https://www.techopedia.com/definition/21300/pci-mezzanine-card-pmc
https://whatis.techtarget.com/definition/mezzanine
服务器主板上，有mezz插槽
mezz是mezzanine的缩写，按字面翻译就是中间层连接器，一般采用PCIe总线，用来扩展PCIe的网卡，SAS卡等。其优点是比标准的PCIe连接器小，安装上mezzanine卡后比较节省空间
What does PCI Mezzanine Card (PMC) mean?
A PCI mezzanine card (PMC) is a printed circuit board that is used to extend the connectivity or function of a compatible computer system, typically for the VMEbus, Futurebus+ and other computer systems supporting the PCI specification. The PMC standard defines a card or board with the characteristics of the PCI bus as well as the physical dimensions of the common mezzanine card (CMC) format.
Techopedia explains PCI Mezzanine Card (PMC)
A PCI mezzanine card is a peripheral card that extends the functionality of a system by providing a means to connect other peripheral devices and serves as data acquisition cards used in industrial settings for sensors, sonars and the like. These functionalities often yield analog signals, so it is the job of the PMC to receive and transform this data to machine-readable and subsequently to human-readable format. It is basically an extension card providing any functionality that is required.
A PCI card is often slotted perpendicular to the motherboard, but a mezzanine card is slotted in parallel to the motherboard, so a PMC serves as a cross between having the CMC format and sporting PCI connectivity.
Mezzanine definition
Mezzanine is a term used to describe the stacking of computer component card s into a single card that then plugs into the computer bus or data path. The bus itself is sometimes referred to as a mezzanine bus. The term derives from the Italian word, mezzano , which means middle. The more common use of this term is in architecture, where it is a low-ceilinged story between two main stories in a building. In theaters, a mezzanine is a balcony projecting partly over the ground floor below it.
A mezzanine card is a smaller form of the more familiar Peripheral Component Interconnect ( PCI ) or Industry Standard Architecture ( ISA ) card. The original and still most popular mezzanine card is the Industry Pack (IP) card. An IP card provides a 16-bit data path. The IP card is 3.9 x 1.8 inches and has two 50-pin connectors that plug into an IP-to-PCI adapter card. The IP-to-PCI adapter card usually holds up to three IP cards. Another popular mezzanine card is the PCI Mezzanine (PMC) card. This card provides 32 or 64-bit data paths and uses 64-pin connectors. Both types of mezzanine cards are widely used with VME bus , which is an expansion bus technology that supports up to 21 cards on a single backplane . VME bus is widely used in industrial, telecommunication, and military applications.
A printed circuit board that plugs directly into another plugin card. For example, a mezzanine card, also known as a "daughterboard," might plug into a VMEbus card, CompactPCI card or PCI card in order to extend its functionality. Adapter cards are also available that enable a mezzanine card to plug into the larger slot of the target bus.
Smaller than standard expansion cards, mezzanine cards are typically designed for rugged industrial use and are often bolted down after being plugged in.