Hardware elements of integrating a PXI system

-September 04, 2012

A previous article entitled Integrating Traditional and Modular Test Instruments focused on the controller and software aspects of integrating box and modular instrumentation into the same system. One advantage of deploying open-system modular instruments such as PXI is the wide set of modules available many different manufacturers. This article will focus on the hardware aspects of integrating multi-vendor PXI instrumentation into a common chassis.

PXI is an industry standard that’s managed by the PXISA (PXI System Alliance, http://www.pxisq.org). PXI standards let maufacturers offer chassis, controllers, instruments, and switches that can be mixed and matched to form a complete test system. PXI itself is an abbreviation for PCI eXtensions for Instrumentation, indicating that the backplane is electrically based on the system bus architecture found in PCs. Indeed, PXI itself is closely related to cPCI (Compact PCI), a modular industrial computing architecture based on PCI.

PXI essentially leveraged the cPCI architecture, but added features critical for instrumentation. These features included hardware timing, trigger and synchronization as well as a set of software specifications to enable multi-vendor integration. PXI defined two module sizes: 3U (100mm high x 160mm deep) and 6U (233mm high x 160mm deep). PXI specifies a 0.8 inch slot width, and modules may be built to consume one or more PXI slots.  While the 3U is the most common module height by large margins, 6U products deliver additional capability and the two may be integrated together in the same chassis.

The advent of high-speed serial communication buses in the early 2000s presented a dilemma for the PXI community. PCIe (PCI Express) promised breakthrough speeds compared to the parallel PCI bus, but PCIe is electrically and mechanically incompatible with PCI because of its serial electrical interface and  high-speed connectors. There was a strong desire to leverage the large installed base of modules based on the older PXI standard (referred to as PXI-1, the original specification), but also take advantage of the capabilities of PCIe.

PXIe and the hybrid slot

In 2005, the PXISA published the first PXIe that included several clever solutions to the problem. A first level of solution is very simple. Because PCI and PCIe behaved similarly and shared the same memory map, it is possible to build a chassis where some number of the instrument slots (technically called peripheral slots) were PXI-1, and some were PXIe. These could be controlled by a new PXIe controller if a PXI-toPXIe bridge were added to the backplane. While technically a solution, it limited the flexibility of users from placing any module in any slot. The number and locations of PXI-1 and PXIe modules would be predetermined by the chassis selected.

This led to a second and more capable solution: the hybrid slot. Technical experts noted that while PXI-1 defined two connectors at the rear of any PXI-1 instrument, it was the bottom connector J1 that was always required. The most common functions of upper J2 connector could be done with half the pins. They also noted that PCIe, being a serial protocol, required many fewer pins as well. So in the space that J2 existed, engineers could place two smaller connectors, called XJ3 and XJ4. X represents that they are defined in the PXIe specification. Essentially, XJ3 delivers the new PXIe functionality, and XJ4 connects selected PXI-1 J2 signals to the backplane. A hybrid slot is a chassis backplane slot that has all three of these connectors: P1, XP3, and XP4 for connecting to J1, XJ3, and XJ4 respectively.

This allowed makes of the older PXI-1 modules to upgrade to a PXI-1 Hybrid Compatible module simply by replacing the J2 connector with the XJ4 connector, which is mechanically similar to J2, but with fewer pins. The XJ4 is intentionally backwards compatible to such a degree that the printed circuit board doesn’t even need to be modified. It is a plug in replacement. The rest of this article will refer to PXI-1 Hybrid Compatible modules simply as PXI-H.

PXI-H modules and all PXIe modules can be plugged into the same slot.  Chassis can now be designed with a large number of hybrid slots that bring back the concept of “any module into any slot.” Not only can the new PXI-H modules work in the new chassis, but they also work in the older PXI-1 chassis. Due to the advantages of this approach, most manufacturers have readily done the conversions from PXI-1 modules to PXI-H.

While it may be tempting to think that the integration problem is solved, there are still some careful considerations. First of all, it is a difficult design challenge to make all slots hybrid compatible, and very few chassis offer this. Second there are two specialty modules that have their own special connectors: controllers and timing modules. Finally, there is the issue that PXI allows two sizes of modules: 3U and 6U. The remainder of this article will focus on how to address these considerations.

The hybrid dilemma

The advantages of hybrid slots are clear: They accept the older PXI-H or the newer PXIe modules. In fact, it’s a little unfair to classify all PXI-H modules as “older”. A vendor may choose to create a PXI-H module in a new design where the speed of PXIe isn’t needed, since that module will work in an older PXI-1 chassis as well. If a backplane were designed composed completely of hybrid slots, the concept of any module into any slot becomes a reality. This gives the system integrators incredible flexibility; they are free to choose any mix of PXI-H and PXIe modules and place them into whichever slots they wish.

However, designing a backplane full of hybrid slots is very difficult. PXIe requires very high speed switching performed by high speed ASICs placed on the backplane. Hybrid slots populate the entire backplane surface with connectors, conflicting with the placement of ASICs. Chassis offered by ADLINK and Agilent have overcome this limitation using undisclosed methods. Suffice it to say that while all-hybrid slots are desirable, they are rare. Figure 1 shows an example of an all-hybrid backplane.

Figure 1. In this 18-slot PXIe backplane, the left-most slot is the System Controller Slot, which accepts an embedded controller or a link to an external controller. To the right are eight hybrid slots, a System Timing slot, and eight more hybrid slots. Each of the hybrid slots can accept either a PXI-H or a PXIe instrument module. The System Timing slot may accept a System Timing Module, or a PXIe instrument module. Courtesy of Agilent Technologies.

Instead, many manufacturers offer a compromise. A chassis will offer a certain number of hybrid slots and a certain number of PXIe slots. This can typically be done with some cost savings as well. As long as the user is certain that the number of hybrid slots is equal to or greater than the number of PXI-H modules, the chassis should work. A final check must be made that the location of the hybrid slots are suitable for the PXI-H modules. Some test systems require modules in a certain physical order for various reasons.

Finally, some chassis still offer legacy PXI-1 slots. They may do this for several reasons, perhaps to accept a module that has not been converted from PXI-1 to PXI-H. If so, this becomes a slot that accepts PXI-1 and PXI-H, but not PXIe, and a similar analysis should be done to ensure the chassis accepts the desired modules in the desired locations. In general, the modules and module types should be determined first, and then a chassis chosen compatible with this.

System and Timing Slots

There are two specialty slots—the system slot and the timing slot—that support these two module types. Every chassis has a system slot, which is typically the left-most slot in a chassis when facing the chassis from the front. This slot typically connects to the test system controller—either via an embedded controller that plugs into that slot, or via a Cable PCIe interface to an external controller. Whether using an internal or an external controller, the logical architecture is the same: the instrument modules appear as part of the controller’s PCIe memory map.

An external interface can easily be built within the confines of a single module slot. Not so with an embedded controllers, which often require more width. For this reason, the system slot often contains space to expand to the left, up to 4 slot-widths wide.  By expanding to the left, a multi-slot controller consumes no additional instrument slots, which are to the right of the system slot. Figure 2 shows a multi-slot controller module.

Figure 2. This PXIe embedded controller is a multi-slot module, N slots wide. The rear connectors are placed on the right side of the module so that, when placed into a chassis, the extra width expands to the left and does not occupy any instrument slots. Courtesy of Adlink.

Because a PXI chassis can be up to 21 slots wide and still fit within the confines of a standard EIA rack cabinet, a common large configuration is to reserve the 4 left-most slots for a controller, with 17 slots to its right for instrument modules.

One more special module is the System Timing Module, which can only be inserted in a System Timing Slot. The PXIe System Timing Module and the respective slot add precision timing and synchronization between instrument slots and, via front panel connectors, even between multiple chassis.  However, it is not a required module and most systems don’t deploy it. To be a useful even when a System Timing Module isn’t needed, the System Timing Slot can accept either a PXIe System Timing Module, or a PXIe instrument module. It can’t accept a PXI-H module as the P1 connector has been replaced by two special high frequency timing connectors: TP1 and TP2. Figure 3 shows an example of a System Timing Module.

Figure 3. A system-timing module distributes precision 10MHz and 100MHz reference clocks to all modules via the backplane, while the front-panel connectors can extend this synchronization to other chassis. With the OCXO option (Oven Controlled Crystal Oscillators) timing accuracy of 1 part per billion can be achieved. Courtesy of National Instruments.

Because the TP1 and TP2 connectors carry precision timing signals, a common location for the timing module slot is midway between the endpoints of the backplane. This minimizes the longest path length. A common configuration for a large system then becomes 4 controller slots on the left, eight instrument slots that are some combination of PXIe, PXI-1, or hybrid, the timing slot, and then 8 more instrument slots. With the conversion of PXI-1 modules to PXI-H, most instrument slots are PXIe or PXI-H. Smaller chassis can also offer significant flexibility in the slot type and location.

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