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Extending USB to Time-Critical Applications

March 1, 2006 By: Peter Foster, Chris Svensrud, Fiberbyte Sensors

By adding synchronization, timing, and determinism, USB is moving beyond PC peripherals and into time-critical DA.

Rack-based systems such as VXI and PXI have long been the mainstay of data acquisition and control systems. Due to the need for greater flexibility and reduced cost of ownership, there is a trend away from the bulky and expensive centralized rack-based architecture toward a portable platform based on the PC, with connectivity and expansion provided through the Universal Serial Bus (USB) interface. Although USB is the most popular external PC interface, dominating the peripheral connectivity market, its lack of determinism has prevented its wider application. Until now.

USB Basics

USB was collaboratively designed by a consortium of industry leaders as the next-generation connectivity interface for the PC. USB did not need to be compatible with legacy interfaces and was therefore optimized for ease of use, reliability, speed, and flexibility while cost of ownership was kept to a minimum.

Designed as a true plug-and-play interface, USB automatically connects multiple devices of different types from different manufacturers and seamlessly controls their enumeration and configuration. Because it is a hot-swap interface you can connect devices on the fly without needing to turn off the computer.

USB is an "asynchronous" bus that allows multiple devices to sequentially interact with the host PC. The nature of the USB protocol prevents devices from interacting with each other in a deterministic way. As a consequence, USB by itself has a very limited capacity to synchronize events or processes on multiple devices and has no provision for providing accurate timing information to multiple devices. Within the standard USB framework there is no ability to provide synchronous real-time control or data acquisition which means that deployment of USB in real-world test, measurement, and automation applications has been limited to single device data-uploading.

A New Twist on USB

USB-inSync technology has been developed to transform USB into a synchronous, deterministic, and expandable control platform. Think of it as an extension to USB that combines the widespread compatibility features of USB with the advanced timing and software features required for an industrial PC I/O communication platform. USB-inSync devices are USB-compatible and possess all the standard connectivity characteristics of USB in addition to accurate synchronization and deterministic timing capabilities. The technology lets you turn your PC into a highly deterministic distributed control platform, enabling the development of synchronized systems for distributed test and measurement, data acquisition, automation, and manufacturing applications.

Synchronization and Determinism

Each USB-inSync device is equipped with a local clock that is accurately phase-locked to that of every other USB-inSync device attached to a given host PC. A deterministic triggering system enables processes to be started and stopped in a precisely controlled manner. The onboard FPGA in each device executes the system triggering while distribution is accomplished via the synchronous clocks; the whole is controlled by software on the host PC.

In passive USB-inSync (so-called because the devices automatically lock to each other without needing any additional equipment), all local device clocks are passively synchronized to within ±5 ns of each other when connected to a common expansion hub (Figure 1). Up to seven USB-inSync devices can be passively locked to within ±5 ns of each other by attaching them all to a generic USB expansion hub.

Figure 1. Passive USB-inSync
Figure 1. Passive USB-inSync

Although all USB-inSync devices automatically lock their clocks together in frequency, there may be a small phase offset between clocks when the devices are located at different points along the USB connection tree. The phase offset for a typical highly distributed passive configuration is on the order of 100 ns.

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