Big brother: AXIe extends PXI performance

4PXI, and a larger format standard, AXIe, are causing a revolution in the instrumentation industry. Traditional “box” instruments with knobs and displays are giving way to faceless modular instruments that are controlled by a computer, such as a desktop or laptop, or even embedded within the modular chassis itself. With this new format come speed, size reduction, flexibility, and lower cost.

In the late 1970s, Hewlett Packard (HP) revolutionized the measurement industry when it introduced a cabled 8-bit bus labeled HP-IB. Adopted by the industry, it was ratified as IEEE-488 and renamed GP-IB, the General-Purpose Interface Bus. It allowed general-purpose instruments from any vendor to be governed by a computer, known as the controller. With this came an explosion of automated test applications in design, manufacturing, and , and GP-IB became a keystone of the modern manufacturing floor.

While most of these products were traditional instruments, some were internally modular. Users could pick and choose modules to configure a system, for example, by controlling the electronic switching of signals between the Device Under Test (DUT) and the racked instruments in a test system [1]. However, these modular instruments were proprietary - a user could not combine modules from different vendors into the same system.

Unlike industrial computing buses that are integrated by sophisticated engineering teams to create other products, instrumentation systems are often integrated by end users who expect "plug-and-play" operation. Thus, the rules for system creation are key to system developers in the test space, but there were no standards in place that enabled an easy integration experience between modular instrument vendors.

This all changed in 1987 when HP, Tektronix, and three others announced the VXIbus standard, simply called VXI. VXI itself was an abbreviation for VME eXtensions for Instrumentation. Leveraging the already popular VME industrial computing standard, VXI added extensions vital for instrumentation: timing, triggering, configuration rules, and communication protocols that eased integration. VXI also added a deeper 6U board size that suited the measurement industry better.

An industry consortium was formed that allowed any vendor to offer products in the VXI format. This formula become the norm for future standards as well: choosing a proven industrial bus architecture, adding instrumentation extensions, forming a consortium to manage the specification, and putting a big X in the name for "eXtensions."

While VXI found wide adoption in mil/aero applications, it was the advent of the PXI standard, proposed by National Instruments in 1997, that really ignited the industry. Formed atop the (cPCI) standard, PXI added almost the exact same triggering and timing buses that were found in VXI, and offered 3U and 6U sizes (the 3U size became very popular for many data acquisition applications, and is the most prevalent module size today). In 2005, PXI was upgraded to PXI Express () by adopting the PCI Express (PCIe) high-speed serial bus. PXI and PXIe are both still known generically as PXI, with both variations of modules often combined within a single chassis.

The PXI standard enabled big speed increases in test systems, not just because of the improved bandwidth of PCIe, but also because of the software programming metaphor. While traditional instruments have largely replaced GP-IB with LAN interfaces based on LAN eXtensions for Instrumentation (LXI), the programming metaphor remains exactly the same: sending English-like ASCII commands that are parsed and interpreted in real time by a processor inside the instrument. What ASCII programming brings for compatibility, however, it loses in speed. Many instrumentation systems consist of thousands of single measurement commands where measurement latency time - not the bus bandwidth - is the principal bottleneck to test system speed. ASCII interpretation speed is typically measured in milliseconds, while a memory-mapped architecture like PCIe brings latency time down to microseconds. Indeed, many instrument classes have shown dramatic speed improvements when moving to the PCIe architecture, often exceeding an order of magnitude. This test speed improvement leads directly to a lower cost of test in production environments, as the number of testers is proportionally decreased.

With Moore's Law, the small PXI form factor has been able to replace many traditional instruments. Outgrowing its box counterparts in the marketplace by double digits for nearly a decade, it is now positioned to become the mainstream architecture of automated test. Its case was bolstered in 2010 when Agilent Technologies, previously a PXI skeptic, entered the PXI market with over 40 products, including capabilities not present in box formats. The transition to modular instruments has not only reached a tipping point, it is accelerating.

AXIe, PXI's big brother

While Moore's Law has certainly enabled the small PXI form factor to address a growing number of applications, it can also be a barrier. A good portion of the test industry is focused on digital verification and test, which is driven by ever increasing test speeds and channel counts (also enabled by Moore's Law). This requires power, cooling, and circuit density. Engineers knowledgeable of industrial bus architectures will be familiar with this conundrum, as it is one of the principal reasons for the variety of board sizes in industrial computing, including large-sized () boards.

With these applications in mind, the standard was born. Based on the ATCA architecture, AXIe is an abbreviation for for Instrumentation and Test. AXIe adds an instrumentation-oriented bus topology to the family of star and mesh topologies available in ATCA in a way that retains backwards compatibility. Limiting the maximum chassis size to 14 slots instead of the 16-slot maximum ATCA size, hundreds of pins are freed on the backplane that are repurposed for key instrumentation functions, including high-performance timing, triggering, and local bus.

AXIe is often referred to as the "" of PXI because, despite its much larger module envelope, it acts logically as a PXIe system. Communication with external or embedded controllers is performed over the backplane through a 4-lane PCIe bus interface. To a controller, an AXIe system is just another PXI chassis, though one with much larger board area and power capability. The continued viability of VXI demonstrates the need for large module formats, and AXIe offers 14 percent more circuit area on a module than the aging VXI standard, and several times that found on PXI.

The AXIe bus extensions

AXIe's bus extensions bring powerful new features to the test and measurement industry. Twelve parallel trigger lines are routed to each slot. Using Multipoint LVDS (MLVDS) signaling, these clocks are capable of synchronizing events up to 100 MHz across each slot of the backplane. When more precision is needed, there is a radial or "star" bus that routes signals to and from each peripheral slot from the hub slot. The star bus structure allows very low jitter and very high frequency, and can be used separately or in conjunction with the parallel trigger bus.

One of the most powerful features of AXIe's bus extensions is the local bus. Local buses are a set of short copper segments that connect pins between adjacent slots (hence, "local" buses). For example, the right side of slot 2 is connected to the left side of slot 3; the right side of slot 3 is connected to the left side of slot 4; and so forth. Because the copper path lengths are short (only a few centimeters), very-high-speed signals can be routed from slot to slot, enabling high-speed private communication between related modules of an instrument set. Since the local bus begins and ends between the modules of an instrument set, another set of modules may use their local buses completely differently. This way, instrument sets from different vendors may co-exist in the same chassis, each exploiting the unique capabilities of the local bus.

The AXIe local bus consists of 124 lines organized as 62 high-speed differential pairs. With 60 pairs dedicated to a data path, and using fairly common Serializer/Deserializer (SERDES) technology, the aggregate bus bandwidth totals 40 GBps (320 Gbps) from one slot to the next, which is an order of magnitude higher than other bus structures (Figure 1). Since AXIe only defines the topology of the local bus, the aggregate bandwidth will increase proportionally with the interface bus speeds deployed.

Figure 1: Pictured here are the additional bus extensions AXIe adds to ATCA. In red are 12 parallel trigger lines capable of 100 MHz synchronization. Black figures represent radial or "star" clock and trigger lines for very-low-skew and -jitter applications. Blue shows the structure of the high-speed "local bus" capable of 40 GBps (320 Gbps) data transfer between adjacent slots.

The Guzik family of digitizers from Guzik Test and Measurement were the first products to exploit AXIe local bus capabilities. The Guzik 6000 series is essentially a family of high-fidelity 8-bit digitizers based on an aggregate 40 GSps sampling rate. Moving data from left to right, any combination of digitizers, digital processing, memory, or waveform generators may be included in a system, as long as any one local bus link is within the 40 GBps limit (Figure 2). As FPGA speeds increase the local bus capacity will also increase, making 160 GBps very feasible in the future.

Figure 2: This example data flow for local bus-enabled modules shows a digitizer sourcing continuous data at up to 40 GBps (320 Gbps). A () module may process the data, stream it to an external RAID array or processor, or store the waveform in local memory. Alternatively, the DSP module may stream waveform data to an arbitrary waveform generator. Any combination of modules may be deployed as long as the local bus data rate does not exceed 40 GBps. With increasing FPGA speeds the link capacity limit will grow as well. Not shown is that all modules also have a PCIe connection.

Other powerful AXIe instruments and systems are available from ADLINK, Aeroflex, Agilent Technologies, Gigatronics, and Test Evolution. AXIe modules are typically mounted horizontally in a chassis, allowing very short chassis heights and maximum rack density (Figure 3). The availability of 2-slot and 5-slot horizontal chassis allows system integrators to deploy AXIe either as the principal modular architecture in a system, or complementing a PXI or VXI subsystem. In particular, small horizontal chassis avoid the typical "two half-filled" chassis drawback of deploying two vertically oriented architectures in a single system. A single 48 V backplane power supply enables modules up to 200 W to be powered from just over 4 A, supporting high-power components found in high-speed digital and data converters.

Figure 3: AXIe is often deployed in horizontal chassis, requiring minimum rack space within a system. Pictured are a 5-slot chassis deployed with a two-channel 12 GSps arbitrary waveform generator from Agilent Technologies, and a 40 GSps digitizer from Guzik Test and Measurement.

New tools to aid AXIe development

The availability of small horizontal chassis has allowed vendors to deliver AXIe-based solutions without the traditional issues that new standards face in achieving critical mass. Indeed, vendors now offer a range of digitizers, waveform generators, semiconductor test equipment, and digital development tools. Since an AXIe subsystem acts logically as a PXI subsystem, the small rack height and PXI compatibility allow AXIe systems to leverage critical mass from its smaller scale brother.

Nevertheless, AXIe faces issues that previous instrumentation bus standards avoided. In particular, the robust system management features of the Intelligent Platform Management Interface () bus leveraged from ATCA present extra development effort to new vendors or end users wishing to develop modules or chassis in the AXIe format. Two new vendors are now offering solutions to this quandary.

Elma Electronic is well known in the industrial bus industry, and now offers AXIe products as well. For chassis developers, Elma Electronic has developed an AXIe IPMI Shelf Manager card with a redundant Intelligent Platform Management Bus (IPMB). The Shelf Manager's system monitoring features include power management, cooling control, event sensor logging, electronic (e-keying), and module hot-swap monitoring.

For module developers, Elma has developed an IPMI controller mezzanine card that fits onto a standard AXIe- or ATCA-based module. This card allows users to quickly deploy the required IPMI functionality, freeing engineers to focus on the unique value adds of their designs.

Elma also intends to offer chassis for next-generation AXIe development and deployment. The company believes its unique insight into the worlds of both AdvancedTCA and AXIe world enable it to offer unique and innovative solutions.

For AXIe module developers requiring a more turnkey development environment, Hiller Measurements plans to release the MiAXIe AXIe Hardware Development Kit this fall. The MiAXIe is essentially a near-empty module with a complete AXIe backplane interface and software driver set. A user merely develops a custom PCB and plugs it into the forward-facing connector on the AXIe interface board at the rear of the module.

Hiller sees the MiAXIe development kit being useful in mil/aero applications where users have functionality in VXI or other formats and wish to migrate to the modern AXIe platform. AXIe's module size and power capability allow it to deliver specialized loads, dense electronic switching, or Radio Frequency Interface Units (RFIUs) on a single module instead of a complete chassis. The rear interface delivers a number of relay drivers, programmable voltages, and -to-Digital Converters (ADCs) to further accelerate module development. Hiller also offers custom engineering design services.

Moving to modular

PXI, with its small size, fast bus, and numerous vendors has captured the industry by storm. Add to that the power of AXIe, now enabled by new development tools, and open modular systems can address a higher percentage of applications than ever before. With the test and measurement industry transitioning to modular instrumentation, PXI and AXIe have a bright future.

Larry Desjardin, President of LLC, is an independent expert in the modular instrumentation industry. Larry is currently a member of the VXI, PXI, and AXIe Consortiums, and holds a BS in Engineering from California Institute of Technology, and an MSEE from Stanford University.

Modular Methods

[1] Test specifications often use the term “module” in place of the term “blade” or “board.”