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Using PLD or FPGA for ISA bus board with DMA

Discussion in 'Electronic Components' started by Guest, Apr 23, 2004.

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  1. Guest

    Guest Guest

    I'm planning to build an ADC board with onboard buffer with DMA transfer
    from onboard buffer to computer's memory.
    Where can I find an ISA bus board design example with DMA functionality
    using PLD or FPGA ?
    What would be the max paractical speed to transfer data from ISA bus to PC's
    memory?
     
  2. KR Williams

    KR Williams Guest

    ISA is trivial. A simple web search should give you all the
    information you need. What sort of DMA do you want? ...classic
    ISA DMA, busmaster? Both are possible, but both rather rot. If
    you really need DMA support, I'd take the plunge and go PCI.
    It's not all *that* hard, if you use the bridges and drivers that
    are available (of course you didn't mention you budget either).
    With a modern PC, I wouldn't expect more than 500kB/s, *maybe*
    twice that. ISA sucks rocks and no one cares about giving it a
    hand.
     
  3. ISA was never designed for DMA, unless you consider the 8-bit 8237 on the
    motherboard. Both ISA and the 8237 are very much obsolete, and even if
    present on current chipsets, they have probably not been verified for
    correct operation for generations (chip generations, that is).

    Unless entirely impossible, I'd suggest designing a PCI board; there are
    many interface chips available, and most of them include some sort of DMA.
    Best of all, the PCI bus was designed to use DMA as its basic data-transfer
    method (it's not called DMA; it's called Master transaction in PCI-speak).

    Designing a board around a PCI interface chip not too bad; the chip makers
    have lots of guides, app notes and possibly even evaluation cards available;
    PCI is doable using 2 signal and 2 power planes - even (if you are very
    careful) only 2 signal planes. OTOH, you can't wire-wrap or protoype one
    very easily.

    If going the PCI route, I'd recommend learning at least the PCI basics,
    since it's very different from ISA, or any single-processor async bus
    (which ISA basically is).
     
  4. Zak

    Zak Guest

    I remember the Adaptec 1542 (busmaster DMA on ISA) to be problematic
    with PCI chipsets.

    Also, DMA over ISA is limited to 16 megabytes of reachable memory.

    Programmed I/O over dual ported RAM (DMA to memory on the card, that is)
    might be the easy way out, and probably fast enough.


    Thomas
     
  5. KR Williams

    KR Williams Guest

    THere are three forms of ISA DMA. The 8237 DMA registers only
    address one. ISA can also busmaster (HOLD/ HOLDA/, IIRC). IBM
    had a series of modem/sound cards that did ISA busmastering. It
    worked rather well, though the drivers had to copy into buffers
    below the 16MB line.
    I wouldn't disagree with this assessment. ISA is dead, and may
    it stay dead. ;-)
    You won't meet spec without the internal planes. It might work,
    but might not. I'd never go below 2S2P for a PCI card, which
    throws it out of the range of the hobbyist.
     
  6. Actually, you had 2 x 8237 chips: one was wired for 8-bit, one for 16-bit
    (even addresses) transfers. The two chips were cascaded (don't remember
    which was master and which slave); one 8-bit channel was used for DRAM
    refresh and one for floppy. Since the 8237 has only 16-bit address,
    there was an external 8-bit register to get 24-bit address (up to 16 MB,
    can't cross 64-KB physical address boundary in one DMA transaction).
    You could also program a 8237 channel to be bus arbiter only, using your
    own hardware to create bus cycles.

    However, none of this was widely used, and even when there was no PCI,
    many ISA DMA cards wouldn't work in all "PC Compatible" systems. IIRC,
    most of these high-performance cards has a default programmed-I/O mode
    (for example: basic ATA-controller PIO mode), which was the one used
    by most users.

    I am not at all sure that modern chipsets still support all these
    modes. After all, who in his right mind will use an ISA bus-mastering
    network or tape interface card TODAY? why waste verification CAD cycles
    to test these modes? I am not even sure if modern OSes still read/write
    floppy using DMA, and using DMA to refresh DRAM is dead as the Dodo.

    I was assuming a hobbyist card, so he wouldn't care about failure rate in
    a 10,000-unit production run; one unit will most probably work - most PCs
    leave a reasonable noise-margin error.

    You may also be able to get a prototyping board with the PCI interface
    ready for use and a prototyping area in the local-bus side.
     
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