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Requirements for system architecture and enclosure solutions for test and measurement systems

Feature

By Alan Cook, Foremost Electronics

There are growing requirements for simulation, test and measurement tasks that are changing the requirements for enclosure and cabinet solutions. This concerns the system architecture with factors such as high computing power, fast data transfer, more accurate synchronisation of individual components (clock and trigger functions), etc., as well as requirements for mechanical properties such as improved EMC shielding, superior signal integrity, improved cooling and suitable configurations, and platform-based developments for the modularity of test and measurement systems.

Depending on the application, it is important to choose a system architecture that provides the desired properties and interfaces. In the area of test and measurement, there are several standardised system architectures acceptable for building corresponding systems. In addition, corresponding mechanical requirements must be considered, depending on the application.

Versatile applications of PXI Express

PXI Express systems are suited for the tough requirements in simulations for autonomous driving, on test stands for function tests in the area of civil and military aviation technology, and for product tests of consumer electronics during the production process. Such complex products require test systems with high data throughput and extremely precise clock and trigger signals for synchronising the functions of the test object. These devices can be easily integrated into IIoT networks and offer enough data bandwidth to connect and synchronise the test devices to and with the rest of the production line. Additionally, detailed data is transferred for quality management as well as for documentation.

Thanks to its fast data transfer rates with PCIe, paired with the implemented high-precision clock and trigger architecture which synchronises the cards with each other, PXI Express is very suitable for such test, measurement and simulation tasks.

System architecture and mechanical systems

PXI Express is an updated PXI platform that provides the user with the widely-adopted PCI Express bus. PXI Express is based on the widespread 19” form-factor and offers backward compatibility with the proven PXI.

PXI Express has PCIe Gen3 with up to eight links and supports system bandwidth of up to 24GB/s for acquiring and evaluating simulated and measured data. A sophisticated clock and trigger architecture supports synchronisation of the measured data. Backward compatibility with PXI-1 enables access to a variety of dedicated measurement and test cards. The PCI Express bus is expanded by one or more PCI Express switches to enable a larger measurement system with more than four slots. A PCIe-to-PCI bridge is required to operate the older PXI cards on the parallel 32-bit PCI bus in addition to the PXI Express cards in the chassis. If the required clock and trigger signals or the synchronisation are designed to customer specifications, then CPCI serial systems can also be used.

Schroff enclosure

Modular design of PXI Express systems

Any PXI Express system is subdivided into two areas: one includes the plug-in cards (processor cards, measurement cards, I/O cards, etc.), which are selected in a wide variety of combinations specific to the application. These represent the actual target functions of the system. The second area consists of the PXI Express chassis. It provides the infrastructure required for operating the plug-in cards such as the mechanical enclosure, backplane, cooling system, power supply, etc. Since a test application finds a variety of uses, manufacturers of test devices prefer enclosure solutions such as the nVent SCHROFF PXI Express chassis, which is modularly built upon the proven RatiopacPRO platform and requires only minor adaptations for various areas of use.

Backplane

To ensure that the backplane can be quickly adapted, it is designed to be as passive as possible, and the PCI bridge, PCIe switch and clock functions are executed as separate modules and arranged on the rear side of the backplane between or above the slots. As a result, a more compact structure is possible in comparison to active backplanes. Thanks to the modular concept, standard components can be used, and any PXI Express backplane configuration can be easily implemented.

Cooling

Depending on the area of use, there are different requirements for the cooling system. In general, attention should be given to providing uniform system cooling with a view to maximising the permitted thermal power loss per slot. The loss depends both on the permitted temperature difference and the achievable air flow. In general, the greater the air flow and permitted temperature difference, the greater the thermal power loss that can be dissipated within the system. When used, for example, as a desktop enclosure in a laboratory environment, the resulting noise emissions should also be taken into consideration. The cooling system for a PXI Express system is usually designed for the maximum power per slot to be installed. However, due to the varying power values of the individual cards, as well as the varying air impedance, the actual air distribution can differ. The cooling system can be adapted to the respective test or measurement task by running a simulation or testing the real system in advance. For applications outside of the lab, an air filter should also be installed to protect the electronics from dust and other dirt.

Power supply

It is necessary to ensure that the power adapters being used will be available, if possible, over the entire anticipated product life-cycle, which prevents costly recertification of systems. For nVent, corresponding industrial-grade power adapters with the conventional PC voltages (3.3V, 5V, ±12V and 5V standby) are used. Unlike the ATX power adapters used in the commercial area, such industrial power adapters are typically available for more than ten years.

The switch-on behaviour and correct switch-off behaviour of the various output voltages are also very important. Firstly, to ensure that the cards are in fact initiated correctly and all functions are detected by the system controller during the boot process. Secondly, it is also necessary to prevent data loss or, in the worst case, damage to a card caused by uncontrolled switch-off of the voltages when the device is shut down. In the best case, this is ensured by a separate controller in the system that switches on the voltages in the correct sequence, monitors during operation, and maintains the specified times and sequences when the device is shut down.

Hardware management

In addition to controlling and monitoring the voltages, this controller can also assume other functions. To ensure the right balance between cooling output and fan noise in every state of the system, the fans are controlled by temperature sensors. The controller uses an I²C interface to transmit the system status to the system controller, the CPU board. Here, too, it is possible to secure a number of advantages through a modular design. By taking the approach of building the system controller from a COM module and a carrier, you can flexibly replace the standard COM module with processors of varying capacity while using the same carrier. It is even possible to transition to the next generation of processors here without redesigning the carrier.

Mechanical systems: enclosures, sub-racks and cabinets

Depending on the area in which the PXIe systems are used, for example, as test and measurement applications in production engineering, in the automotive field, or in aeronautical engineering, there are different requirements for modularity, durability and EMC characteristics. It is therefore advantageous when you can rely on a broad product portfolio of enclosures, sub-racks and cabinets that are established in the market and satisfy these requirements.

At the mechanical level, then, modular platforms such as those of the EuropacPRO sub-rack and the RatiopacPRO enclosure from nVent Schroff, which are available from specialist electromechanical distributor Foremost Electronics, satisfy the test and measurement application requirements for modularity, stability, EMC protection and unique customer branding or customised aesthetics. They flexibly adapt to the modular system infrastructure of PXIe and, by having a broad standard product range, enable an enclosure solution to be implemented quickly and easily. This means every PXI Express chassis can be implemented quickly and with little risk. PXIe systems are used in various form factors – as a desktop unit in a lab and in the development department or as a sub-rack, e.g. installed in the modular nVent Schroff Novastar electronics cabinet, when multiple systems are combined.

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