Retro Computing

Back when microprocessors were a new thing, I was fascinated by the Intel 4004 chipset, originally developed for the programmable calculator market. I got the manuals for it and wanted to build a system. But things progressed rapidly, and Intel released the 8008, quickly followed by the 8080. There was an explosion of DIY computer kits such as the Altair 8800, introduced in 1974. This was rapidly followed by the IMSAI 8080 in 1975, pretty much a clone of the Altair. These were popularized by construction articles published in magazines like Popular Electronics and Radio Electronics and largely started the personal computer craze. These were considered hobbyist kits since they required a lot of hands-on building and low-level programming. Similarly, Apple released the Apple I in 1976, based on the low-cost MOS 6502 chip. It was not until the release of the Apple II in 1977 that a "finished" computer was available to the personal computer market at a reasonable cost. This was joined almost simultaneously by Radio Shack when they launched the TRS-80 the same year, based on the new Zilog Z80 chip. Both the 6502 and the Z80 were cheaper and simpler to implement at the systems level.

In 1974, Motorola introduced the 6800 microprocessor. To me, it had a much more straightforward architecture than the Intel chips, which seemed to have been heavily influenced by the earlier 4004 instruction set (to this day, the X86 architecture is something of a hodgepodge, but that no longer matters because the hardware is buried in layers of software). In fact, the 6800 was heavily based on the Digital Equipment Corporation's PDP-11 minicomputer architecture, originally developed in the late 1960's. Since my business partner and I were looking to get into microprocessors for industrial control applications, we started looking at the 6800. Motorola published a huge applications manual that we obtained, and it was really helpful in learning how to use the microprocessor.

We acquired a development board to learn how to program at the assembly level. This was a Micro68 from Electronic Product Associates. It had 7-segment LEDs to indicate hexadecimal addresses and data and a hex keypad for data entry. There was a monitor program in ROM to control input and display. Programming this thing in hex assembly code was extremely tedious!


We set out to develop a set of boards to build 6800-based computers for industrial controls. Here is one of the original schematics for the CPU board I designed. These "blue line" drawings (as in blueprints) and all hand-drawn on large sheets of vellum paper.

The board had clock circuitry, bus drivers for the 8-bit I/O bus and address lines, and sockets for EPROM chips.

Ironically, no computers were used in the process of designing these computers! 

This is artwork I created to make photo mask images for PCB preparation, in this case the CPU module. These were hand-taped by me on mylar film at 4X actual size. We also created boards with additional memory, digital I/O and serial communications, including a modem for dial-up.

Sadly, our partnership ended before we could get into production with this series of computer parts, and I went off to work for Digital Equipment Corporation (DEC) as a support engineer. But I had learned a huge amount about microprocessors in the process. I knew there would be a solid career for me in computers, so this was a good move for me.

Once I got to DEC, I was immersed in minicomputers and large systems and did not have access to tiny systems to play with at home. But I still had that bug and when Byte magazine started publication in 1975, I became a charter subscriber to keep tabs on what was happening in the hobby. Around 1981 I acquired a TI 99/4A because it had a 16-bit processor! OK, I was a word-length snob and 16 bits were certainly better than 8 bits, right? It had built-in BASIC so you could type in programs directly on the screen (a TV was used as a monitor).

Was 16 bits important for a home computer? At that point in time, not really. 8-bit machines like the Apple II, the Radio Shack TRS-80, and lots of others were doing just fine with 8-bit processors, and it was really all about the software. So when Commodore released the C64 computer console (the 64 referred to the fact that it had 64 Kbytes of memory, quite a lot for its day), I sold the TI and bought one. The C64 became the largest-selling home computer of all time, so there were lots of software programs available. I just had the basic console, but when I relocated to DEC headquarters in 1984, the nice folks I worked with in Pennsylvania chipped in and got me a Commodore floppy drive as a going-away present to go with the console, making it a more complete system.

Like the TI, it had built-in BASIC as soon as it was booted up. Also, cartridges could be inserted in the rear, and it had ports for joysticks since gaming was a big deal back then.

I still have the computer and floppy drive, along with lots of software in a box, just waiting to be brought back to life. 

As the IBM PC became a de facto standard, everyone wanted a PC clone at home, so I built a system from parts (an Intel 286 motherboard, power supply, case, modem, etc.) acquired from a local guy and built a system to run MS Windows in the early 1990's. So the Commodore was relegated to the attic. Our kids grew up using Windows machines and since I worked for a computer company, newer hardware was not an issue (DEC had finally got around to making IBM compatibles, so I had access to them). Neither was software since I had an MSDN license courtesy of DEC and access to all the software Microsoft produced. Nowadays, we use MacBooks and iPhones at home because, well, they just work.

To bring things full circle, a couple of years ago I learned of a project to produce a kit that replicated the DEC PDP 11/70 from the 1970s. The project was documented here and brought to life by a guy named Oscar Vermeulen. Oscar had previously made a kit that emulated the DEC PDP 8 based on a Raspberry Pi running simulation software and it had become very popular. When he announced the PiDP-11 (the name incorporated Pi) I was first in line to order one. In my early days at DEC, I had cut my teeth on PDP 11s and the 11/70 was my favorite machine. This kit is a 2/3 scale replica of the PDP11 front panel, complete with working blinking lights and switches that function identically to the original.

The simulation software running in the PiDP-11 is called SimH (and on GitHub here), a brilliant piece of code created by Bob Supnik, who worked at DEC and was famously responsible for a VAX CMOS chip project that obsoleted the enormously expensive VAX 9000 mainframe project less than a year after its introduction (whole 'nother story). SimH is an open-source project that has been enhanced over the years by others to allow the emulation of all sorts of computers, mostly obsolete today (even the Altair).

In the case of the PiDP-11, there is a whole host of legacy operating systems that can be booted up. These include:

Each of these has programming languages, games and utility programs so it is an endless playground for retro computing idiots like me.

Oscar is currently (2023) working on releasing a kit that simulates the PDP 10, complete with blinking lights and switches. I am looking forward to getting one.

Here is a photo of his prototype - beautiful molded panel and very authentic looking - just 2/3 scale.