RFC ATOM

I wanted to expand my collection with a microcomputer from the 1970s, but since these are rare collector's items, their availability and high price made it difficult for me to acquire one.

However, after a short online search, I discovered a small but very active online community that is keeping these machines alive and even developing them further within their hardware limitations.

+++This is an early prototype, there can be imperfections in the overall design.+++

About the development

I would like to specifically highlight s100computers.com, which contains a wealth of resources, including guides, technical documentation, circuit diagrams, Gerber files, and firmware—essentially everything I need to recreate a vintage machine.

After some thought, I chose the IMSAI 8080, a very popular microcomputer from the late 1970s, known for its compact size, affordability, and expandability. It was used primarily by small and medium-sized businesses (including Microsoft), the banking sector, scientific institutions, and even the military.

My main reason for choosing this machine was its beautiful and unique design. As a film enthusiast, I first saw the IMSAI in "WarGames", and as a kid, I thought it looked incredibly cool.

Technical Overview

The IMSAI 8080 is essentially a clone of the Altair 8800, with full compatibility. It is based on the S-100 bus, which allows for various expansion cards to be installed.

The front panel serves as a hardware status display and also allows manual programming using switches.
Initially, it was sold with an Intel 8080 processor card, which was Intel’s first major 8-bit CPU, running at 2 MHz.
The base configuration included 4 KB of RAM, but the processor could handle up to 64 KB.
An I/O card was necessary to connect the system to a terminal or teletype.
The machine originally had no operating system, but in 1976, it received support for CP/M, the predecessor of DOS.

Work Process:

During the preparation phase of the design, I had to consider the dimensions of the case, as the backplane, front panel, and expansion cards all needed to fit within it.

I didn’t want to deviate too much from the original IMSAI dimensions, but at the same time, I wasn’t aiming to create a precise 1:1 replica, as that would have required too much additional effort. Instead, I aimed for a compromise between authenticity and practicality.

Fortunately, a local IT enthusiast was selling two identical ATX cases, and when placed vertically, they were almost the same size as the IMSAI. So, I bought both as a base for my project.

Step 1: Designing the S-100 Backplane

Since I wanted to stay close to the original, I aimed to create a 21-slot backplane.

I used a relatively late revision of the S-100 circuit schematic, which connects the slots in a daisy-chain configuration. This resulted in a very simple backplane with just a few dozen electronic components.

I designed the PCB in Altium Designer, though KiCad would have been sufficient as well, given its better library for through-hole components.

The main challenge was the footprint of the S-100 edge connector, as very little information is available about its origin or exact specifications. Fortunately, a forum member found a technical drawing for me, which allowed me to manually create both the male and female footprints—although I wasn’t entirely sure if the pitch was correct.

Step 2: Sourcing the S-100 Connectors

S-100 (2x50) connectors are incredibly expensive (~$16 per piece) from UK and US electronics distributors, which almost discouraged me from continuing the project.

However, after checking Alibaba.com, I was pleasantly surprised to find that some Chinese manufacturers still produce these nearly 60-year-old connectors in small quantities—for just $4 per piece.

After a quick email exchange, I ordered 20 pieces from a Chinese supplier.

Step 3: Manufacturing & Assembly

By the time my package arrived, I had already sent the Gerber files to Nyákáruház for PCB fabrication.

When the two-layer backplanes arrived, I was pleased to find that the manually designed connector footprints fit perfectly—confirming that my measurements were accurate.

The only thing left was to solder everything by hand.

Since the PCBs had around 1,300 solder joints, I decided to invest in a Weller SMD soldering station, which turned out to be a fantastic upgrade—a pleasure to use compared to my old Polish soldering iron.

Before starting, I had ordered five large spools of leaded solder from China, after reading about its many advantages.

Post-Soldering Testing

Continuity Testing
- I used a continuity tester to check the circuits, and thankfully, I found no manufacturing defects.
Power Rail Testing
- I tested the power rails using a laboratory power supply.
- The system consists of two 8V and two 16V power circuits, and everything worked perfectly.

Designing and Building the Front Panel

The next step was designing and fabricating the front panel, which presented some challenges.

In the original IMSAI design, the flip-flop switches were soldered directly onto the PCB. Over time, due to years of use or excessive force, they often ripped out of the board, causing irreparable damage.

To prevent this issue, I made two key modifications:

Replaced the switches with 3x1 headers to allow for a modular connection.
Designed a sturdy aluminum mounting bracket to hold the switches securely.

Sourcing the Rare Rocker Switches

Before designing the PCB, I first needed to source the custom switches.

These rocker switches are extremely difficult to find and very expensive to import from the U.S..
Since I previously had success with Alibaba.com, I gave it another shot.
After several hours of searching, I finally found one Chinese manufacturer willing to produce a custom batch of these rare switches.

These switches came in four configurations, available in red and blue:

ON-NONE-ON
ON-OFF-ON
(ON)-OFF-(ON)
ON-NONE-OFF

Designing and Assembling the Front Panel PCB

I designed the front panel PCB using Altium Designer, based on the original schematic diagram. However, I made one key modification:

I added an unpopulated S-100 connector in the top corner for diagnostic purposes.

The design process was more complex this time, as the board included logic ICs. However, I applied my trusted routing strategy—directing all traces toward the edges of the PCB to keep things organized and manageable.

Despite the increased complexity, I completed the design over a weekend. However, due to the large board size, I sent it to JLCPCB in China for manufacturing.

Component Sourcing & Soldering

I ordered the ICs, DIP sockets, and all other BOM components from AliExpress at a reasonable price.
Once the board arrived, I soldered all components.
A continuity test showed no issues.
The board performed flawlessly under power-on testing.

Building the Front Panel Switch Mounting Bracket

The next step was creating the mounting bracket for the front panel switches.

Designed it in Fusion 360.
Imported the 3D models of the panel and case (which I found on GrabCAD).
Adjusted the standoffs and cutouts to match the panel and chassis dimensions.

Milling the Front Panel Switch Bracket

At the local metal supply store, I purchased a 1000x2000mm, 16mm thick aluminum sheet. To fit my CNC router’s work area, I had it cut into eight 50x50cm pieces on-site.

I had specifically purchased the entry-level TwoTrees TTC-450 desktop CNC and engraving machine for tasks like this.

CNC Setup & Initial Tests

I exported the G-code for each machining step from Fusion 360, as this CNC requires manual tool changes, and I needed to preserve the starting point during each swap.

First Attempt & Adjustments

I initially used a 6mm end mill with a 500W spindle, but the results were not great.
After further experimentation, I optimized the settings:
- End Mill: 3mm, 4-flute
- Feed Rate: 100 mm/min
- Spindle Speed: 10,000 RPM
- Coolant: Continuous WD-40 spray

With these settings, I was able to cut the 16mm aluminum cleanly using the small spindle motor

Planning the Processor Card

The processor card design was one of the most challenging parts of the project.

The schematic contained numerous logic ICs, along with the 8080 processor, UART, and memory chips.

This is a modern all-in-one card, with the following specifications:

Intel 8080 CPU @ 2 MHz
SD card reader
EEPROM bootloader chip
64KB RAM
RS-232 UART
CP/M operating system compatibility

Designing the Processor Card in Altium

The first step was creating a 2x50-position S-100 edge connector in Altium, as this processor card is inserted into the first slot of the backplane.

Following the schematic diagram, I placed the components on the pre-dimensioned PCB in a logical arrangement:

Left: UART chips
Center: CPU and RAM
Right: More UART chips
Scattered throughout: Logic ICs, mostly serving as chip selectors and signal amplifiers

The editing process took nearly two weeks since I had to fully optimize space usage on the PCB.

Once completed, I sent the Gerber files for manufacturing.

Testing & Debugging

Continuity testing revealed one issue:
- I had forgotten to connect a ground trace.
- This was easily fixed by soldering a small wire.

Prototyping: Painting & Assembly

The next step was painting and assembling the prototype machine.

All fittings aligned perfectly, thanks to precise design work.
The next phase will involve installing the remaining components and proceeding to the testing stage.