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Parastream’s IMSAI Gen 1 System Go back IMSAI Development System, Closed on Workbench

In honor of Parastream’s twentieth birthday, founder Robert Weatherford is finally realizing his dream to replace his original IMSAI 8080 machine. His original machine, which was a hand-fabricated clone from a real kit, was lost in the 1980s. Thanks to some hard work from members of the S-100 Computers group, building a 21st century replica is now possible. The stock photo on the right will be replaced with one from the actual machine once construction actually begins.

This system will be the home and development platform for older S-100 cards that existed before the IMSAI 1977 IMM extended address revolution, some of which eventually wound up in the IEEE 696 specification. It is an exact replica of the IMSAI 8080 chassis except that it is only 12 inches deep instead of 18. This was done to reduce the shipping weight and that very few users ever used all 22 slots of the original system. It will have:

  • 9-slot motherboard without terminations installed. A separate terminator card will be available to use when appropriate.
  • Switching power supplies in place of the PS-28 (which wouldn’t fit in this chassis anyway).
  • A 2022-release of the CP-A (front panel) clone with many updates to make it behave better electrically and a couple of functional updates.
  • An MPU-A reproduction.

There will be cosmetic changes to the original, mainly in the colors used. The chassis comes completely raw, so it must be primed and painted. While the original IMSAI colors are known (IBM blue and IBM gray), matching them with commonly-available paints will be challenging. The other challenge is the paddle switches. We don’t have enough of the original C&K switches to flesh out a CP-A. The NKK switches that are being made available through a special purchase don't exactly match the IMSAI colors. The red is darker, and the blue is way lighter. So given that we can’t match the switch colors and the cabinet match will be difficult, we decided on a deliberate color scheme “update” - black and gray. The red switches will be black and the blue switches will be gray. The cabinet top will be black with a similar crinkle finish to the original IMSAI. The escutcheon under the paddle switches will be a darker gray than the switches. The LEDs will still be red. Part of the inspiration for the color changes came from the paddle switches we were able to find from All they had in new old stock C&K were black and gray. When we ordered the black and gray switches, we also ordered a set of NKK paddles to compare against the C&K. The results of this comparison is available here.

To make the system complete, we need a modest (8 to 32K) RAM card and an SIO or equivalent serial card. An IMSAI RAM 4A would also be a nice addition.

The switching power supplies for the system are three separate enclosed units.

Supply Make and Model Current Dimensions
+8V Mean Well HRPG-150-7.5 20A 6.66 × 3.82 × 1.50
+16V Mean Well HRP-75-15 5A 5.55 × 3.86 × 1.50
−16V Mean Well HRP-75-15 5A 5.55 × 3.86 × 1.50

All supplies are adjustable via a trimpot near the terminal block. The +8V unit has a +5VSB output. It is used to bias an NPN transistor whose emitter is connected to ground and collector is connected to the three units’ remote control input. The power switch from the CP-A is connected between the transistor's base and emitter. This arrangement prevents the use of line voltage on the CP-A board.

The chassis fan is a 12VDC unit TODO: maybe we want to use a PWM fan controller? At least something adjustable.

The following is the timeline of the system’s initial construction in reverse chronological order. You can see what’s planned for the system once it’s complete here.


We have built the MPU-A from the PCB replica and the spare parts we had on hand. Testing will wait until the chassis and CP-A are built.


We have the chassis, motherboard PCB, CP-A PCB on order waiting for delivery. The CP-A switches and motherboard connectors have been ordered and are awaiting delivery. We have the MPU-A PCB and all parts ready. As soon as we get the CP-A BOM, we will order the necessary parts.

Future Plans

Once the system is complete, it will be used to bring up other Gen 1 cards in the queue.


The FM-1 was Robert’s first S-100 design, done in 1977. It had 3K of 1702A EPROM and 1K of 2102 RAM with power-on jump to the EPROM. It was intended as a bootstrap and monitor for a system without a front panel. A companion FM-2 was planned but never produced. The FM-2 was to add an assembler and debugger and the ability to single-step and breakpoint a program under test, using the FM-1’s jump circuit.

There were five prototypes produced. One was sold at a Byte Shop on consignment. Parastream will keep one. The other three are in various stages of completion and will be built and sold.


We need a PIC-8 for our system. If we cannot find one, we will wire-wrap one.


You can read up on the MPU-85 here. The original design had some shortcomings and will be evaluated in the Gen 1 system with static and dynamic RAM cards. There is one completed MPU-85 that we will be evaluating, and one bare board. The fate of these boards is unknown at this time.

There are plans for a second revision to the MPU-85. The Gen 1 and Gen 2 systems will be used to validate and debug the new design.


By the time the IEE 696 specification was released, there were almost no systems being built with front panels. We’d like to change that. This will be a complete replacement for the CP-A including new acrylic panels, escutcheon, and mounting hardware. There are many difficult challenges to creating a card like this. The development cost and the lack of demand at the time is undoubtedly why it hasn’t been done before. It will not be inexpensive either with around 30 paddle switches and more than 50 discrete LEDs.

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