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View Poll Results: Which system architecture would you like to see in a 4P-compatible Z380 system?

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  • Z380 plus a modern ARM MCU for storage and systems management

    1 11.11%
  • Z380 plus a Zilog CPU (such as eZ80F91) for storage and sys management

    0 0%
  • Z380 using either a CPLD or discrete chip-based storage and sys management

    3 33.33%
  • I don't care, just get it done already!

    5 55.56%
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Thread: System architecture for a Z380 TRS-80 compatible.

  1. #1
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    Default System architecture for a Z380 TRS-80 compatible.

    So, after a lot of investigation and thought, I have found that it really isn't feasible to make a 100% compatible super TRS-80 using an eZ80F91 as the core. I denied that in myself, thinking that a fast eZ80 would be attractive enough on its own, but this is a vanishingly small group here that would even be interested. I have gradually warmed up to the idea of a Z380 design. The poll I put out last March, and the thread I started way back in 2017, show that the users who really care about upgrades and such also care about 100% Model III/4 hardware compatibility.

    So I'm in the early design phases of a Z380 system to be 100% Model 4P compatible (4P, since then it only needs a boot ROM and could then use the MODELx/III files for Model III compatibility).

    This poll lists the options I have for this system. I am interested in the group's thoughts on the advantages and disadvantages of each system type.

    Thanks in advance!

    EDIT: Note that I am planning on making the complete design open source.
    Last edited by lowen; January 13th, 2021 at 05:44 PM.
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  2. #2
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    What are you planning to use for the video output hardware? And keyboard input, so far as that goes. (Interface to an actual matrix, or some kind of PS/2 or USB converter?)
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  3. #3

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    Yeahhhh, I was looking at the eZ80 too, but it just doesn't look like it's compatible enough to be a drop-in super-replacement in these old boxen. All those built-in peripherals get in the way.

    Quote Originally Posted by Eudimorphodon View Post
    What are you planning to use for the video output hardware? And keyboard input, so far as that goes. (Interface to an actual matrix, or some kind of PS/2 or USB converter?)
    I have a plan for doing that on the Model II, actually. With the way the z80 wait states work, it ought to be trivial to just wire some kind of fast microcontroller's gpio to the z80 bus with a minimum of support logic. Then program it to respond to the same IO ports in the same way, and act like RAM when the vram is paged in. Maybe not 100% compatible as I am sure the timing would be slightly different, but ought to be fine for stuff that isn't super tightly timed. Then can bit bang video and keyboard on other pins. There are several different libraries for bit banging VGA on various uCs, so surely it would be more than possible to bit bang 640x240 TTL monochrome out to the video analog board too.

    That'd be the easiest thing for me to do personally anyway, as I don't know a thing about complex cplds or fpgas.
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    First, thanks for the question, because one of the biggest lessons I learned in 2020 is that 'feeping creaturism' is the enemy. I want to be as simple as possible; ideally, I'd like a Z380 CPU, some RAM, a medium-sized FPGA, CPLD, or even discrete logic for glue, boot ROM, and video framebuffer(s), and then the systems management and storage hardware would manage the disk storage, the actual video output, and keyboard input. For 100% 4P compatibility, the keyboard has to present itself as a memory-mapped matrix and the framebuffer has to present the same way.

    For the external interface, I would want a USB port for keyboard and maybe storage, and either VGA or some digital output (I would lean towards DisplayPort myself). The systems management hardware will handle the mapping of the USB keyboard to a matrix, similar to Inkey/80, and the mapping of the TRS-80 framebuffer to a region of the output video (I say a region; could be upscaled to the whole screen, could be 'windowed' but I'm wanting to guard against initial feature creep while not painting myself into a corner like the programmers of TRSDOS 1.3 did with disk support). Storage would be emulated FDC and HDD, with DMK and VHD disk images on SD card or similar; I have NO DESIRE to try to build a real FDC board. I'm thinking half a dozen chips, maybe a couple more. I want SIMPLE.

    For prototyping I have four Z380 chips, two soldered to carrier boards for breadboarding, and smattering of Altera DE1 and DE2 FPGA boards which have VGA output and PS/2 input. I also have an assortment of STM32 Nucleo boards; the STM32 has 5V-tolerant I/O. Some 74LVC245's and the Z380 can talk to the FPGA on the DE2. The DE2's little brother the DE1 has TRS-80 cred, being the base for the Coco3FPGA project, and Coco3FPGA shows how to accomplish a bunch of this stuff.

    For a production board I would want to target the Mini-ITX form factor, making cases, power supplies, etc trivially easy to find. I want SIMPLE: like plasmo's ZZRCC, which is three chips (Z280, RAM, CPLD). Simple is easier to troubleshoot.

    If I only ever build and document the prototype, with the code and design open, I would be mostly satisfied; the likely userbase is just too small to get a reasonable return on investment for the amount of time it will take. The response I get here will tell me if it's a feasible product or not; regardless, I'm planning to make it a project for the next year or two.
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  5. #5
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    Quote Originally Posted by bladamson View Post
    Yeahhhh, I was looking at the eZ80 too, but it just doesn't look like it's compatible enough to be a drop-in super-replacement in these old boxen. All those built-in peripherals get in the way....
    For something to drop in to an existing system, you basically have the Z180 (in either Z80180 or Z8S180 form, or even the HD64180) or the Z280. These chips do the RAS-only refresh cycles like the Z80 can and like the DRAM in these old systems needs. Z380 can work with somewhat newer fast-page-mode and EDO DRAM, but does a CAS-before-RAS refresh instead of RAS-only, and thus won't refresh the RAM in the old systems. eZ80 has no dynamic RAM support at all; you'd need a DRAM controller tacked on.

    That's part of the reason I'm looking at a whole new, but small and SIMPLE board.

    EDIT: And Ian Mavric already does an HD64180 board, the 4cellerator, and I wouldn't dream of competing with him in that segment; Ian's done too much good for the community. A Z280 board is a possibility, though.
    Last edited by lowen; January 13th, 2021 at 07:20 PM.
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  6. #6
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    Quote Originally Posted by lowen View Post
    First, thanks for the question, because one of the biggest lessons I learned in 2020 is that 'feeping creaturism' is the enemy. I want to be as simple as possible; ideally, I'd like a Z380 CPU, some RAM, a medium-sized FPGA, CPLD, or even discrete logic for glue, boot ROM, and video framebuffer(s), and then the systems management and storage hardware would manage the disk storage, the actual video output, and keyboard input. For 100% 4P compatibility, the keyboard has to present itself as a memory-mapped matrix and the framebuffer has to present the same way.
    I fully understand the "Get the damn thing going!" sentiment. I just wonder where you see it going after that. Given the Z380 can have a flat 32-bit address space and multiple register banks (just to mention two features) you could do something interesting down the track with it. But for real 4P compatibility you'd just leave it in Z80 mode.

    Also, if the end goal is "just" a modern simple 4P, why not just stay with the z180? You could create a "4P + XLR8er" system that way and, with an FPGA, handle everything that was possible with the real hardware. I wouldn't see this compete with the 4cellerator; it's a totally different beast to an exact re-creation of the original.

    Not criticising anything here; rather trying to understand your choices.

    PJH

  7. #7
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    Quote Originally Posted by pjhacnau View Post
    I fully understand the "Get the damn thing going!" sentiment. I just wonder where you see it going after that. ...
    You mention two of the Z380's killer features. The greatly expanded instruction set is another. This is an excellent question, by the way, as too many of my projects have been more "this will be fun to put together" rather than "this will be fun to use." It's like putting together a CP/M 68K machine; what do you do with it once it is together, or is it the perpetual project you're doing mostly to say you're doing something?

    Anyway, by sticking with a basic strict 4P compatibility once the boot ROM has finished setup, there is automatically a large archive of software that will just work.

    And then this becomes a springboard for other projects, like a newer operating system that could run multiple virtual TRS-80's, a Z380 hypervisor so to speak. But I have had a tendency in the past to make a project so comprehensive and complicated that I don't finish it. But I'm warming up to the idea of at least adding a memory expansion bus; while I doubt I would try for a 4 GB system, 128MB is pretty easy on one 72-pin SIMM.....

    Also, if the end goal is "just" a modern simple 4P, why not just stay with the z180? ....
    These are excellent points, by the way. The complexity levels of a Z180-based design and a Z380-based design are comparable; there might even be a bigger market for a 33MHz Z8S180 design. It's worth thinking about, for sure.

    Not criticising anything here; rather trying to understand your choices.

    PJH
    You're points are valid, and I appreciate the questions, as they do make me think about why I'm choosing certain things. In the end analysis, I am wanting the fastest and most featureful strictly upwardly compatible to the 4P machine I can actually get finished. Z380's features are superb, but compatibility needs to be tested. Last year at this time I had convinced myself that the eZ80F91's lack of strict compatibility wasn't an issue, but I was just plain wrong, because backwards compatibility does matter, unless you're just wanting to build it for the sake of building it.

    And the ability to say that your TRS-80 is a full 16/32 bit system has a definite coolness factor.

    Thank you for the thoughtful feedback!
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  8. #8

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    Hi lowen...

    I'm with you.

  9. #9
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    Hi, Fritz! Good to see you around!
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  10. #10
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    Z380, CPLD, bus extension / expansion. (my 2c)

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