RE: OT - limitation of PC's parallel port (was: Logic analyzer)
Date: 2002-04-18 09:01:56

  • Next message: Christer Palm: "Re: OT - limitation of PC's parallel port (was: Logic analyzer)"
    Hi Spiro,
    >> LPT vs ISA speed limitations
    I've looked at this a fair bit having baselined dozens of PCs and LPT card
    combinations, ISA, VLB, PCI, on-board, etc
    The fastest access I've achieved is 1MHz. ISA cards are a little slower than
    PCI, but it seems that access to the LPT port is intentionally limited.
    Perhaps a legacy issue for ensuring printers work?
    On one machine which allowed the ISA speed to be selected as a ratio of bus
    speed I tried various settings. The difference between the 8MHz and the
    "bus/2" option (which should have been 16.666MHz) only increased access
    speed by 30% for 8bit ops, but nearly got the 100% increase for 32bit ops.
    This machine had a 32-bit VLB IO card.
    My most recent thinking is that IDE (newer ATA protocols) must have overcome
    these speed limitations, especially for PCI cards. I've yet to build a test
    PC (I don't feel like accidently erasing my HDD), but my theory is to use a
    secondary device or channel for access. Ruud pointed out to me that IDE a
    bus, not a  device so it will need latches added to the end of it before it
    can be implemented for LPT sytle devices.
    It is possible however that the IDE bus is still limited to ~1MHz since all
    HDD (even old ones) work with newer cards. The improvement in speed may be
    due DMA by the newer devices.
    enough ramblings from me....
    - Nick
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