Modcomp Minicomputers

by Douglas W. Jones
THE UNIVERSITY OF IOWA Department of Computer Science

Translated to Romanian by Alexandra Seremina of

A Modcomp advertising poster from 1980 (author's collection). The very small text across the bottom of the poster gives Modcomp's business addresses and does not include a copyright notice.
 [photo of Simpler terminal]

Modcomp, of Fort Lauderdale Florida, began making 16 bit minicomputers in the early 1970's, targeted to compete with machines like the DEC PDP-11 and the HP 2115. In 1974, they came out with the Modcomp IV minicomputer, an upward compatable machine that included support for 32-bit integers, a 64-bit floating point unit and a memory management unit. The Modcomp IV came on the market slightly before the Dec Vax 11-780 and was, in some ways, a decent competitor. The Modcomp IV was followed around 1980 with the Modcomp Classic, which added support for 32-bit addressing to the architecture.

Ironically, the photo shown here includes no Modcomp products. The Plato V intelligent terminal is from CDC (it's an IST-II), and the output shown on that terminal is from the Simpler system, shortly after the University of Illinois spun off Global Information Systems Technology to market that software. Simpler was an implementation of the Tutor programming language from the Plato IV system at Illinois. Modcomp did have an interest in marketing Simpler because it ran on a Modcomp IV or a Modcomp Classic minicomputer. Later, Simpler would be renamed ACCORD.

Programmer's Reference Material

A programmer's pocket reference card for the Modcomp IV copyright 1976 by the University of California at San Francisco. This is a scan of a photocopy of the original. Bootleg copies of this card were widely used, as there was no official concise reference card for the machine.


Lyle Bickley has posted to the web a nice photo collection showing a Modcomp II system. As can be seen in these photos, Modcomp's minicomputers of the 1970 were constructed of wire-wrapped boards that were connected hinges along one edge and arranged to open like pages of a book. Interconnection between boards was done by ribbon cables along the spine edge of the book, so it could be opened to any board while the machine was running. A minimal Modcomp II computer required 4 boards, two for the CPU and one 32K word core memory plane. The machine shown in this photo collection has 64K words of memory, so it has 2 core planes.

While Modcomp's smaller computers were packaged with the spine of the book horizontal and opening upward, computers that required more boards were packaged with a vertical spine so that the pages opened outward from the front of a relay rack. This can be seen in the photo collection at DVQ; scroll down to the bottom of the page to see the Modcomp II photos. This machine is larger for two reasons: It has a floating point unit and the main memory is built of 4 16K memory modules instead of 2 32K modules. This implies that it is older, but there is no manufacturing date visible in the photos.