Computer History 101 & the Mark-8 Minicomputer

The Mark-8 Minicomputer was the third microprocessor based computer ever made available to the public, second in America, and the first to grace the cover of a popular magazine. Arguably, the first widely known. Here's my short version of computer history.

Although many computers were built prior to the advent of the microprocessor, the cost of the individual components and the cost of labor required to build them was tremendous, ranging from hundreds of thousands to millions of dollars. One of the first "affordable" minicomputers was the PDP-8, a 12 bit computer made by Digital Equipment Corporation in 1965. It was affordable, at about $18,000. That's about $117,000 in today's dollars! (Mmmm... Affordable!) The idea of having a computer at home was little more than science fiction to most, but in the 1970's, things began to change...

In January of 1971, Federico Faggin completed the worlds first microprocessor, the 4004. This was a Harvard architecture computer, with 4 bit bytes. Good for a calculator, not so good as a general purpose home computer. About the same time that work began on the 4004, an 8-bit CPU-on-a-chip was commissioned by a company known as Computer Terminal Corporation (CTI), to both Texas Instruments and Intel. TI failed to make a functional chip, and Intel was only successful after the 4004 was completed, and Federico was put on the task. The 8008 was released in 1972, and became the worlds first 8-bit microprocessor.

Also in 1973, Jon Titus, a graduate student at Virginia Tech working on his PhD, designed the Mark-8 out of an interest in computer control and data acquisition. Using the Intel data book on the 8008 as a basis, he designed the Mark-8 to suit his own needs. His unique front panel design made the computer fairly simple to understand and program. The prototype was built using Veroboard, with a total of 768 bytes of RAM. Jon recalls that it worked the first time he powered it up--at about 2:30 AM, no date mentioned, but presumably mid-late1973.

Radio Electronic Magazine

The published article in Radio Electronics magazine was interesting but sparse. It described the basic operation of the machine with an explanation of the operation of each of the boards, and offered some programming examples. The article also contained a parts list so those interested could start hunting for parts. Clever, since the Mark 8 was not offered as a complete kit-- this was a real do-it-yourself project! The only thing offered was a board set, scrounging of parts was entirely up to the builder. 

If that didn't scare you off, and you still wanted to build a Mark-8 computer, you had to order a set of plans from the publisher. This was played as a plus, since "interested readers" would get "full details on the computer in a single package." As opposed to offering the construction plans in the magazine pages over a period of several months. This made counting the possible number of Mark-8's builders a little easier.

Mr. Titus estimated from tax records that about 7500 plans were sold, and about four hundred PC board sets. Still, estimates of the number of Mark-8's actually built varies widely. Some have estimated the original number built at between 1000-2000. I think those numbers sound very high for two reasons: 

First, the Mark-8 was a very complicated machine from a hobbyist standpoint, and required of the builder a skillful ability to read and follow schematics in order to assemble and connect all the boards (especially since connection information was not included with the plans!). Further, the circuit boards did not have plated-though holes. This required builders to solder numerous connections on both sides of each board. Their were also a few unreported errors on the PC boards that required modifications to correct.
 

This proved too challenging a project for many builders. Many who have emailed me to tell their Mark 8 stories, never got their machines to run, or never finished. I suspect that a lot of Mark 8 computers were abandoned, half finished, or cannibalized for parts, and I suspect that most of the plans sold never went far beyond a good first reading. Too bad, because the Mark 8 is a soundly designed and beautiful machine. 

The article and the Mark-8 computer lit a fire in the hobbyist world. Clubs and newsletters were spawned, mostly to exchange software, construction tips, and troubleshooting hints. One reason Popular Electronics rejected Jon Titus and the Mark-8 was that they already had a microprocessor based project on the burner. According to Forrest Mims, they had envisioned a microprocessor "demonstrator" project of sorts, but when PE caught wind of the growing Mark-8 revolution, they were eager to trump RE. They dumped the lame demonstrator idea, and in January of 1975 they stole the crest of the wave with the introduction of the Altair 8800. Bill Gates saw the Altair on the cover, and we were off and running.