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Thread: Z80 Hello World example in hex

  1. #21
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    Here are pics of the machine in question if it helps further:

    https://drive.google.com/drive/folde...hk?usp=sharing

    It's quite a well done little machine I think. It has a seven segment led display (should be 8 characters, but one LED has fallen out). The LEDs have a ribbon cable that goes to a plug but no idea where it goes.

    There are four cards - one is a 'mystery' card that has two big open sockets and a wire off.. I had to remove that card for the machine to 'post' properly. Then we have what I assume is some sort of I/O card, then video card, and finally the CPU card.

    Someone went to a lot of effort to build this, probably to have something to throw wood/veneer cladding on. I'm wondering what the purpose of the LEDs would have been though - perhaps were the original display before he wired up a video card?

    The machine only appears to have 1KB of RAM total (2x2114s), so I'm wondering if this were devoted to a specific purpose - perhaps EPROM programming or similar. There are a couple of open sockets on one of the other cards too. Bit of a mystery machine.

    There is also what appears to be a 3.5mm audio jack at the back. Wire coming off it going nowhere.. thought it might be for loading/saving except there's no second jack (usually would have one for MIC/output, SPKR/input?).

    Love the Clare Pendar keyboard.. types really nicely.

  2. #22
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    Quote Originally Posted by dreNorteR View Post
    Noticed that too. Bit rot?

    ...
    Re: IRQs, enabling them before RETI may be standard, but whatever is in the A register will be overwritten when you press a key. So interrupts should really only be enabled when the program is halted waiting for a key, and turned off again until it is ready for the next one.
    The bug is that A is used directly in this manner, not that IRQs are re-enabled..... IMO.
    --
    Thus spake Tandy Xenix System III version 3.2: "Bughlt: Sckmud Shut her down Scotty, she's sucking mud again!"

  3. #23
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    I dug my little Z80 wire-wrapped board out...

    Z80 CPU, 8K RAM, 32K EPROM, Z80 PIO (no SIO though), SP0256 speech synthesizer, LM386 amplifier, MM74C917N driving 4 off 7-segment LED displays, SP1000 speech recognition and syntheses chip, 7805 Voltage regulator and MM58274 RTC (although I appear to have 'borrowed' that for another project).

    I also found the hand-drawn schematics AND the source code listing tucked in a Radio Shack catalogue!

    Not enough for CPM - but I could get a a system going with a mini-monitor using some PIO bits as a UART...

    The wire-wrapping appears to have survived - apart from a couple of pins on an unused socket.

    Dave

  4. #24
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    Quote Originally Posted by falter View Post
    Here are pics of the machine in question
    That is certainly one heck of a thing. If you ever feel like tracing it out I'd love to see a schematic of the video section.
    My Retro-computing YouTube Channel (updates... eventually?): Paleozoic PCs

  5. #25
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    It's pretty involved. I have it on my list of 'trace-it-outs'. But it'll take me a while.. I go crosseyed just looking at it.

    Is it me or is it strange that they only went with 1KB of RAM? Would that have been the norm for a z80 homebrew project in 1977 or so? Seems like he went to way too much effort here.

    You know, come to think of it.. I wonder if that spare card is a RAM card? Or if there are other cards that simply weren't with the machine when it was found and sold.

  6. #26
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    I suspect there would have been an additional RAM card that you don’t have...

    Fig 13 of Don Lancaster’s TVT article (https://www.tinaja.com/glib/Kilobaud...rdware_pt2.pdf) describes the CG5004 on your video board. It is a character generator plus an integrated video shift register.

    If there are some LED displays, I wonder how you drive them? Possibly not via the built-in monitor but via a separate I/O port I suspect from user program.

    The 2758 is only 1K, so should occupy the memory from $0000 to $03FF.

    The original NASCOM-1 debug monitor (NASBUG or T2BUG) was only 1K, and that packed more functionality into it and had cassette read/write, VDU driver and a keyboard matrix scanner. Although, if this is homebrew, then it was probably developed on a budget in someone’s own time of course.

    Dave
    Last edited by daver2; May 19th, 2020 at 01:43 PM.

  7. #27

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    Quote Originally Posted by falter View Post
    It's pretty involved. I have it on my list of 'trace-it-outs'. But it'll take me a while.. I go crosseyed just looking at it.

    Is it me or is it strange that they only went with 1KB of RAM? Would that have been the norm for a z80 homebrew project in 1977 or so? Seems like he went to way too much effort here.

    You know, come to think of it.. I wonder if that spare card is a RAM card? Or if there are other cards that simply weren't with the machine when it was found and sold.
    If your 2758 has a "low" on it, you can use a 2716 to put new code on. You might look at what is wired to the A10 location for the socket. If it is connected to the processors A10, there would be no issue using 2716's. That way you can keep the original EPROM intact.
    Dwight

  8. #28
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    Quote Originally Posted by Dwight Elvey View Post
    If your 2758 has a "low" on it, you can use a 2716 to put new code on. You might look at what is wired to the A10 location for the socket. If it is connected to the processors A10, there would be no issue using 2716's. That way you can keep the original EPROM intact.
    Dwight
    Thank you! I might give that a try, actually. Wouldn't mind fixing that 'GH TO' message, and just playing around with it a bit. I'll see where the A10 location goes.

  9. #29
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    Something else I noticed with this machine - it has a 'Page' button. When you hit it, the monitor prompt disappears and the PAGE B light goes on. You get a blank screen and can't do anything. Hit Page again and you're back to the prompt. It's sort of like the TV Typewriter where you'd have Page A and Page B memory and could switch to Page B when you filled up Page A. I'm wondering if that points to this machine possibly having had more RAM than it does? I'm gonna trace the wires for the empty sockets and see if I can surmise what sort of chips might have been there. Really curious now.

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    Quote Originally Posted by falter View Post
    Something else I noticed with this machine - it has a 'Page' button. When you hit it, the monitor prompt disappears and the PAGE B light goes on. You get a blank screen and can't do anything. Hit Page again and you're back to the prompt. It's sort of like the TV Typewriter where you'd have Page A and Page B memory and could switch to Page B when you filled up Page A. I'm wondering if that points to this machine possibly having had more RAM than it does? I'm gonna trace the wires for the empty sockets and see if I can surmise what sort of chips might have been there. Really curious now.
    So there's actually 1K video RAM at 0C00 - as daver2 pointed out, the "clear screen" function is called twice during init to clear both pages.

    You could try entering some bytes at 0E00, can the CPU read them back or is it write-only?

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