Some people think that a PC (ISA/PCI) board, crowded with components,
including RTC's and processors, would do the same as a ADC chip in a
small board connected to the PC parallel port... now you understand why
you need an independent real time measurement circuit with buffers and
all.  In this case, the PC act just as the equipment's front panel and
storage system.  Of course, to measure the time observing paint dry, any
PC with those zillions of Windows interrupts can do, but to observe a
high freq jitter noise around 2ns, you will need something more
expensive...
Wagner.

Chris Fanning wrote:
>
> >         I think one is going to have some trouble measuring real time
> > accurately for any period under a second in any sophisticated
> > operating system, be it any of the Windows flavors, DOS, or Linux.
> >
> >         The tricky thing about exact timing on a modern computer is
> > that interrupts are constantly being serviced from many different
> > systems such that a finely-accurate timing routine may be put on hold
> > for a millisecond hear and another there every time the disk drive
> > needs servicing or the operating system must tend to another user.
>
> What do most of you consider real-time?  I have lab software that has
> a typical latency of < 1ms under Windows [NT,95,98].  I only measure
> milliseconds so it may be substantially less.  Occasionally there's
> a 1.x to 2.x ms latency spike.
>
> Is this real-time enough for most of you?  If so there's a few things
> that you can do to make performance more predictable.  One is to make
> sure you have no devices that will disable interrupts for a long time.
> Careful selection of hardware with good drivers helps a lot.  It's
> largely irrelevant now, but make sure you have a bus mastering hard
> drive and no other programmed IO devices.  Disable networking support
> under Win95 (maybe 98 too).  The last thing is that you absolutely
> cannot poll.
>
> I'm using National Instruments aquisition boards and callbacks.  Works
> very well for me.
>
> If 1ms latency isn't enough or occasional higher delays aren't
> acceptable then a more dedicated solution is definitely in order.
>
> How good is PIC AD?  I'm used to working with these National Inst.
> boards with no missing codes with great accuracy and precision.  Is
> the AD on PICs up to serious work?
>
> Chris