ENCLOSURES
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.
Modular design of PXI Express systems
Any PXI Express system is subdivided into two areas. One area 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 airflow. In general, the greater the airflow 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. This prevents costly recertification of the 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 10 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
36 DECEMBER/JANUARY 2023 | ELECTRONICS TODAY
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 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.
nVent SCHROFF products are available from specialist electromechanical distributor Foremost Electronics.
Foremost Electronics
www.4most.co.uk
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