorder (a rarely used feature) or a RS-232 port. The Comm-80, a separate Micromint product, did offer a RS-232 option.

The main appeal of the Disk-80 was its complete reliability. Unlike the Radio Shack Expansion Interface, the Disk-80 could just be plugged in and forgotten about. This reliability resulted from a very well thought out design, which featured a number of good ideas:

  • fully buffered address and data buses.
  • CMOS chips to help reduce power usage to eight watts
  • a built-in data separator to ensure reliable floppy transfers
  • a floppy cabling system that required no termination packs
  • a small sized board to help reduce electrical noise

The Disk-80 documentation included full schematics and anyone who wanted more information about how it worked could refer to the BYTE article.

Probably the best summary of the Disk-80 was written by Dennis Báthory-Kitsz in his review in 80 Microcomputing:

Reviewing Disk-80 is almost incongruous, because any comments can be summarized with the sentence, “It works.”

Micromint continued to advertise the Disk-80, along with their other Model I products, until early 1984.