JRC-65 Computer

1. Introduction and Design Objectives
2. Clock Circuit

Tech Info

• IMSAI 8080 Replica
• Greaseweazle Floppy Controller
• Debian on an Asus Eee
• Gentoo on an Asus Eee
• Virtual Network Test Environment

NerdWorld Computer Museum

• Fuse Box circa 1910
• DEC PDP-8/I Replica
• Osborne 1
• TRS-80 Model 100
• Apple IIe
• IBM PC 5150
• Sharp EL-5400 Pocket Computer
• HP 95LX Palmtop Computer
• HP Vectra VEi7 & OS/2 Warp
• IBM 340 & Windows 95
• Compaq Evo N400c & Windows 2000
• Eee PC Netbook

Essays

• Robert A. Heinlein: What I Believe
• Global Warming Fails the Smell Test
• The Speed of Light and Time Dilation
• Nattering Nabobs of Negativity
• Lessons of the Civil War

Links

• Interesting Links I use Regularly
• My Lean Manufacturing web site

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IMSAI 8080 Replica
& VT132 Terminal

May 6, 2025

The IMSAI 8080 was one of the first "Personal Computers" that actually looked like a computer. Released in 1975 in both kit and assembled versions, this system had that marvelous front panel loaded with LEDs and toggle switches.

This is the computer that was immortalized in the 1983 movie WarGames with Matthew Broderick and Ally Sheedy. Unfortunately, for some reason they obscured the prominant IMSAI logo on the front panel. Also left unexplained was how a twerpy high school kid had an IMSAI, dual 8-inch floppy drives, keyboard, monitor, and acoustic-coupled phone, plus his own phone number in his bedroom. (His parents must have been extremely wealthy.)

Real IMSAI's are still available, but they are expensive, huge, and rare, so I opted to buy and build the replicas offered by The High Nibble. Designed and sold by Dave McNaughton from Australia, it includes an excellent design and full documentation. Dave has really matched the look of the real IMSAI. And unlike the PiDP8, this is a full size replica, except that it's only about 3" deep. (We don't need all that S-100 buss and transformer-laden linear power supply stuff anymore.)

And instead of a wimpy 8080 running at a glacial 2 MHz, this system uses an ESP32 (complete with WiFi) for it's brains.

The real IMSAI could theoretically be operated from the front panel, assuming you met these criteria:

• You knew 8080 Assembly language and could hand-compile it to octal or hex data
• You had plenty of time (and patience!)
• You had severe OCD and could do EXTREMELY tedious work for hours at a time

For the rest of us "normal nerds", you needed a serial terminal. No graphics for this, of course, just a keyboard and a CRT display.

This IMSAI replica can be operated via its fantastic web interface over it's WiFI interface, an ability never dreamed of for the original IMSAI. Alternnately, you can use one of the 2 standard RS-232 ports on the back panel. I opted for some realism and so wanted to go serial. I could have used a program like minicom and utilized one of my old laptops as a serial terminal, but I wanted to be more authentic. I therefore bought a VT132 serial terminal from The High Nibble. The VT132 serial terminal comes in 2 versions: 1) a board for the RC2014 Z80 Computer; and 2) a stand-alone terminal. I chose the stand-alone version. I plugged in a USB keyboard to the VT132, and then connected to the IMSAI with an RS-232 cable.

Building and Using the VT132 Terminal

The stand-alone VT132 is a pretty simple kit. It consists of a single small circuit board with an ESP32-WROVER module, and several RS-232 connectors. It emulates the magnificent DEC VT100 terminal. (The DEC VT100 is a piece of hardware I'd LOVE to get my hands on. I have VERY fond memories of this terminal.) The VT132 even does the delightful "smooth scrolling" that I loved so much on the DEC terminal. Actually, however, the scrolling is not perfectly smooth on the VT132 and does not quite match the scrolling quality of the DEC VT100.

Even though this kit is small and relatively simple, there were a few things which gave me some challenges. These were:

• The ESP32 is surface-mounted, which made soldering it to the circuit board challenging. My shaky hands were a real obstacle here. Luckily, I watched several Youtube videos on surface mounts, used copious amounts of flux, and utlized a vacuum de-solderer serveral times, so my system worked fine when I powered it up.
• Unlike Dave's IMSAI 8080 kit, there are no step-by-step instructions for the VT132, and the guidance given by The High Nibble's web site was a little chaotic. I got it built, but would have appreciated more info. I presume that Dave McNaughton assumed that anyone who wanted one of these would probably have the skills to put one together. For me, that was true, but still...

Also confusing was "which RS-232 connectors go to which ports on the VT132" question. The VT132 is designed to hook up to a computer and/or a modem. Unfortunately, the RS-232 "standard" is inconsistently applied in the wild, so whether you use a male or female connector, or a straight-through or null-Modem cable is up to you. Luckily, the VT132 has jumpers so you can make any cable work for either port. I eMailed Dave for guidance, and he quickly replied with his recommendations. I followed his suggestion, and did not have any signifigant troubles hooking the VT132 to the IMSAI. (I won't say I had zero trouble because RS-232 always seems to confuse me. [sigh])

Dave sells this system with 2 options for hooking up a keyboard. You can get it with a standard PS/2 connector, or a USB connector. I elected to use the USB, since that seems much more common these days.

Once I had built and tested the terminal, I bolted it to the back of an old LCD monitor, added a power switch, and got it ready to connect to my IMSAI. I selected 9600 Baud connection speed, since that was about the fastest an original IMSAI could do in 1975. I also selected "smooth scrolling" for the output.

Building and Using the IMSAI 8080 Replica

The IMSAI 8080 replica is a marvelous kit. The first thing you notice when it arrives is how well all the components are packaged. Small things like resistors, LEDs, etc are in an anti-static bag. Other components are securely placed in die-cut cardboard spaces. The shipping box, which is a custom size, also holds the finished replica if you want to store it after assembly. Dave is apparently an electrical engieer, but he also has excellent Industrial Engineering and Poke-Yoke (Error-proofing) skills.

Unlike the VT132, there are detailed, step-by-step asembly instructions for putting together the replica. Dave has also made a series of assembly videos that are on youtube. (Look for the tag "THN" to find his official videos. There are also tons of other assembly videos available.) My only quibble is that Dave has improved the kit over the years (For example: He removed the need for surface-mounted components), and the assembly instructions can sometimes get confusing because of this. The instructions reference these changes, but then still leave the original instructions in place also. This makes sense so that the user can better understand all the available instruction materials, but it does make things a little more complex.

This replica is a wonderful recreation of the original IMSAI. The switches and LEDs function as they did on the original, and they look fantastic. The entire front panel looked very faithful to the original IMSAI (at least as far as I could tell; I've never seen the real thing in person). My color-blind eyes couldn't tell if the colors were correct, but they sure looked good to me. The switches were solid and appeared to be custom made. Likewise, the graphics on the front panel were gorgeous. The metal "enclosure" was beautiful and solid.

Electrically, the kit went together easily. The weakest part was the mechanical assembly of the kit. Even though all the parts are VERY well made (laser cut to high precision), I still had some trouble during final assembly. It took a lot of fiddling to get all the plastic pieces from the front panel to line up with all the screws and spacers. (This difficulty is acknowledged in the instructions.) It looks to me that Dave stayed true to the original front panel construction, including the poor choices that were made by IMSAI in 1975. Actually, it wasn't too bad; it just took some finesse and a small amount of cussing to get it all together. The worst aspect of mechanical assembly was the clear plastic rear panel and how it was attached to the metal enclosure. The slots in the plastic that hold the nuts were clunky, and prone to breakage. But they were still effective, and held up if you were gentle with them. Dave has added reinforcements to those slots, but a redesign of the whole rear panel attachment would probably be a good thing. Still, these are quibbles. Dave did an excellent job on the kit.

Oh, One other quibble. The USB power socket is butted up against the inside of the enclosure. You can squeeze in a short USB-C power plug, but a right-angle plug adapter is really needed to properly fit a power plug without stressing the plug.

I had almost no trouble putting this kit together. All the testing steps went well, and I had a functioning IMSAI 8080 in no time. I had a little trouble understanding all the capabilities of the Web interface provided, but mostly that was just me getting used to the IMSAI and the CP/M environment.

Unlike the real IMSAI which included the huge S-100 bus and a massive linear power supply, the replica is slim, only a few inches deep. Since the ESP32 replaces the CPU, RAM, I/O, and Disk Drives, most of the replica is actually just the front panel. The thinner form factor makes it a lot easier to put it on a desk or shelf, so it is a welcome change. The original IMSAI weighed about 50 pounds. This replica is only about 3 pounds. Nice!

Interestingly, the cost of the replica is less than the cost of the full kit in 1975. The basic IMSAI itself, with only minmal RAM, was $439 if purchased as a kit. (Equivalent to $2,700 in 2025) The assembled version was $621, equivalent to $3,800 today. Of course, at a minimum you had to add a serial terminal and a cassette interface to make it work. This makes it an expensive purchase for a hobbiest in 1975. (This explains why I never owned one, but does make one question how Matthew Broderick was able to afford his setup in WarGames.)

Because the CPU and system are emulated in the ESP32, you have lots of options to how the replica runs. You can select either the original 8080 or an enhanced Z-80 CPU. You can also select the original 2 MHz, or the later 4MHz clock. I chose the original 8080 at 2 MHz.

Running CP/M on this thing is a wonder. Dave has included CP/M and a host of games and utilties. I LOVED watching the LEDs flash as programs run. This is how a "real" computer should look!

Brief "Blink'n Lights" video.

IMSAI 8080 & VT132 in use.