The Mark-8 and Me

By Jon Titus
Inventor of the Mark-8

In September 2013, we featured a customer submitted story by Bob Bell about building his first Mark-8 computer. After reading the article in the Jameco newsletter, we were surprised and excited that Jon Titus, the inventor of the Mark-8, reached out to share his own story.

Making the Mark-8

In 1971, while in grad school at Virginia Tech, I had my first computer experience with a PDP-8/L minicomputer from Digital Equipment Corporation. It struck me at the time: I could design and build my own computer...and the idea for the Mark-8 was conceived. Basically, I wanted my own computer to work with at home.

When Intel came out with the 8008 microprocessor in 1972, the future looked brighter. The 8008 instruction set seemed powerful and the 8-bit processor would be practical for an engineer or student. I procured the 8008 manuals and began conceptualizing a design (today, we would call it a "reference design").

Instead of EPROMs, I opted for SRAM for my Mark-8 prototype. Some companies sold 1101 chips for just a few dollars each, so with my modest budget I assembled 768 bytes of storage. My next challenge was transferring the information into the 1101 SRAM chips.

I came up with the memory-loading and memory-examining approach and used switches for data and address loading, with LEDs as readouts. This design aspect would let hobbyists eliminate the need for EPROM chips because they could use the front-panel controls to store information directly in the SRAM chips. I bow to the PDP-8L front-panel light and switch design as my inspiration.

DiagramMark-8 Bus Connection Diagram by Jon Titus

I programmed the Mark-8 in assembly language only. I wrote code on paper and "assembled" a program by writing the appropriate 8-bit instruction and data on each line. After a while, I had memorized many octal codes and could quickly assemble a program on the fly. While most programmers preferred hexadecimal, I felt comfortable with the 3-bit "digits" I had used to program a PDP-8/L minicomputer.

Unlike my concepts for the computer architecture, my concept for the name "Mark-8" just popped into mind. I had approached Radio-Electronics magazine with the idea of a computer construction project, and when editor Larry Steckler pressed for a name, I had to respond with the first name that came into my head. The magazine covered the Mark-8 project for its cover story in July 1974.

Mark 8 MinicomputerThe Mark-8 on Display

After the Mark-8

I never had plans to develop the Mark-8 beyond the original design. I can't remember why I set the 4 Kbyte memory limit, but some people changed the design slightly to address all 16 Kbytes the 8008 could address.

I regret not designing the circuit boards with plated-through holes and edge fingers that would plug into a motherboard. I wanted to save cost and cutting these corners met that objective, but it caused some hobbyists problems. Also, I could have put more circuits on each circuit board, so the design would use fewer boards.

In retrospect, I should have abandoned the original Intel input-port design and used instead an open-collector or three-state bus for the input of data. That data could come from three sources: input ports, memory or an interrupt vector.

In 1975, I designed a successor – the Mark-80 – based on the Intel 8080 microprocessor. The Mark-80 used dual-width, dual-height Flip-Chip printed circuit boards. Digital Equipment Corporation had adopted the same size boards for some of their computers, and the size and density of the edge connector contacts seemed like a good fit for the Mark-80.

The Fruits of My Labor

As a grad student, I had no financial means to turn the Mark-8 into a commercial venture. I just wanted to show other hobbyists they could build their own computer too. I laid out the printed circuit boards by hand and a New Jersey company called Techniques sold a complete set for about $50. The Radio-Electronics article could not provide all the building details, so the publisher offered a manual that sold for $5. Some of the money I earned from these ventures went to buy an IBM Selectric typewriter, of all things.

Although the Mark-8 didn't make me rich, it forged many friendships over the years. People still tell me how their life changed when the Mark-8 got them interested in hobby computers and digital electronics.

Jon Titus has worked as a technical or contributing editor for several electronic-engineering magazines. He designs circuits and, these days, works mainly with microcontrollers. Jon lives in Herriman, UT and uses the ham radio call KZ1G.