Re: Re[2]: emc on microway alpha pc?

[stratton-at-mdc.net]

> jon> You should know that the ISA bus is a HUGE bottleneck, and
> jon> getting worse as CPUs get faster.  You might do well to at
> jon> least have the stepper motors on a PCI parallel port.
> jon> The auxilliary stuff should be OK on the ISA.
> jon> 
> 
> now i have two people tell me two opposing points-of-view.
> one person is saying isa should be fine and the other is
> saying that isa parallel cards would be a bottleneck.
> 
> a dual parallel port pci card must share an irq.
> to allow for individual irqs i would need to single
> parallel port pci cards.
> 
> for now isa cards are the choice since the budget is tight.
> pci parallel cards may come later.

The ISA bus may well be a bottleneck for many things in today's
computers, but motion control I/O is not one of them.  No one runs a
parallel port anywhere close the 8mhz or so limit of the 8bit ISA bus.

Neither the ISA bus nor the PCI bus is directly connected to the
processor.  Both come out of bridge devices, often part of the
chipset.  It is possible that a poor bridge design could make writing
to a slow ISA device more time consuming than writing to an equally
slow PCI device - but hopefully no design is this poor.

Reading on the other hand may be more of a bottleneck.  If you have
one parallel port sourced out of the motherboard chipset it could be
faster for reads - so it might make sense to use that one for all of
your inputs.

If you do buy PCI cards, it may make more sense to buy real I/O cards
than lots of parallel ports.

Chris

-- 
Christopher C. Stratton, stratton-at-mdc.net
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