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.

Some people require much tighter timing capabilities than this. USB-inSync can actively compensate for these phase offsets and synchronize all clocks to within ±5 ns regardless of their connection location. This is called active USB-inSync and it uses a USB-inSync master hub to measure and compensate for the clock phase offset of each device (Figure 2). This is all done through the standard USB protocol and does not affect the operation of other normal USB devices, such as a mouse or a printer.

Figure 2. Active phase compensation

Notice in Figure 2 that only one USB-inSync master hub is depicted. The master hub operates as a timing controller to phase-align all device clocks, regardless of the number of standard USB expansion hubs used. This phase correction process occurs automatically during the device enumeration process and is repeated each time a new device is connected to the system, maintaining accuracy while preserving USB's hot-swap capability. As soon as the host PC's Windows 2000 or XP operating system recognizes the device, it is available for synchronous operation.

The system can be tied to an external reference clock signal of known accuracy to provide a reference clocking signal that can then be distributed around the plant. This enables you to specify the absolute accuracy of your own clock reference (perhaps accurate to 1 ppm). All USB-inSync clocks will then lock to the supplied frequency.

Deterministic control of multiple devices is achieved by harnessing structures that are already present in the USB, but by using them in new ways. Command sequences are queued within the separate devices and executed upon receipt of specific data structures that are broadcast to all devices. Those data sequences are also phase compensated to ensure trigger accuracy of ±5 ns across the system.

Distributed Synchronous Advantages

Existing architectures have limited expansion that is normally dictated by the number of bays in the mainframe chassis. Once the chassis is full, synchronous expansion can become very difficult. For instance, if you have 16 inputs attached to your 16-channel data acquisition device you can't expand unless you buy a 32-channel device. With USB-inSync you simply add another 16-channel device and synchronization is automatic.

A centralized architecture may result in many hundreds of control lines for a moderate-sized plant. Cabling looms can be expensive to install and maintain and the many mechanical connections can introduce possible signal integrity issues. USB-inSync can often reduce these large cabling looms to a single USB cable. The USB-inSync device then provides the "branching point" somewhere within the plant where multiplexed digital commands are deterministically routed to the many local control signals required in a given installation.

Distribution of the test equipment can also reduce unwanted noise. A centralized measurement system results in signal wires being fed back from distant equipment. This is far from ideal, and sensitive measurements can be polluted with noise from nearby machinery. When a USB-inSync data acquisition unit is placed next to the signal source on the factory floor, it synchronously acquires data and then transfers the results back to the central PC through the noise-immune and error-corrected digital USB interface.

The Limits of USB-inSync

One of the main drawbacks of USB is the limited range of the interface. USB was designed primarily as a desktop interface with a maximum range of 30 m from the PC. This means that, in principle, a USB-inSync synchronous network is limited to a circle with a radius of 30 m. Numerous methods have been developed to increase the range of USB to well over 120 m and some of these may also be applicable to USB-inSync.

Another limitation of USB is the number of devices that can be attached. The USB specification defines a total of 127 discrete devices that may be attached to the bus. This includes expansion hubs; in practice, almost 100 usable devices can be attached. New technology under development allows synchronization of multiple PCs, which effectively means that an unlimited number of devices can be used in a synchronized system.