A PXI Chassis is More Than a Box With Slots

PXI modules all use J1 for the basic PCI bus. J2 handles 64-bit PCI transfers, local bus signals, and trigger signals. XP4 in a PXIe hybrid slot duplicates the power, trigger bus, star trigger, and 10-MHz clock signals defined in PXI-1 but does not support the reserved pins on J2 used for 64-bit transfers and the local bus. This is the reason the same color is used for both XP4 and P2 connectors in Figure 1.

According to the PXI-5 standard, “The 3U PXI Express Peripheral Module has two connectors, XJ3 and XJ4. A simplified description of the connector functionality is that XP3/XJ3 are for PCI Express and Differential Triggers and Timing and XP4/XJ4 is for instrumentation signals that are defined in the PXI-1 specification [basic PXI].”

Connector numbering increments from the bottom to the top of the backplane. Differences among connectors in similar positions are denoted by their prefixes and highlighted in Figure 1 by separate colors. For example, connectors 1 and 2 in a system timing slot are called TP1 and TP2 where the similarly located connectors in a system controller slot are XJ1 and XP2.

Additional connectors may be used to provide more power or for other signals in 6U chassis. Geotest-Marvin Test System’s GX7005A 6U Chassis can supply 4.5 kW via the J5 cPCI connector that is not used for standard PXI cards. Also, PXI-5 defines a stacked 3U configuration in which two 3U PXI or PXIe modules can be used within a single 6U slot.

Electrical
Regardless of the number of slots in a PXI chassis, a maximum of eight peripherals forms a 33-MHz PCI segment. In larger chassis, multiple eight-slot segments are connected by PCI-PCI bridges. The left-most slot in a chassis houses the system controller, and if a star trigger controller is used, it must occupy the second slot.

Triggering via the star trigger controller is particularly accurate because it uses equal-length backplane traces to ensure <1-ns skew among up to 13 peripherals. Triggering also is supported by the eight-line PXI trigger bus, which allows triggers to be passed from one module to another.

A 10-MHz system reference clock is distributed to all module slots for synchronization and timing. It is possible instead to use a 10-MHz clock distributed by the star trigger module, which ensures very low slot-to-slot clock skew.

To maintain compatibility with legacy PXI modules, but also to support the higher speed of PXIe, many PXIe chassis provide PXI slots and separate PXIe slots as well as hybrid slots that accommodate either hybrid-ready PXI or PXIe. A PXIe chassis, by definition, uses PCIe on its backplane to communicate with the controller. This means that PXI and hybrid slots must connect to a bridge device to become compatible with PXIe.

Also, a chassis will be designed to support a number of PCIe lanes, typically from one to four. Present high slot-count, four-lane chassis use PXIe switches to select which module is connected to which lane at a particular time. There are other configurations, however, and the Agilent Technologies M9018A 18-slot PXIe Chassis shown in Figure 2 supports different controller capabilities through a configurable backplane interface.

According to Dave Brannon, PXI platform product manager, “Two x8 links provide either 2-GB/s or 4-GB/s per-slot bandwidth depending on whether only x4 links are used for the peripheral slots or a mixture of x8 and x4. Alternatively, a single x4 controller link ensures the greatest compatibility among controllers but limits bandwidth.”

PXIe eliminates PXI’s star trigger controller, replacing it with a high-performance system timing module. This module must plug into a special backplane slot having the necessary complement of connectors for trigger and timing signal distribution.

In addition to supporting PXI triggering and the 10-MHz clock, the PXIe system timing controller generates a clock or event synchronization control as well as differential star triggers and a 100-MHz differential clock. The 100-MHz clock is distributed using low-voltage positive emitter-coupled logic (LVPECL), and the timing skew between any two peripheral slots is specified to be <200 ps.

The system controller must occupy the left-most slot, but no restriction is placed on where the system timing controller may be located within the chassis. Obviously, because a special connector layout is used, there is only one slot into which it can be plugged. This slot is identified by a chassis-slot glyph consisting of a white number on a dark circular background all enclosed by a black-outlined white square. The square black outline signifies a timing module and is used by itself to identify a PXIe system timing module.

As shown in Figure 1, the different types of slots have a distinctive glyph. A white number on a dark circular background with an H above and to the right of the circle signifies a hybrid slot. A white number 1 on a dark triangular background marks the controller slot. Dark numbers inside a circular outline are used for basic PXI slots.

The PXI-5 hardware standard supports peer-to-peer communications along with the higher data transfer speed provided by the PCIe serial architecture. Within a four-slot switch, data can be streamed from one of the slots to another without first being sent to the controller. Streaming also is possible between modules attached to different switches, but because the signal path is through two switches and some of the controller’s hardware, the maximum speed may be reduced.

In chassis with a configurable controller-to-backplane interface, such as Agilent’s M9018A Chassis, the connections needed in addition to those within the peripheral slot switches can be made on the backplane. This means that peer-to-peer communications can occur between any two modules without involving the controller at all.

Peer-to-peer streaming capability greatly enhances the performance of systems that need to tightly couple data acquisition and subsequent processing. Of course, the PXIe hardware must be designed to take advantage of peer-to-peer communications, and this feature has started to appear in some digitizers, signal analyzers, and FPGA-based processing modules.

As well as high-speed signals, several hundred watts of power must be efficiently distributed. PXI-5 specifies that a chassis must supply at least 30 W for each PXIe and hybrid slot and 25.6 W for each PXI slot. In addition, from 30 W to 140 W must be provided for the controller depending on the number of slots it occupies. However, these are minimum values, and the maximum available power is one of the factors that differentiates one chassis from another.

Pickering Interface’s David Owen, business development manager, explained, “The PXI standard defines minimum requirements for the power supply currents on each rail based on chassis slot count, but vendors often provide significantly more capacity than the minimum.

“Modules…such as power supplies and battery simulators deliver some of their power to an [external] load…. These modules can require the chassis to deliver more than the minimum power requirements,” he continued, “and the module [design may have to be optimized] to a particular chassis specification that exceeds the minimum. This is particularly true if these power-consuming modules are replicated in many locations in the chassis, an example being battery stack emulation for automotive applications based on the Pickering 41-752.”

Thermal
Cooling is an integral part of PXI-5 and directly linked to the power dissipated within a chassis. PXI-5 recommends that 3U modules not dissipate more than 30 W and 6U modules not more than 60 W. Nevertheless, some modules do, and the chassis must remove the required amount of heat while maintaining the modules within their temperature limits.

The alternative is to derate a chassis so that its available power is reduced when operating at high temperature. Unless the chassis cooling capabilities match the power rating, the combination of high dissipation at high temperatures is not allowed.

National Instruments PXI and PXIe chassis with eight or more slots use NI-designed power supplies that do not require derating even at the highest specified operating temperature. According to Patrick Webb, PXI systems product manager at the company, “Using our own design means that NI can guarantee long-term availability for these power supplies and fewer design changes to its chassis. Relying on a standard PC power supply can cause issues because of the relatively low quantities involved with PXI compared to those in the PC industry.”

Thermostatically controlled fans save power and reduce noise if the power being drawn is low and the ambient temperature reasonable. They can have the same effectiveness as a noncontrolled fan when running at full speed. Some chassis allow you to override the thermostatic control to ensure adequate cooling for modules with special requirements.

The PXI-5 specification states, “Chassis SHALL provide forced airflow that flows from the bottom to the top of a module….” ADLINK’s Model PXIS-2719 uses four 68-cfm fans in the rear of the chassis. As the company’s Catherine Wu, senior director of the measurement product center, described the operation, “Cool air is pulled in through apertures on the bottom and exhausted through the back. This design gives uniform airflow on each PXI slot and minimizes the drawing of hot air in from the rear area where all other devices or instruments exhaust.”

This type of design doesn’t use filters. Over time, the modules will get covered in dust and dirt because there’s nothing to prevent it from happening. On the positive side, there’s no fan filter to become blocked, which they usually do because nobody remembers to clean them, so airflow will be maintained. On the negative side, the actual cooling may become less effective as the modules gain a layer of insulation.

NI’s Mr. Webb explained that large NI chassis bring air in from the back, deflect it downwards and under the module connectors, and finally exhaust the air out the top. In chassis that support 38.25-W dissipation in PXIe slots, higher capacity 110-cfm fans are used to increase cooling. Nevertheless, the acoustic noise actually has been lowered by careful choice of a fan model and by using vibration-dampening fan mounts. Pulse-width-modulated (PWM) speed control further minimizes fan noise at low dissipations and temperatures.

The topic of fans is complicated: How many fans are used, how and where they are mounted, how they are monitored to ensure efficient operation as well as that of the power supply, and whether a chassis operates with positive pressure or with a partial vacuum are fundamental chassis design choices.

Some chassis have multiple fans mounted directly below the modules. This arrangement obviously ensures good airflow, but it could become very uneven if one or more fans failed. Electrical noise from the close-proximity fans could be a problem. These concerns are mitigated by mounting fans some distance behind the backplane and using a separately regulated fan power supply. Additionally, for positive pressure chassis, it’s common to insert dummy panels to block unused slots, diverting more cooling air to the occupied slots.

Conversely, for chassis that draw air in at the front, rather than blocking unused slots, it can be advantageous to open them to allow additional air to be drawn in. This is especially beneficial for unused slots on either side of a high-power module.

Mechanical
One of the benefits claimed for chassis that draw cooling air in from the front and exhaust out the back is a reduced rack mounting height. ADLINK Technology, Agilent, and others specify only a 4U height, claiming that this cooling arrangement eliminates the need to provision a spare 1U space above the chassis for heat dissipation.

Geotest’s 4U-high GX7600 Series draws air in from the front with two 65-cfm fans to cool the peripheral slots and a third dedicated fan for the power supply. The nine-slot chassis allocate five slots for PXI modules, one for the PXIe controller, another for the PXIe system timing module, and two more for PXI hybrid slots. The chassis also are available for use with an MXI-to-PXIe interface and an external controller.

Most of NI’s chassis also measure 4U in height, but it’s important to distinguish between the chassis height and the height occupied by the chassis when placed in a rack. According to NI PXI system guidelines, “Your chassis must be installed such that the cooling clearances meet the specifications stated in the user manual. A typical example for an NI PXI chassis with a rear air intake and top/side exhaust would be to provide a minimum of 3″ of clearance from the air intake on the rear of the chassis and 1.75″ of clearance above and on the sides of the chassis.”¹

Aside from simply saving space, if you don’t need to provision cooling clearance above your chassis, you can use a 1U industrial PC instead of an embedded controller and still only occupy 5U of rack height. Agilent’s Mr. Brannon makes this argument, explaining that a 1U PC usually costs less than a comparable embedded controller and often will use a more recently developed computer chip set. Based on this reasoning, you could assemble a lower cost and more powerful solution in the same 5U height required by a more conventional chassis plus cooling clearance.

When a chassis is mounted in a rack and populated with modules that are interconnected and wired to other things as well, it’s impractical to remove the entire PXI/PXIe chassis to replace a fan or PSU. Instead, many chassis have separately removable/replaceable shuttles containing the fans and PSUs.

The Tracewell 14-slot S32-RH PXI Chassis is a good example of this type of construction. According to the datasheet, “An integral power and cooling module with an RS-485 interface monitors all PXI output voltages, inlet and exhaust air temperature, and fan speed. The fully plugging module provides mean time to repair (MTTR) of <1 minute.”

Chassis do much more than ensure a suitable electrical and thermal environment. They also must secure and protect the peripheral modules plugged into them. To ensure a rigid structure, Tracewell’s S32-RH Chassis uses welded laminate construction the company claims provides high strength, low flex, and reduced weight. Type 5052-H32 aluminum alloy in 0.062″ thickness is used for the inner and outer chassis with 0.125″ thick rack flanges and a 0.080″ rear panel. This aluminum alloy is well suited for chassis construction, being very strong yet capable of being formed with reasonable bend radii.

PXI/PXIe-Based Test Systems

Some manufacturers have developed standard preconfigured test systems based on PXI and PXIe chassis. The Geotest basic automated test system (GBATS) TS-700 Series is a good example, available in several versions covering commercial and military avionics, digital test, functional test, and mixed-signal test requirements.

A 14-slot GX7102A PXI Chassis supports a mix of 6U and 3U modules, a system controller with 1-GB memory and 160-GB hard disk, and a 960-pin Virginia Panel iCON high-density UUT interface providing access to all core and optional system resources (Figure 3).