这是我在网上查到的解释。可惜看不明白。那位高手能再解释一下，并说明一下PMC的应用情况，让我有个感性认识。
The PCI Mezzanine Card (PMC) standard defines a family of slim modular mezzanine cards, for a variety of computer systems, based around the PCI (Peripheral Component Interface) specification.
PCI is a high speed local bus used by a variety of microprocessors. The PCI specification calls for a board that plugs into a carrier in a perpendicular fashion. Perpendicular boards are not usable for low profile computer applications, so the PMC standard defines a slim, modular, parallel mezzanine card. I/O can be via the front bezel and/or through the connector to the host computer for backplane I/O.
PMCs are intended to be used where slim, parallel board mounting is required - such as in single board computer host modules with the addition of expander cards or option cards. For maximum utilisation of component space, the mezzanine card is typically placed so the major component side (side one) of the mezzanine card faces the major component side (side one) of the host board.
The Pn1 and Pn2 connectors are always present and contain the signals for the 32 bit PCI Bus. When the PCI Bus is expanded to 64 bits, the Pn3 connector is used for these signals. User defined I/O signals are assigned to the Pn4 and Pn5 connectors.
The Pn3, Pn4 and Pn5 connectors do not need to be present on either the PMC or the host when their signals are not used.
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