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PMC - The Super
Mezzanine Standard |
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The VME,
CompactPCI® and Multibus suppliers have joined
forces to agree on a common standard for mezzanine boards. The result is PMC (PCI
Mezzanine Card) - a mezzanine bus based on the well established high performance PCI
(Peripheral Component Interconnect) specification. PCI is now used by most high
performance PC system suppliers and add-in card vendors. Hence, it is very well supported,
low cost and readily available. |
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What is PCI |
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PCI was
developed by Intel® as a very high performance local bus to provide a
means of inter-connecting components together on a board but with no
external access. Its purpose was to provide a simple low-cost way to add
high performance peripheral components like graphic controllers and I/O
devices to a CPU. Following the initial release of this interface, a
modified definition was derived which provided an external interface -
this became known as the PCI bus. This bus allows for PCI boards to be
plugged into the PCI bus connector. PCI provides many features including
data transfer rates of 132Mbytes/s (264Mbytes/s for 64-bits), support
for a system similar to Multibus II's Interconnect and 3.3V interfaces.
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Mezzanine Problems ... and
Solutions |
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Mezzanine cards have long been a
popular method of adding additional or specialized I/O to
processor boards. In the Multibus II market, there already
exists standards such as iSBX and MIX from Intel, but these
interfaces are relatively low performance and do not have a wide
multi-vendor support. Concurrent Technologies overcame the
performance problem by adding high performance I/O directly onto
the board therefore achieving very high data transfer rates and
low latency between the CPU, Memory and I/O. This is much better
than using either of the existing mezzanine standards because
the processor does not have to sit idle (waitstates inserted)
waiting for these slow expansion interfaces to respond. With new
CPUs, like the Intel® Pentium®
processor a lot of CPU time can be wasted with
these slow interfaces. However, if a very fast expansion
interface was available, that did not force the CPU to sit idle,
then high performance mezzanine systems could be built using
this expansion interface. PCI provides such an expansion
interface and with its wealth of support from any different
processor and peripheral component manufacturers as well as PC
vendors, the bus is an ideal choice for use as a low cost
mezzanine system.
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The PMC Standard |
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PCI boards have
a form-factor very similar to that of the conventional AT (ISA) boards
so that they remain compatible with existing PC packaging. In a PC
system, PCI boards (like ISA boards) are mounted at a 90° to the board -
as this would occupy about 5 (Multibus II, VME or CompactPCI) slots! However, by
mounting the PCI card in a plane parallel to the Multibus II, VME or
CompactPCI
board and using different low profile connectors, a PCI board can be
fitted onto Multibus II, VME or CompactPCI boards - and within a single slot. This
connection system together with PCI interface is called PMC.
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The group of companies defining the PMC
specification wanted to use as many existing standards as possible.
Therefore the PCI (Version 2.0) electrical and logical specification was
selected together with the IEEE's proposed CMC (Common Mezzanine Card)
standard. CMC defines the size of the slave mezzanine cards, the type of
connector to be used and the position of the connectors on the master
board and on the slave board. Multiple 64-pin low profile connectors are
utilized. The PMC standard defines which connector pins are used for
which PCI signals; in addition it defines 64 of the connector pins for
use I/O signals. The PMC standard is known as the IEEE P1386.1.
A single size PMC module measures 74mm x
149mm. Two such modules could therefore fit side by side on a Multibus
II, VME or CompactPCI motherboard. Depending upon the specific implementation of a
slave PMC board, the user I/O can be accessed either through the P2
connector or directly through the front panel of the motherboard.
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Benefits |
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As an IEEE
standard, PMC assures users that any host or module complying with the
standard will function with any other module or host designed to the
standard. This offers users flexibility to mix and match different host
cards and modules. PMC is an open standard. It brings to Multibus II, VME
and CompactPCI a wealth of I/O products and capabilities which have not been
readily available before. For high
performance Multibus II users, it offers an extremely high performance
I/O interface for their specialized I/O requirements. More graphics
options will be available. And yet, since the boards will essentially
have originated from the large volume word of PCs, the price of these
modules should be relatively low.
Mezzanine cards will continue to provide
a method of customizing and optimizing the use of Multibus II, VME and
CompactPCI
motherboards. PMC brings mezzanine technology up to date.
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