My IMSAI System


(My system right after I got it)

    I have an IMSAI 8080 machine in my collection as well. The IMSAI could be considered to be the first "clone" machine, in this case, of the Altair 8800. It was configured similarly (8080 CPU and 4k of SRAM) but it had a splashier front panel graphics and larger, bi-color paddle switches on the front panel. The power supply was larger and less electrically noisy, and the backplane was better designed than the Altair.

    My system didn't come with the stock 8080 CPU card installed (although it was provided). Rather, it came with the TDL ZPU Z80 CPU card (replacing the CPU card was a common upgrade as the Z80 became more prevalent as it was faster and had a more capable instruction superset). It came with 48k of installed memory in three Vandenberg 16k boards. The disk drive system was the iCOM "Frugal Floppy" system. The serial cars is a Solid State Music 2p-2s card.

    The picture above shows my IMSAI system (Serial #003526 from the San Leandro facility) on top of the iCOM Frugal Floppy system (Serial #1154). Based on my discussions with Thomas "Todd" Fischer, he places the manufacturing date of my IMSAI between March and June of 1976. I received this system and related parts in December 2005 and January 2006.

    Here are some manuals relating to my IMSAI machine:

            Core System Manuals

            Manuals relating to the hardware installed in my machine

            Digital Research CP/M Manuals that came with my system

            Other Manuals and Documents that came with my system

     CP/M and Microsoft Manuals



The Frugal Floppy system that came with the system is a bit flaky. Actually, it's not the system per se, but the Pertec FD400 floppy drives. So, I wanted to replace this floppy system with one based on 5.25" disk drives. Not only is it difficult finding a working controller capable of 5.25" disks but if you're lucky enough to find one, it's even harder to find a pre-made working boot disk.

In many circumstances, you can make an image of an 8" diskette using ImageDisk and take that image and write it onto a 5.25" high-density (1.2mb) diskette. The drive RPM (360) and transfer rate (500kbps) is the same as an 8" drive, and the 5.25" drive is capable of stepping 80 tracks, which is greater than the 77 tracks on 8" disks.

In advance, I owe a load of thanks to John Singleton, Herb Johnson, Mike Stein, Tony Duell, and many others on the ClassicCmp list for their assistance with this project.


Trying to Fix the Frugal Floppy System

The iCOM "Frugal Floppy" System is a nice dual-8" floppy system. The actual system is manufactured by Synetic Designs Company but its based on the iCOM FD360 controller-formatter and Pertec FD400 drives. These are the same drives used in the original Altair 88-DCDD. FD400 drives have direct-drive DC spindles and a soft-sectored geometry of 77 tracks containing 26, 128-byte sectors, for a total of 256,256 bytes.

The controller-formatter is a two-board, totally TTL (i.e., no LSI logic) controller which connects to the host through a 50-pin non-Shugart-compatible interface. This cable connects to a host interface card in the host machine. This card contains some glue logic, buffer RAM and an EPROM.

In this system, the first drive (the top one in the picture above) worked OK, and the system booted CP/M 1.4 with no problems at all. The bottom drive worked for a while when I first got it but suffers from CRC errors. There is a light on the drive cabinet that indicates CRC errors, and I get nothing but BDOS errors when accessing that drive.

Given this, there are a few procedures that I've done and still have yet to do:

Since I had no floppy drive system, I stopped using the IMSAI and moved to other projects.

In April, 2008, I came across a post on the Vintage Computer Forums ( which mentioned a software-based disk drive emulator called the Altair Peripheral Emulator ("APE") ( APE uses two pieces of software to emulate a CP/M compatible 4-drive floppy disk system. A Windows program runs on the host machine, which connects to an S-100 machine through a serial cable. A small boot loader is toggled into the S-100 machine which loads a secondary boot loader that does the heavy lifting to get CP/M booted.

I contacted Frank and he is very helpful and was able to create a custom floppy that matched my S-100 serial port configuration (I/O port location and status bits). It's a little slow because of the 9600-baud serial connection but absolutely very functional. If you have a fast enough (4MHz) S-100 machine and serial card, it is capable of running at 19,200 baud which should be more than adequate for daily use.

I usually use a Windows-based laptop as a terminal for my classic machines. For this setup, I use two USB-to-RS232 adapters connected to the two SIO ports on my IMSAI and I run but Hyperterm and APE on the laptop. Works like a charm. Thanks, Frank, for a great tool. This held me for a while until I decided that I needed to refocus on building a floppy system.

May, 2013: as a follow-up, I've pretty much given up on the iCOM floppy system. The 5.25" drives work well and back in February 2013, I picked up a Lobo dual-drive 8" floppy drive system which uses SA801R SSSD drives which can connect directly to the CompuPro Disk 1 controller. The CBIOS will need some changes, but that's it.

June, 2013: I've donated the entire iCOM drive system to the MARCH retro-computing group at InfoAge to which I belong. Although the drive isn't yet connected to MARCH's ComputerMart IMSAI, it's on display there in the Microcomputer Room.


Implementing a New Disk System

Since the iCOM system didn't work, I was kind of stuck. APE worked but was kind of slow because of the serial connection. With the help of another hobbyist (John Singleton) who had the exact same system and had gone through the same process, I was able to re-implement an 8" disk system from scratch using his CBIOS and method and 5.25" drives. I wrote a complete paper on this, available as a FAQ here (MS Word).

Essentially, this project required a few things:

Overall, the project took about two months, but that included acquiring parts and doing testing. Once I got going, it only took a couple of weeks to implement and test. Now, I have a complete and fully-working dual-drive system that uses 5.25" floppy drives to emulate 8" drives. I originally mounted my drives in an old DEC RX-180 external drive case -- convenient, but not IMSAI obviously. I subsequently upgraded the case to a slim-line dual-drive case that fits the drives better than the RX-180 case, which was designed for full-height drives.

In 2010 and 2014, I made a presentation at two MARCH (now Vintage Computing Federation) meetings about my experiences. In short, the process involves the following:

The presentation files from the meeting are here.



Other Projects for my IMSAI

Cromemco Dazzler

I pulled out one of the Cromemco Dazzler boards I have and see if I could get it to work on my IMSAI. Well, with some minor adjustments, it works just fine. Manuals and software for this graphics board is on my Altair32 site (Altair32 Docs on Classiccmp).


Cromemco D+7A

Over time, I've been able to acquire a few Cromemco D+7A analog/digital boards (pdf). This is a good board, containing 7 analog channels and 8 bits of digital I/O. It requires calibration, which is a bit time-consuming but doable. I put this in my IMSAI and ran a DE25F to the back of the chassis to connect to the JS-1 joystick (pdf) replica I built. Using the test software in the manual, it works reasonably well but the screen size is small and it's a bit touchy. I'd say "demo only" rather than daily use for the Dazzler games.



This is my first 2013 project. I pulled out a Cromemco Interfacer 1 (pdf) and configured both ports for use with the IMSAI. The first port is used as the LPT device under CP/M and the other as a modem port for MODEM37. This port connects to an old Hayes Smartmodem 300 which I use to connect to my demo Mystic BBS system. The printer port connects to a Practical Peripherals Microbuffer (pdf) which handles serial->parallel conversion.

I don't have any serial printers -- only Centronics parallel printers from Radio Shack and IBM. So, I purchased a Centronics 739-1 printer since it's reasonably period-appropriate (1978). I can't locate on-line any copies of a 739-1 manual, but Bill Dromgoole (of the MARCH list) supplied me with a scan of the 737-1 manual (pdf). On the surface, it appears to be the same except the paper feeding mechanism and the lack of a dip switch on the control board. Thanks Bill!

To digress a bit, there weren't many dot matrix printers advertised for sale in 1976 and those that were, were mostly over $1,000. So, if you had an IMSAI in 1976, you probably didn't have a printer unless you were able to get a cast-off DECWriter or a Teletype that you also used as the console.

By 1978, things were a bit better, with Centronics having several models but of course, with a parallel interface. OKI (80 or 132 columns), MPI (40-column receipt-style) and Ohio Scientific had "generic" dot matrix printers. There were of course serial Teletype machines like the ASR33 or 43 desktop model. The microcomputer brands Radio Shack had a parallel printer or two that one could use. The other brands (Apple, Commodore and later, Atari, Coleco and many others) had vendor-specific printers but few that used compatible interfaces.

By 1979, Diablo had the 1641 daisywheel printer ($2900!!), and Texas Instruments had the 810 dot matrix ($1599) as well. With these, you could put that spare parallel I/O port on your console board to use :-) Later OKI and Epson printers had serial interface board options for some of their printers.

The Centronics 739-1 (serial# 005769, purchased in June 2013 from a gentleman who worked for Centronics' sales and marketing group) is a compact 80-column tractor feed printer. It does not use a ribbon cartridge. Instead, it has a removable Mobius-loop ribbon called a "zip pack" ribbon. That makes it easy to replace (or now) re-ink. As a side note, the Atari 825 printer was a rebadged Centronics 737-1 with a custom ROM. I was able to find ribbons at Best Electronics ($6@, limit 2).

I'm still looking for a good re-inking formula. Some use WD-40 to reinvigorate old ribbons. I might try that first before trying to formulate an ink from stamp pad ink and sewing machine oil. I've also heard that black shoe polish might work. I've been contact with a few typewriter restorers about this topic...maybe I'll get the secret formula!

6/14/13:    I finished building the cable for the printer. It's actually pretty easy but you can't use a DB25 IDC connector on the buffer side because the pinning isn't right. The printer side is a 40-pin card edge connector (such as CW Industries CWR-170-40-0000). It's not as common as the 34-pin one used for floppy drives but it is available. Use 25-conductor ribbon cable and a DB25M connector with solder cups. Basically, all even pins (2-24) from the printer connect to ground. All odd pins connect sequentially to the DB25M (C40::DB25, as follows: 1-1, 3-2, 5-3, etc.). The remaining pins should remain unconnected, especially 35, 37 and 39 since they carry 5v and 17v, respectively, from the printer.

After making up the cable, I connected it to the buffer, fired up CP/M and printed a text file. Success!


GI Orator Speech Board

In mid-2012, I picked up a General Instruments Orator demonstration board (pdf). This board uses the SP0256-AL2 speech processor in a demo or "reference board" configuration. I haven't tried it with the IMSAI yet, but soon. I did some basic testing and it works so it's a matter of connecting it to the IMSAI.


Reimplementing an 8" Drive System

2016: I had some free time so I began restoring and implementing an 8" floppy drive system for the IMSAI. This builds heavily on the existing CBIOS and previous implementation work for the 5.25" system. Part of the game plan here is getting the system to read the 100 or so 8" disks that I have from the IMSAI. I'm not quite sure it will work, but I'm hopeful.

The system will continue to use the CompuPro Disk 1 controller with the Lobo drive system. The Lobo drive system is very simple -- a power supply and two Shugart SA-801R SSDD drives (Maintenance Manual; User's Guide). The drives are jumpered to force them to single-density to be compatible with the existing disks. Maybe eventually I'll make them double-density.

Changes this time involved:

Overall the CBIOS changes weren't too difficult. However, I discovered that one of the drives drops in and out of READY. Fortunately the other drive works well, so now I have a single-drive system. I was able to get another drive on eBay, so I'm hopeful I can get a second drive up and running and give me the chance to fix the other drive.

I've shelved this part of the project for a while in favor of building a Seattle Gazelle-replica system for an early MS-DOS project. That project is detailed here.



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Last Updated 02/12/2024 19:44 -0500