PXI is a modular instrumentation platform designed specifically for measurement and automation applications.
With PXI, you can select the modules you need to integrate into a single PXI system from multiple vendors. Communication between the modules uses familiar PC-based technologies such as the 132 MB/s PCI bus, allowing high performance communication that leverages widely available software. PXI also integrates timing and synchronization into the system, so that you can pass signals between instruments for high performance and accuracy, without additional cabling.
The PXI bus combines the high-speed PCI bus with timing and synchronization designed specifically for measurement and automation.
The PXI trigger bus consists of 8 shared trigger bus lines, a low-skew star trigger, and a common 10 MHz system reference clock. Using these synchronization features, you can easily pass trigger, clock, and other signals between PXI modules to make the accurate, high-performance measurements that you need.
PXI supports two module form factors, 3U PXI and 6U PXI. The 3U module form factor defines two interface connectors.
J1 carries the signals required for the 32-bit PCI local bus. J2 carries the signals for implementing PXI electrical features including trigger bus, star trigger and 10MHz clock. On a 3U cPCI module J2 is usually missing, but if included it carries the signals for 64-bit PCI transfers.
The 6U module form factor also defines two interface connectors and in the future may carry one additional connector for expansion of the PXI Hardware Specification.
To summarise, the J2 is reserved for PXI synchronization feature, such as trigger bus, star trigger, 10MHz reference clock and other signals. So a cPCI board with only a J1 connector can be used in PXI system; however, it is unable to implement the PXI-specific features.