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Thread: IMSAI 8080 noob, how to use?

  1. #1

    Default IMSAI 8080 noob, how to use?

    Hello all!

    I've owned an IMSAI 8080 for a number of years now -- but I have never quite figured out how to use it / get it working.

    My system has 6 x 8K boards in it, that seem to be addressed properly.

    There is a Northstar Floppy controller which I'm ignoring for now, it's hard sectored disks. I did buy some widget that is supposed to emulate the hard sectoring but haven't assembled or touched it yet.

    There is a video output board. Processor Technology VDM-1. I have tried it with and without it

    There is a serial IO card that also includes a monitor ROM. It also has a cassette interface as well.

    I have hooked a terminal up to it, and from the manual for the serial IO card it says to execute at C800. I have done this, and the lights on the front panel start to run. Code is executing.

    But I never see any serial output on the terminal. I'm supposed to get a prompt.

    I've replaced the line driver chips after the UART, but no difference. I've watched serial output with an oscope and I don't think anything comes out of the serial port.

    I walked thru examining the first bytes of the monitor ROM versus the program listing in it's manual and it looked good, so I am able to see that.

    Any thoughts as to what next? I pulled out the card that has the composite video output. I hooked up a monitor to it in the past and don't remember seeing a prompt there. The system came complete to me, but no floppy disks.

    Any pointers appreciated. I'm a S-100 newbie, and it's been kind of a long journey piecing together how this is supposed to work.

  2. #2


    The monitor ROM should show what is needed to connect a serial board. It should also have notes on what is needed for the N* board. You'd most likely need to use a 5.25 inch floppy drive with a direct drive. The belt drive ones don't maintain enough consistent speed to work with most hard sectored adapters. Of course, it is possible to make hard sectored disk from soft sectored. You'll still need 360K disk and drive. You can not use 1.2M drive or 1.2M disk easily.
    Most people implemented CP/M. There are books on setting up CP/M on a machine and most of the *.COM files you'd need are on the web. You'd likely have to write your own BIOS level code. Many SOL users had CP/M on their machines using the N* controller. One might look for them to see what can be had.
    First it would be useful to get the serial to work. I recommend getting one of the RS232 light boxes to figure why your serial isn't working. I have no idea what monitor you have and what it was set up for.

  3. #3


    Oooh serial comms, that old chestnut. Use the 'scope to determine where the signal breaks - tracing forwards from the UART with reference to the schematic. Maybe it is set up for hardware handshake? What's the BAUD rate (etc)? Do you have the manual? I'd start by pulling as many cards as possible to leave a minimal configuration, then try to get the monitor talking to the serial port.

    Did you observe output from the UART before replacing the line drivers? You could use a USB RS232 adapter like this one to connect direct to the UART input / output pins if the drivers are suspect.

  4. #4


    Thanks both for the information!

    I finally got my other laptop to dig up the info. It's a National Multiplex / Computer Aid 2SIO.
    Glitch hooked me up with the manual a while ago, props to him:

    I need to probe at it some more. One issue is I can't really probe the card with it in system as there isn't enough room. I can pull out the video board (I hooked up to it before and never saw output, although I did just look at the manual for it yesterday online.)

    I have some 5V TTL Serial dongles and such for my micro-controller stuff. I will prod at it again.

    IIRC I didn't see output from the UART, but the last troubleshooting was a while ago. I tend to rotate through back burner projects and now this one is dead center and progress must me made

    Also, the cards are very very very difficult to remove. Any tips? I was thinking of looping something underneath a card on both sides then pulling upwards?

    Thanks again

  5. #5


    Getting an extension card is what you want. In places where I need to monitor several places with the scope but have no extension card or easy access, I tack wire wrap wire onto the PC board. I may have as many as 10 wires at times. I lead them out to a piece of cardboard and tape them down. I use a sharpie to note which is which. It helps to have a large spool of wire to use. I just got about 5 lbs worth of 4 wire uncovered telephone wire. I'll be using that on my next project repair.
    You might look at making something simple from plumbers tape. You want a loop at the bottom that can be supported from both sides of the board. Maybe a screw that will catch one of the holes on the loop of plumber tape to be able to pull up on both sides at the same time. Then, a wood handle with two of these loops for both ends of the board.
    One might give some more though to the way to catch the other side of the loop than a screw but you still need the something to separate it when you lower it around the board and easily catch it to the hooked piece of tape.

  6. #6


    Quote Originally Posted by telemonster View Post
    Hello all!

    I've owned an IMSAI 8080 for a number of years now -- but I have never quite figured out how to use it / get it working.

    There is a video output board. Processor Technology VDM-1. I have tried it with and without it

    There is a serial IO card that also includes a monitor ROM. It also has a cassette interface as well.

    Any pointers appreciated. I'm a S-100 newbie, and it's been kind of a long journey piecing together how this is supposed to work.

    Maybe I can help because I don't know too much about the IMSAI. But I do know a little about Northstar disk controller cards and the VDM-1. (I only learnt something about S-100 computers after I got a Sol-20)

    From what I can gather the IMSAI 8080 (and I'm sure I'll be corrected if wrong by forum members) was intended to run with a dumb terminal, via the serial link. So that terminal provided the video display for the text and the keyboard for the operator.

    The VDM-1 card on the other hand was like 1/2 of a dumb terminal, in that it provides the text display as a composite video output for a monochrome monitor (or modified TV set). The circuitry in the VDM-1 is practically identical to the video text generator in the sol-20 computer. Also, along with that in the sol at least, it has a keyboard and an elaborate monitor ROM called a personality module which has the firmware which supports the keyboard, I/O functions for the serial/parallel ports, the tape interface and the video circuitry.

    Therefore, the only reason I can think of why you would find a VDM-1 in an IMSAI 8080, would be if somebody was trying to make the computer independent of the need for a dumb terminal. If that were the case, it would seem there would also have to be some sort of keyboard interface and a suitable monitor rom, running the firmware to support it. I have no idea if this hardware/firmware exists in your IMSAI 8080 or on another s-100 board in there, but I'm sure others who do know will shed light on it.

    The Northstar disk controller card, can most likely also be used with more modern 5.25" 360kb disk drives (preferably direct drive types as they are more stable), and soft sectored discs, if you get/buy Mike Douglas's virtual sector generator (VSG) board, which is just a small pcb that plugs in line with the drive cable.

    For my SOL-20 I put two YD-580 drives in a box, with Mike's VSG:

    The Northstar card has all the firmware on it (in the way of three OTP roms) to work, so you don't have to write anything to get the disk drives up and running, but you need the CP/M 2.2 version software on a 5.25" disk which is configured to work with it, and Mike Douglas might be able to give you some pointers on how to do it after you get the VSG. So probably, unless you want to be stuck with the hard sectored media, the first plan would be to get the VSG and a suitable disk drive unit. And try to get the VDM-1 working, but like I say I'm not sure (have no idea) how the keyboard fits into the equation with the IMSAI computer, or if you will be stuck with the need for a dumb terminal.

    The cassette interface will be very straightforward, its probably the CUTS one from Processor Technology.

  7. #7


    To bad you don't have a Polymorphic video card. It also has a parallel input port that is designed to work with a parallel keyboard. There are parallel port boards for the S100. As I recall, some of the serial cards also had a parallel port on them.
    There are also methods of changing 360k disk into hard sectored disk, to work with the N* controller, without the sector creating hardware.
    As for keyboards, it is possible to make a simple I/O to take a AT type keyboard and use it. It would take a lot of the processors time and memory resourses to do this but it is possible. It is better to use a separate processor as it typically done in a PC where a 8035 ( as I recall ) does the work of making characters. There are also several examples using various small processors on gethub as well.
    There is also no reason one could not just stay with the serial I/O. I often use my laptop as a dumb terminal, with the ability to download and up load data.

  8. #8
    Join Date
    Oct 2016
    Dutchess County, New York, USA


    You might try jointing the S-100 Google Group since they might be able to supply a copy of a ROM for your system that allows it to talk to the serial port.
    Crazy old guy with a basement full of Pentium 1 laptops and parts

  9. #9


    Hello! I made some progress... but still have one burning question.

    I bought the 88-2SIO board which has serial ports and a ROM monitor. I dropped that in my IMSAI and everything seemed to work no problem when I execute the ROM. I filled RAM and read the RAM back and it looked like it was behaving.

    The serial board is an Altair clone, so it should be compatible with Altair BASIC. I so took the Altair BASIC binary and converted it, then ASCII upload it into the ROM monitor (had to slow things way down.) It doesn't seem to execute though? It's been a few weeks since I've touched it but I started at 0000.

    I noticed that in the Altair world they would do a loader, then use the loader to read the program off of tape. Does this mean that jamming the binary into RAM there is some other start point?

    Progress is happening...

  10. #10


    Okay, some updates...

    I bought a 88-2SIO serial/rom monitor board and it works. I am able to fill the memory and read it back fine. I have serial access that is similar to the Altair.

    So I tried so use some linux-fu / shell script to take the Altair BASIC and feed the hex over serial into the RAM of the system. But when I execute it doesn't seem to run (no serial output.)

    IIRC there is a loader that is often used first, that in turn reads a paper tape or cassette (if not in ROM.) Is there any kind of padding or execute address offset that is usually used?

    I feel like I am close to successfully executing something on it.

    I am also interested in finding NorthStar DOS floppy setup to work with the Altair serial port address, either on a hard sectored 5.25" floppy or a not hard sectored one (I have a board that is supposed to emulate the hard sector timing pulses.)



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