CompactPCI extenders and PXI extenders are boards used to improve or extend the functions of the standard CompactPCI /PXI system. CompactPCI extenders and PXI extenders are designed to isolate all the voltages and the signals between the PCI motherboard and the Unit-Under-Test, while PC power can be allowed to stay on. This isolation can be accomplished by either using the on-board mechanical On-Off switch or via an external input. This feature eliminates the requirement for power cycling the PC system and repeated rebooting during the test and characterization process.  

Though CompactPCI extenders and PXI extenders are often used, they are not exactly logical interconnects, as they simply merge two crates into one, allowing the insertion of additional modules into the same logical crate (consisting of two physical crates). This solution works for moderately sized systems, where total physical bus load is not an issue and all modules fit into the common address space, but free physical slots are at a premium.

CompactPCI is a high performance industrial bus based on the PCI electrical specifications. Its form factor is specified to be the rugged Eurocard format. Card format is given in terms of (height x length). The height is expressed in units of 'U'.  The 'U' is a unit used for the front panel of cards.  Each 'U' is equal to 1.75 inches.  The length normally is in mm (millimeters). 

CompactPCI extenders and PXI extenders interface with two types of cards: 3U and 6U.  Normally the height of the card is measured in units of 'U'.  The length of the card is measured in terms of millimeters (mm).  A single height board, also known simply as a 3U card, is 100mm x 160mm with one 96 pin DIN connector (the J1connector) on the rear that plugs into the backplane.  A double height board, also known as a 6U card, is 233mm x 160mm and has a second 96 pin DIN connector (the J2 connector).

PXI (PCI eXtensions for Instrumentation) is a specification developed by National Instruments that extends CompactPCI for measurements and automation systems. CompactPCI / PXI is used in many industrial applications including computer telephony, industrial automation, real-time data acquisition, real-time machine control, instrumentation, military systems, and others. Compared to desktop PCI, Compact PCI supports 8 PCI slots (the desktop PCI standard supports only 4) that can be expanded by using PCI-PCI bridges, and is more robust and sturdy for use in industrial applications.


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