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Thread: Mostek Z80 CP/M Computer System from 1979

  1. #1

    Default Mostek Z80 CP/M Computer System from 1979

    Hello everyone,

    I recetly got aware of a very old (and very rare?) Mostek Computer (from what I could find out its called SYS-80FT), featuring:
    - a MK3880 Processor @4.916
    - 4x 2708 EPROMs
    - 8x(+24x on RAM Expansion Board) MK4116 DRAMs
    - 2x 8" Single Sided Floppy Drives
    - several Serial/Parralel Interfaces
    - a Video Interface Adapter for Keyboard Input and Vido Output

    I found the Computer stored away in a shelf at a Universit, probably last "used" 1998, someone tried to boot it up but wasnt sucesfull at that and wrote "Scrap" on its side. There was absolutley no documentation left about the Computer, and it probably has been tossed out while the University scraped like 90% of its inventory in order to make space. Luckily the Screen, the Keyboard and a PROM-prgrammer, as well as a bunch of Floppys were still there. They (hopefully still) contain Operating systems (CP/M, FLP-80DOS, MOS 80) and Programs(A Chess Game, BASCOM-80 & Several other disks).
    When i Power up the Computer the (very loud and very big) fan and the floppy drives spin up, the keyboard makes a very short beep-tone and a blinking cursor is visible on screen. Thats it. The floppy-drives dont seem to get activated at all, not even when a disk is inserted before or after "booting".

    I already did a bit of measuring, the voltages are all only a maximum of 0,5V off, and the data pins on the floppy controller dont seem to change. The EPROMs and some bus-driver-chips get pretty warm, but main chips lik th CPU, the floppy controller itself and RAM chips dont seem to gain any temperature. I feel pretty lost with this thing, i dont have a Idea how i could troubleshoot this Computer, i would really appreciate your help.

    Some Pictures of the System:
    IMG_2202.jpgIMG_1761.jpgIMG_1762.jpgIMG_1804.jpgIMG_1801.jpgIMG_1772.jpgIMG_1777.jpgIMG_1766.jpgIMG_1764.jpgIMG_1831.jpgIMG_1881.jpgIMG_2159.jpg

    Here is some information about my computer and some files i found in the web that have some information about it:
    -Information about some boards installed: https://usermanual.wiki/Document/197...313031759/view Page 622-639
    -Information about individual boards, the whole System and some Software: http://the-eye.eu/public/Books/Elect...k1979_text.pdf Page 59-134
    -Information about some boards: http://www.bitsavers.org/components/..._Data_Book.pdf Page 362-397
    -Pictures and ROM contents (go to pictures/downsized/ for viewig pics): https://github.com/Maggi9295/Mostek-Z80-Computer
    -Video of the Computer "Booting": https://www.youtube.com/watch?v=bJr7...&index=2&t=66s

    I would really like to get this Computer back to run, so i appreciate every piece of help, and if its already that you read this very long post.
    Kind Regards, Maggi

  2. #2
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    Does it actually have the serial I/O controller board? It looks like two of the DB-25 connectors might be wired for RS-232 serial. If you attach something to them, is anything output on the serial ports at power on?

  3. #3

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    Do you have access to an oscilloscope or a logic analyzer? Also, you may want to disassemble the ROMs. These development systems are cool but often the documents are lost to the wind. There is likely some single point failure that has stopped the entire thing.
    You can make a switch type CPU emulator to do static test but you won't be able to run any DRAM ( I'm not sure what the RAM was. Pictures are too small. ).
    What programming skills do you have?
    Have you worked with any of the Arduino stuff?
    Do you have access to a programmer that can program 2716's ( easier to deal with and get than 2708's )?
    Dwight

  4. #4

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    Thanks a lot for your answers,

    Does it actually have the serial I/O controller board? It looks like two of the DB-25 connectors might be wired for RS-232 serial. If you attach something to them, is anything output on the serial ports at power on?
    The Ports you mean are probably the two top ports, they are connected to a handmade Circuitboard on the inside that was used by the University for some other hardware stuff, but i dont plan to leave this board in there. afaik th board is controlled by software, and as long as its not booting any software it doesnt output anything.
    There are two other serial ports, the one is the "VDI Serial Port" a serial port controlled by the Video Interface Controller. The other one is a "Serial Terminal" connection, that sounds very hopefull. i connected them correctly to my pc and also hooked them up to a oscilloscope, but no signal, only a continious high signal.

    Do you have access to an oscilloscope or a logic analyzer?
    I "only" hav a two channels analog/digital oscilloscope, i can connect it over RS232 to my computer.

    Also, you may want to disassemble the ROMs.
    I tried to with several programms, but had no "really" nice dissasembly. An online disassembler dissasembled it well, but had no other functions and just listed the instructions one after another.
    Then i found Ghidra, probably a very good programm, but it only dissasebles a very first bit of the code and then gives some error messages, like "cant accsess adress something", i have to play around with that one more.

    There is likely some single point failure that has stopped the entire thing.
    Yeah, i have the same feeling, but finding it will b the problem.

    What programming skills do you have?
    Its decent, i programmed a bit with everything, but probably forgot most of it again(i could re-learn it if neccesary)

    Have you worked with any of the Arduino stuff?
    Yeah, i did a lot with it one or two years ago, i have some basic equipment and homemeade expansions laying around.

    Do you have access to a programmer that can program 2716's ( easier to deal with and get than 2708's )?
    I have a Batronix Batego II, but it cant ahndle the 2708 because it needs three voltages and a programming volatge. it can do the very similair(especially in programming, because its very special with theese chips) TMS2716, i planned to make an adapter to read and programm 2708's, i already did a prototype reader from connecting a breadboard with wires to the programming socket to and using external voltages to read the EPROMs.

  5. #5

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    This computer contains some of the exact same boards, PROMs and software as in the Mostek AID-80F. See http://deramp.com/mostek.html and especially the restoration document referenced there.

    Two completely different bus connectors, and therefore, board layouts are used between the AID 80F and your machine, but the schematics for the boards are essentially identical other than connector PIN numbers. I used the manuals I found for your versions of the boards to figure out how my boards worked. The new variable in your system is the video board and keyboard - I don’t have that. Mine uses a serial port console.

    Let me dig through the manuals I have and see what I can find for you. I’ll also check and see if your PROMs are the same as mine. The PROMs were originally designed for a FLP-80DOS system, which was later replaced by a much more powerful CP/M clone called M/OS-80. M/OS-80 was based heavily on CDOS, Cromemco's CP/M clone. I have ported CP/M 2.2 to the machine, but CP/M was not on any of the disks that came with my machine.

    With a serial port console, no disk activity takes place until you press return on the keyboard (the PROM waits to autobaud). If your keyboard and video board are treated like a serial terminal by the system (like MITS did with their video board for,the Altair), then maybe your computer is also waiting for return to be pressed before it attempts to boot the disks.

    Mike

  6. #6

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    I've done a little digging. Your PROMs match mine except for three locations. The mismatches do not appear to be intentional. However, the failure is bits reading zero instead of one. Bit rot is usually the other way around.

    If your hardware is functional, I believe we can get your system up and running. You mention you have FLP-80 disks, M/OS-80 disks, and CP/M disks. I also have M/OS-80 disks, and as I mentioned, I wrote my own CP/M for the machine, but I have no FLP-80 disks. I have utilities for archiving and restoring floppy disks to/from a PC through the serial port, so I'm excited we can archive your FLP-80 disks and then try them on my machine. I'd also like to archive the CP/M disks you mentioned.

    It appears your video board is actually just an external serial terminal, however, the video board for the terminal happens to be housed inside the main computer chassis and uses the bus connector only to obtain power. So technically, your console is just a serial terminal like with my AID-80F machine.

    The red toggle switch on the CPU board is used to determine the auto-start address. In one position, the CPU jumps to zero on reset. In the opposite position, the CPU jumps to E000 on reset (the EPROMs). You want the switch setting that jumps to E000. Unfortunately, it doesn't appear that your board has the switch labeled (mine has a "0" on one side and an "E" on the other side).

    Here's what I recommend to try booting again.

    1) Never have the disks inserted when you power up the machine or turn it off. It's not unusual to clobber a disk during power on/off.
    2) Set the red toggle switch in the up position - towards the reset push-button. That's the E000 position on my board.
    3) Turn on the computer
    4) Insert a boot disk into the right drive (just a guess for now that it is drive 0)
    5) Press the reset button
    6) Press RETURN on the keyboard (may take two presses, but usually one does it)
    7) You should see one of the drive lights come on as it tries to boot.

    You can boot directly into the DDT-80 monitor in PROM by following the boot procedure above but do NOT install a disk into the boot drive. After reset, press RETURN as always, you'll now seek "DSK ERR" and get the "." prompt of DDT-80. There are some notes about DDT-80 in the M/OS-80 manual: http://deramp.com/downloads/mostek/A...s%20Manual.pdf

    Mike

  7. #7
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    Quote Originally Posted by deramp5113 View Post
    It appears your video board is actually just an external serial terminal, however, the video board for the terminal happens to be housed inside the main computer chassis and uses the bus connector only to obtain power. So technically, your console is just a serial terminal like with my AID-80F machine.
    Does it require an external cable to make the connection between the connector marked "Serial Terminal" to the "VDI Serial Port" to function?

  8. #8

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    Mike is great with things you'd just never expect. If you have some 2716s, it is simple to make the adapters. I usually stack two machine pin sockets under a 2716. If you take a small piece of wood with a hole in it is easy to pop the machine pins out where you don't want to make connections. The pins pop out the top. On the top socket I run the extra wires for the 2716. I use 30ga wire and after making the connections, I trim the excess pins on the ones to not connect, just in case.
    As for cables, I believe the boards bring the signals to the back connectors as well so that such connections are out of the way.
    For most things, the scope you have sounds like it is enough. Being able to program 2716s will make trouble shooting much easier. You should be able to write simple targeted loops to test the different parts of the system. Use the spec sheets for the parts. If you don't follow what is needed, let us know. A handy trick when writing small test programs is to use an I/O address that you've proved to be working. You can then use that address to sync the scope by putting the one channel on the select lead for that I/O. If it is an output, you can also read and data information, either from the device or just scope the data bus pins. Don't make the program too complicated.
    The first thing to check is the RAM. It looks like you have DRAM but I can't see the numbers on them. Your init needs to turn on the Z80 refresh ( look at the spec sheet ), if they are DRAM. Make simple test. Just writing and reading 55 and AA will find most RAM failures.
    Dwight

  9. #9

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    Thank you so much for your answers, i cant express my feelings.
    It was a really really good decicion to share my problem here... i never thought anyone has so much knowledge about this computer.

    ive seen the AID-80F system while googling for documentation about my computer, but i was disappointed when i saw that the board layout was different. i never thought about comparing the chips, but after you said they are identical i checked again - and you are damn right. one question - where did you found the schematics? i havent sen them on the page you meantioned, could you provide links for them?
    I looked at the Video board and discovered that a lot of traces are connected to the bus connector, including A0-7 and D0-7. BUT, and thats a big but, almost every trace from the bus conector goes to unsoldered/unused Chip "scoekts, no chips are soldered in it. along the fact that a few other chips are "missing" i assume that it was a extra function to have the board talk directly to the computer, that this board doesnt have.
    Like cruff already mentioned, do i have to connect the VDI Serial port to the Serial terminal Port?

    I have a nuch of Disks, including a original CP/M 2.2 disk by digital research, and another one labled the same, probably a copy to dont use the original disk. I also have two FLP-80DOS disks. Could you maybe explin me how you archieve your disks?i dont have any 8" drive left, so i would have to use one out of this computer. i feel really scared that those disks might get damage, and i would hope to have several copys on them.
    Its nice to hear that the proms are the same, that makes things way more easy.
    I already discovered that the button resets the system and the toggle switch changes the start adress, but i wasnt able to figure out wich positon is which adress, now i at least know that.

    I tried your tutorial on booting it up, but it ddnt worked, a blinking cursor like always. im scared that i might damage something when i conect the serial terminal and the vdi serial port together.

    Again, thanks a lot that you participate in this discussion you help me a lot with this computer.

  10. #10

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    Quote Originally Posted by cruff View Post
    Does it require an external cable to make the connection between the connector marked "Serial Terminal" to the "VDI Serial Port" to function?
    I'm not sure how your VDI board, the serial port on your CPU board, and your rear panel are wired, but you can probably figure it out with a few experiments. Or I can look at manuals tonite and try to figure out which serial port (CPU board or VDI) is jumpered to match the port number used for the console port in the monitor PROM.

    For now, don't bother with disks at all. Let's just try to get the "DSK ERR" and the "." prompt from the monitor in PROM. Then we know the console is working.

    Here's what I'd try first:

    1) Connect the keyboard to the VDI Serial Port
    2) Set the red toggle switch in the up position - towards the reset push-button. That's the E000 position on my board.
    3) Turn on the computer
    4) Press the reset button
    5) Press RETURN on the keyboard (may take two presses, but usually one does it)
    6) Look for the "DSK ERR" and dot prompt on the monitor.

    If that doesn't work, connect a terminal or terminal emulator to the "Serial Terminal" port and perform steps 2-6 and see if you get anything. However, I imagine your system has been configured to use the video board, monitor, and keyboard.

    Mike

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