Image Map Image Map
Page 1 of 2 12 LastLast
Results 1 to 10 of 14

Thread: Superbrain - repair - help needed

  1. #1
    Join Date
    Jan 2012
    Location
    Transilvania, Romania
    Posts
    33

    Default Superbrain - repair - help needed

    Hello everybody.

    I am working on restoring and repairing a superbrain, the first version (i assume) and at this moment i am stuck with a non working /wait line

    what i allready did: i had to replace the 74123 chip as the reset line was held low by this faulty chip, right after replacing it, the reset line works as it should have but now i am stuck with a low /wait line so the processor won't start. the /wait signal is generated by the same chip i replaced and worked before replacing as the chip was bad and I was lucky. the /wait signal is generated by the CLICK signal and here is my problem. The same CLICK signal enables the keyboard encoder witch is never enabled so it can't work. (Ok, what i did to get this far, i socketed the 74123 and the /wait pin in bended so the processor runs.) Now the CLICK signal is generated by a 74138 address decoder that should be ok as i also socketed this one and replaced it with a good known one.

    At the moment i am wandering in the dark as i also had no bootdisk and managed to write one using a .imd image i found online, it starts with the message "64k compustar model 30 stand-alone dos ver 0.2 for cp/m" so i assume it is not necessary the right disk image but the other one (.td0 sbrain32) i found only displays garbage.

    What i don`t know is if this signal should be generated right after start-up, after the rom is read or if the signal should be generated only after CP/M is loaded? It would be of great help if somebody could point me to the right answer or send me a correct disk image for this machine. i was thinking maybe the keyboard is scanned only after a successful boot (as it boots without the need of a keypress). If someone who owns a superbrain, this first version, and a scope or a logic probe could check the signal on pin 4 of the keyboard encoder before boot would be great, at least i would know if it needs a correct bootdisk to enable the keyboard encoder or not, anyway i suspect there should be also an other problem with this superbrain as it would still not start up with the /wait signal connected.

  2. #2
    Join Date
    Oct 2016
    Location
    Burnley, UK
    Posts
    284

    Default

    Hi - I'm a superbrain owner too - also attempting repairs. I've got mine to display a white raster and a block cursor so far. It's sounds like your troubleshooting skills are more advanced than mine.

    The real Superbrain expert on here is JonB Hopefully he will chime in eventually and offer you some guidance.

    Mine is a Superbrain QD - so a different boot ROM to yours and different boot disks? I have a load of floppies that came with it I have yet to make Images of, wanted a working brain' first.

    Do you have any pics you could share of yours? What boot ROM version do you have? Where did you get it from? Just inquisitive!

  3. #3
    Join Date
    Oct 2016
    Location
    Burnley, UK
    Posts
    284

    Default

    Quote Originally Posted by alexfloca View Post
    It would be of great help if somebody could point me to the right answer or send me a correct disk image for this machine.
    I will try and Image all my SB floppies this coming week - you and others may find something of use in there if I post the .imd images.

  4. #4

    Default

    Hello Alex

    Alan's alerted me to your plight...

    Because your machine is displaying the boot prompt we can be pretty sure it's a Superbrain I, otherwise with that particular boot disk it would not initialise the screen properly and you'd see garbage. Incidentally, that disk is fine for a Superbrain QD - it is the same machine as a Compustar 30 and I use it routinely on my QD because its BIOS implements a type ahead buffer and auto key repeat. So far, so good. Next we need to determine what sort of disk drives you have. Are they SS or DS? Have a look at the head mechanism. If SS then you have the wrong boot disk - you need a SS boot disk (although this should not prevent the A> prompt from appearing). I've got images of every boot disk found online plus some others. You can try them all, but I think the CS30 one is good for your machine, at least for now.

    Do you get the A> prompt? And are you saying the keyboard is not active? The keyboard encoders in these machines often fail... maybe that is the problem.

    Looking at the schematic, CLICK is generated by the 138 as you say, but only when the CPU tries to access port 50h to retrieve a character from the keyboard encoder. But it doesn't attempt that unless it knows there is a character ready (which it determines by polling the PB1 line of the 8255 PIO chip, which the keyboard encoder generates). So stepping back a moment, we first need to see if the keyboard decoder is generating the key ready signal (AK0 on pin 5). If you have a simple logic probe, you can test this by holding the point on pin 5 of the KR3600 and pressing a key. If it changes state momentarily (LOW to HIGH), then you know the decoder is detecting a keypress. It will do this even if its /CE input is inactive. If nothing happens, you have a faulty decoder - and if that is the case, you might be in a spot of bother, because these chips are basically unobtanium as they were customised for the Superbrain. There's a workaround I designed that uses a generic encoder and a ROM chip, but we need to do the diagnosis on your existing IC before trying that.

    If you don't have a logic probe, you ought to get one - they are not expensive.
    Last edited by JonB; May 25th, 2019 at 01:30 AM.

  5. #5

    Default

    A little bit more about the /WAIT generation. The LS123 is a dual one shot monostable and with the resistor and capacitor values shown on the schematic it gives a /WAIT pulse of ~560ns based on the timing equation Tw=K x R x C where Tw is ns, K is a constant value of 0.37, R is resistance in KOhms and C is capacitance in pF. Now the question is, why would you want to make the CPU wait every time it accesses the keyboard encoder? I'm guessing, but it might be to overcome keyboard bounce as the decoder has a delay between the key press output (AKO) and the data being ready. See the timing diagram (attached).

    KR3600 timing diagram.jpg

    So back to /WAIT. It seems to me that if you disconnect this signal your keyboard will not work reliably. It might be a good idea to check the values of the resistor and capacitor that are soldered to the 123 (on my SB they are actually soldered to the pins of the 123, not on the PCB).

  6. #6
    Join Date
    Jan 2012
    Location
    Transilvania, Romania
    Posts
    33

    Default

    Hi JonB, the info really helps. i expected the keyboard controller to be enable to generate the keypress code in first place. i had also suspected the 8255 but i swapped it with a known good one and this leaves the keyboard controller.

  7. #7
    Join Date
    Jan 2012
    Location
    Transilvania, Romania
    Posts
    33

    Default

    I forgot to mention, i am using an oscilloscope for diagnosis, i think the 74123 replacement might be also bad as accoring to the logic state chart the /wait signal should have been reversed allowing the processor to start working, i ordered a new one. Later today i will test the other signals on the keyboard encoder. At first i tought the processor would issue a enable (click) periodically to test the keyboard state and the /wait signal would have stipped the processor to let the keyboard encoder read and write the key value to it's output port.what i didn't know was if the code to issue this periodical signal should have resided in rom or should have been loaded from diskette. As it seems tl function differently i have new ground to check everything up. By the way, the disk drives are SSIMG_20190523_235041.jpg
    A picture of the boot screen

  8. #8
    Join Date
    Jan 2012
    Location
    Transilvania, Romania
    Posts
    33

    Default

    Ok, so the kayboard encoder is 99% faulty as there is no change in the output signal on pin 5. I can see a change on the keyboard pins when ipress a key but here all voltages are a little odd (-5.6 to -3 v),b oth +5 and -12 v used by the keyboard encoder are present and ok. i have also tested my 74123, on the old chip from the sb the part with the /wait line is working ok, on my nos replacement chip, the part with the reset line is working ok but the other part is bad. I ordered a new one anyway.the capacitor and resistor that were soldered on the pins of the 74123 i have soldered on the pins of the socket so i can easyly swap the chip.

    JonB could youbhelpnme out with the keyboard controller replacement ? I can build it myself or buybit from you

  9. #9

  10. #10
    Join Date
    Jan 2012
    Location
    Transilvania, Romania
    Posts
    33

    Default

    Thank you JonB! This is really helpful, i don't have to reinvent hot water . The nearest source for the AY-5-3600 pro is china, this means my superbrain project will be put on hold for at least one month until the parts hopefully arrive. Thank you so much for the support, i would have never thought the keyboard encoder worked when not enabled.

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •