Bringing USB to the PXI Test Environment

    Much has been said about the integration of PXI products with LXI and VXI to create hybrid test systems, taking the best of each of those standards and combining it with the premier standard for modular instrumentation. Pickering Interfaces has certainly seen much synergy between PXI and LXI for switching applications with LXI offering better solutions for large switching systems with high performance and PXI offering better solutions for smaller and more diverse switching systems

    Very little has been said about combining PXI and USB products in a test system though - yet USB is an extremely common interface on consumer and professional products, and ubiquitous on many mobile devices. That means that it may be native control connection on a device that needs testing - so a test system ideally needs the capability of supporting USB.

    Diagram of module 40-737-901
    The 40-737 is a relay based USB switch that allows a front panel USB common port to be connected to any one of 8 USB output connectors
    Front panel of 40-737

    The USB standard has also been used in creating elegant and cost effective solutions for test purposes, for example USB based RF power meters are available from a number vendors while others have created data acquisition systems and even basic scopes and DMM's. These instruments often benefit from being able to be supported by USB power alone, avoiding the need for AC power sources. They are therefore typically instruments which have modest power requirements and size.

    They can also typically be placed more directly on a connection to be measured, for example a power meter sensor will be positioned at the RF connector, rather than be fixed in a chassis. USB devices can be placed closer to the point of measurement than devices that are chassis mounted as long as they are within the reach a distance of 5 meters. That means that USB devices can bring a different set of features to a test system, and as with LXI there are advantages in being able to add USB to a system with PXI content.

    Clearly a USB device can be added to a test system by using the USB ports on the system controller. If the system controller is external to the chassis that means the USB devices are not part of the chassis system. If the PXI embedded controller is used chances are it has limited numbers of USB ports because of the limited panel space so it requires the addition of a separate USB hub - and another AC power connection to power it. That is not always convenient.

    In the case of a device to be tested it may not be a wise move to connect a test device directly to an expensive PXI controller - whether embedded in the chassis or remotely connected. Devices to be tested are expected to have faults or there would be no need to test them, and some of those faults could prove damaging to the controller. Odd things can happen as well when a faulty device is added to a system, including hanging the controller up.

    USB has also gained a reputation for very occasionally not being very robust, for example failing to connect properly to the controller. In manual systems this is easily fixed - simply unplug and reconnect - and many users have had this experience of USB. It is not quite as simple in a test system, they are intended to be mostly automated and manual intervention is not a help test system throughput.

    Pickering Interfaces has provided PXI based solutions for USB switching, the 40-737 for example offers a USB switch which allows a single USB connection to be switched to one of 8 USB devices. That allows a test system to programmatically select one out of a number of USB devices to connect into the USB common by using Pickering Interfaces standard switch driver

    It does not however allow simultaneous connection to all the USB outputs - that requires a USB hub - since it simply provides a means connecting a USB "common" to one of 8 USB output connections through a mechanical relay based switch. The 40-738 USB hub from Pickering Interfaces seeks to change that and adds the ability to simply attach USB test devices and USB test equipment directly to a PXI chassis.

    40-738 USB Hub

    Functional diagram of module 40-738

    The 40-738 USB hub allows a USB connection to one or more USB outputs to be made from the PCI bus of the PXI system controller

    Front panel of 40-738

    The 40-738 is a single slot 3U PXI module that combines an 8 port USB 2 hub and USB data/power switching to provide a controlled connection between the PXI controller's PCI interface (the PXI backplane) and USB products. Unlike the 40-737 the 40-738 connects to the PCI backplane of the PXI chassis rather than a USB port on the controller. The 40-738 is connected via the chassis PCI bus to the controller's internal PCI bus in the same way as other PXI modules are. The hub appears on the controller's device list in exactly the same way as any other USB hub would, but the 40-738 is fully integrated into the chassis.

    USB products connected to the 40-738 initiate the same driver install procedures familiar to all USB users when first connected. The PXI controller, remote or embedded, recognises the USB device and installs the vendor's drivers as though the product was connected to the controller's native USB ports. The process is entirely familiar to all PC users, it requires no software support from Pickering Interfaces.

    The 40-738 offers additional control features, it provides a means to independently connect or disconnect the USB data and power connections, allowing the user to simulate fault conditions on the USB interface or to completely disconnect the USB product - as though the USB connector had been physically unplugged - under programmatic control through Pickering Interfaces PXI switch drivers. 

    As well as providing a programmatically managed USB connection, the 40-738 eliminates the need for free standing powered hubs in a system - it creates a system where USB connectivity is provided through the chassis rather than separate USB hubs. This method also assures users that the connected USB device on a physical port is released at the time of disconnect.

    Summary

    In addition to aiding the testing of USB products the 40-738 allows the addition of USB instrumentation to be added to PXI - such as power meters, scopes, and data acquisition products - expanding the range and types of test products that can be supported. In some cases this adds new instrumentation capabilities such as power meters, in other cases it adds the possibility of integrating products such as low cost USB data acquisition wired directly into the chassis instead of into the controller.

    The 40-738 USB Hub simplifies the integration of USB devices and test equipment into the PXI test environment and represents a significant expansion of user choice for adopters of PXI test solutions.

    Diagram showing the connections the 40-738 can make to USB test equipment
    Diagram showing the connections the 40-738 can make to USB test devices

    Other useful links:

    Controlling LXI from USB

    USB and Ethernet Switching

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