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Thread: Microkit 8/16

  1. #11
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    My guess would be corrupted boot PROM(s), which CPU and what display adapter would govern the appropriate code.

    Further looks show an 8080 and 8216 PROMs, AFAICS. So, maybe it's the display control, unless that's via serial terminal (I see the RS232 keyboard interface, but where's the display out?).
    Last edited by WBST; December 7th, 2019 at 06:50 PM.

  2. #12
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    The ENABLED lamp on the keyboard refers to the interrupt enabled status of the CPU.

    The fact that this light changes state is probably good news .

    The CPU card doesn’t have any ROM on it. The D8216 devices are 4-bit bidirectional bus drivers.

    The only ROM is on the serial card in the form of a 256*8 EPROM bootstrap.

    The keyboard is parallel not serial.

    The VDU interfaces to the system RAM via DMA (i.e. there is no dedicated VDU RAM).

    I see from your pictures that there appears to be 8 cards fitted, but I don’t see 8 pictures of cards. What else is there?

    Looks an interesting project (as yours always are)...

    Have you found any more documentation anywhere by any chance (other than what is on bitsavers)?

    Dave

  3. #13
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    I see from the original e-bay photographs that you have 5 memory boards (making 40 KB in total) plus the CPU, VDU and EIA cards.

    As the memory devices are Intel 2107 dynamic ram devices (4K * 1), and are triple-rail devices (+5V, -5V and +12V relative to 0V/GND), are all of the supply voltages present? With them being DRAMs, they require a 2ms refresh. Can you see any refresh activity on either the /CS or CE pins?

    The MITS Altair 8800 4K DRAM card schematic may help out with the pinout for the 2107.

    Can you also post a photograph of the CRT board please? I think that is the card that is missing from the set.

    Dave
    Last edited by daver2; December 8th, 2019 at 05:37 AM.

  4. #14
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    Quote Originally Posted by daver2 View Post
    The CPU card doesn’t have any ROM on it. The D8216 devices are 4-bit bidirectional bus drivers.

    The only ROM is on the serial card in the form of a 256*8 EPROM bootstrap.

    The keyboard is parallel not serial.

    The VDU interfaces to the system RAM via DMA (i.e. there is no dedicated VDU RAM).

    Dave
    My apologies, I don't automatically recognise chips from that era, despite having had an S-100 bus system once upon a time.

  5. #15
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    No need to apologise, I was just filling in the details as I saw them.

    Dave

  6. #16
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    I've posted the CRT board now. Under the original google drive link I posted.

    As far as I can tell, the voltages are all there and well within spec. I'll check out those memory refresh pins shortly.

    I've not found anything other than what bitsavers has. I've not found anything with a really good description of what the system is supposed to do on start up. I'm wondering if the bootstrap even has a prompt or anything to suggest it's active, or if that screen of 90s is just what you get until the tape it loaded in, after reset.

  7. #17
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    I'm also curious why all the memory boards except one have one jumper strap? The last one has two. I'm assuming that's nothing nefarious - just some method for telling the computer that's the last board?

  8. #18
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    There's a schematic of the CPU board here... unfortunately this is after the Genrad/futuredata takeover and the board is being used in some other device. No mention of the 8/16 or boot or anything like that.

  9. #19
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    The jumpers on the memory board will be nothing more than setting the base address for the card. As simple as that...

    The CPU schematic you have found is the Rosetta Stone! It allows us to work out what pins perform what function on the backplane!

    For example, pin 68 is an active low DMA request, so this should be oscillating when the VDU card requests some memory. Pin 69 is an active low DMA acknowledge to tell the VDU card when it is allowed to access its memory. Both of these pins should be oscillating.

    Thanks for the VDU board photograph. I can see the white Fairchild 3258ADC. This is the 64*5*7 character generator! The data sheet is here http://www.citylan.it/wiki/images/5/58/3258.pdf.

    I see the CPU card has pull-up resistors on the data bus, so if memory doesn’t respond to the VDU card, the character selected will be whatever is assigned to character $3F (=‘?’).

    The tape cassette should have ‘motor drive’ signals, so one of these should operate when you hit ‘BOOT’.

    I’ll have another think.

    This project is turning out to be a doozy!

    Dave

  10. #20
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    I can see an NE555 timer on the CPU board (U22). That looks like a power on reset. I will print the schematic out tomorrow and have a bit more of a look.

    It would be interesting to see what is in the boot EPROM; but my advice would be to not destroy it if you are not sure you can read it successfully!

    Can you show us exactly what the display looks like (or describe what the screen looks like):

    A) when the power is first applied.
    B) after a reset.
    C) after a load.
    D) after a further reset.

    If you remove all of the memory cards and power up, what do you see on the screen?

    Dave
    Last edited by daver2; December 8th, 2019 at 01:27 PM.

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