The time has come. DSP board and system vendors have a decision
to make: to move to the PCI-X bus or not. PCI-X, the extension of
the venerable PCI bus, will happen. The infrastructure is falling
in place to support PCI-X and the system makers are on their way to
incorporating PCI-X into their servers. For them, PCI-X offers
higher and more stable bandwidth, overcoming some of the key
weaknesses of the original PCI bus.
That's the good news. The bad news is that PCI-X is not 100%
compatible with the existing PCI bus. Yes, it does run existing PCI
cards or boards. But in order to do so it degrades its performance
so that it's just another PCI bus variant. The result? No
performance advantage. So it's really PCI-X or PCI, take your pick.
But you can't have PCI compatibility (i.e., running PCI
cards/boards) and PCI-X performance.
For awhile it looked like PCI-X wasn't going to really happen.
For one thing, it was developed by the Big ThreeCompaq, HP,
and IBM and their allies, not Intel. Intel, who basically
single-handedly developed PCI technology, was not happy about
PCI-X, especially when PCI-X was caught up in the Intel's NGIO Vs
Big Three's FutureI/O wars.
Intel definitely has a NIH attitude when it comes to PCI and its
architecture initiatives. And PCI-X was not on its long-term
Pentium chipset roadmap. The problem is that Intel is crucial to
the acceptance of a PC hardware technology for Intel carries a lot
of weight as the leading PC uP vendor, leading PC chipset vendor,
and leading PC motherboard vendor.
Also, PCI-X was perceived by many to be a system company's
solution to its server I/O bottlenecks, a bus narrowly defined by
system companies like Compaq, HP, and IBM. It was not expected to
be a robust, multi-level bus like PCI with heavy application in the
embedded space. For while the PCI bus has a lot of drawbacks, it
has become the ubiquitous bus, with a PCI variant in every 32-bit
or bigger system.
On a Roll
PCI-X is now on a roll. Perhaps folks figured it was better to
fix the PCI bus, rather than keep trying to go faster and wider.
Infrastructure is now falling in place to make PCI-X a reality, not
just for PC server systems, but also for embedded systems as well.
PCI-X controller cores and chips are on the way, including FPGA
PCI-X looks to be a winner, and not just as a PC server I/O bus.
Work is underway to explore a CompactPCI version for embedded
system servers and platforms. Current research findings show that a
PCI-X running at 66 MHz and 64-bits wide can drive between five to seven
slots (relaxed timing), opening it up for cPCI class operation.
In engineering, there is old joke: "You know when a technology
is going mainstream; that's when the tester people decide to build
equipment for it." Well, the bus tester folks are now climbing on
the PCI-X bandwagon, providing test equipment for PCI-X
implementations. Agilent, Catalyst, VMEtro, and others are all
anteing up PCI-X test equipment. The wind is blowing right. When
you build PCI-X-based equipment you'll be able to test it.
The DSP community was slow to adapt to PCI. For one thing it was
the PC's I/O bus. For another, many board and system vendors had
proprietary buses or were using extensions of their DSP's main bus.
Nevertheless, most people liked the advantages of using a PC
standard product technology, which included low cost, wide product
selection and software compatibility. They've learned to live with
PCI's flaws and reap its benefits.
Today, another choice is emerging. To PCI-X or not. On the
positive side, PCI-X offers a PCI done the way it should have been:
split transactions, large status word, byte-count transactions,
wait-state restrictions, and so on. On the negative side, PCI-X
means leaving the PCI cocoon with a new standard. But it may be
worth it. There's a good chance that PCI-X will follow the PCI path
and emerge as a universal bus at multiple levelsSystem
(cPCI), I/O (PCI), Mezzanine (PMC, PC-MIP). PCI-X has what DSP
engineers needbandwidth. It's time to choose.