PXI

PXI uses PCI interfaces on the backplane. where most modules operate at 33 MHz and 32 bits wide to provide a BW sufficient for their intended application. there are only three types of slots:

System slots, which are used to accept a controller or a remote interface to a controller.

Star Trigger slots, which accept peripheral modules or Star Trigger controllers that drive modules that use the Star Trigger feature. Star trigger slots can be used as normal peripheral slots. It is not common to use trigger systems on PXI switching modules because the trigger model used is usually IVI (software) based.

Peripheral slot that accepts any peripheral PXI module.

Backplanes are shared between modules and are displayed with the devices on that bus as a set of bus numbers (corresponding to each segment of the PCI bus), usually numbered from 15 down. All devices in a particular bus number share the same 32-bit PCI bus segment. The standard limits the bus number to 256.

PXI Express

The PXI Express (PXIe) chassis uses a PCI Express serial interface to connect to peripheral devices from its system slot. The system slots are not compatible with PXI, so you need to use a controller or PCIe interface with a sufficient number of PCIe connections to support the peripheral devices. The use of serial interfaces improves the available BW for peripherals because in principle it is not a shared BW - each peripheral gets one or more serial connections with a serial rate of 2.5 Gb/s (GEN1). Since PCIe is a point-to-point connection system, each connection is defined as a bus number and device 0 (no other devices are shown on that slot). Like PCI, it has a 256 bus limit. As a result, the maximum number of modules is lower than for PXI.

Using PXI Express does not guarantee faster system speeds, and the most common system speed bottlenecks are not related to backplane speed, but speed advantages can be seen on modules that receive or transmit large amounts of data.

PXI Express mechanical and electrical interfaces are different from PXI. To take full advantage of the fast PCI Express lanes, chassis typically contain a mix of PXI and PXI Express slots (hybrid chassis). As a result, chassis can detail many slot types. Typically, slots are defined as:

Controller slots. Accepts only controllers designed for PXIe and hybrid chassis.

PXI Express slots. Accepts only PXI Express modules for which only a relatively small number of types are available.

Hybrid slots. They can accept either PXI Express or PXI modules, which have two sets of connectors, but the second PXI connector is shortened to allow installation of the PXIe connector. All Pickering switching modules are compatible with Hybrid slots.

The Legacy slot, which accepts only PXI modules.

Trigger slots, which can only be occupied by modules designed to populate the slot, and therefore do not apply to "normal" peripheral modules. For this reason, some chassis do not include timing slots.

PXI Express chassis also vary in the number of PCIe lanes supported per lane. This is transparent to the user, but for high-speed applications there is considerable advantage in ensuring that some PXIe slots have a large number of PCIe lanes, as this is the main reason for specifying a PCIe-enabled chassis.

There are other electrical differences as well, notably different power supplies. PXI Express lacks both 5 V and -12 V power supplies. 5 V power supplies are a particular problem because the most commonly used relays have 5 V coils - and are the components that distribute the storage in the channels and are therefore the easiest to get service support for.

The vast majority of peripherals are PXI rather than PXI Express.