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Prevent Hindenburg-level USB meltdown in harsh industrial environments

Posted: 05 Jul 2012 ?? ?Print Version ?Bookmark and Share

Keywords:Hindenburg? USB? electrostatic discharge? electromagnetic interference?

USB cables aren't helplessly vulnerable to electromagnetic interference (EMI). They do use twisted pair wire on the data lines and there should be some copper braid and aluminum shielding (figure) In desktop applications any EMI issues will be insignificant. It's a different story when a length of USB cable is exposed to the strong magnetic fields associated with industrial motors. Powerful currents will be induced on the USB cable and the results may include anything from data loss and downtime to burned circuitry in connected devices.

Arcing
One of USB's great advantages is simultaneously one of its weaknesses. USB hubs provide 5 VDC power to downstream devices at up to 500 mA. Current is allocated in units of 100 mA, and devices may draw up to 500 mA per port. If the hub is bus powered, it can't provide more than four 100 mA units of current to downstream devices, as it needs one unit for itself. And it can't have more than four downstream ports. But if a hub has its own power supply, it can provide the full 500 mA to every port. (Computers normally have a powered USB hub built in.)

This ability to power gadgetry like cameras and external hard drives via a USB port is quite convenient. But conditions can get rough in the industrial world. Shock and vibration are common occurrences and USB cables can work themselves loose. They can also be dislodged accidentally, when workers are moving things about. When that happens, the same 5 VDC that is so useful in benign environments becomes a serious fire hazard. If it arcs, and flammable gases or materials are present, you've got trouble.

Solution: Range extension
USB doesn't have great range. Even with hubs, the best you can do is 30m. As cable is normally laid above, below and around working spaces, and as industrial working spaces can be very large indeed, 30m isn't much. Your network will most likely involve various kinds of extension and conversion, and some of them automatically resolve USB's electrical issues while they're extending its range.

Fiber optics, for example, not only give data communications incredible range, they eliminate the risk for grounds loops, power surges and EMI. As data travels on a beam of light, rather than a copper core, fiber optic cable is immune to electrical issues of any kind. Single-mode fiber has the greatest bandwidth, but is also more expensive than multimode. And in both cases the cabling, transceivers and receivers are more expensive than the materials used in a copper cable installation. A large part of any installation, fiber or copper, is actually going to be labor cost. Fiber's immunity to damaging electrical currents, and the protection it provides to connected devices, can more than make up the difference.

Solutions: Wireless
Wi-Fi is rapidly becoming another viable option for industrial installations. Wireless networking used to be plagued by issues like multipath propagation, the phenomenon that occurs when transmitted signals bounce off intervening objects. Different parts of the signal would arrive at the receiver at different times and out of sequence. If multipath propagation got bad enough, early Wi-Fi devices couldn't distinguish between the signal and the noise floor. The current 802.11n Wi-Fi standard provides for the use of multiple input multiple output (MIMO) technology.

MIMO devices anticipate multipath propagation and turn it into an advantage. They deploy multiple antennas at both the transmitting and receiving sides of the wireless connection, and they split the data into numerous spatial streams. The streams are transmitted through separate antennas and collected by corresponding antennas in the receiving devices, where onboard software uses signal-processing algorithms to correct and interpret incoming data. The new Wi-Fi devices also use such precoding and postcoding techniques as spatial beamforming, and the 802.11n standard adds frame aggregation to the MAC layer. These new techniques give Wi-Fi the range and bandwidth it needed to become a reliable option for M2M data communications and industrial networking. And, as is the case with a fiber optic link, a radio link creates a barrier that damaging electrical events cannot cross.

Solutions: Isolation
There will be many situations in which fiber optic cable and Wi-Fi connections will be impractical. But manufacturers have found several ways to interpose isolation in data communications streams. Isolation works by altering the signal on USB D- and D+ lines and transforming the 5 VDC power on the other pair. The isolator converts the data signal, either to pulses of light that work like a very short fiber optic connection, or to an electrical field. It then converts it back to an electrical signal again. Data can pass through, but the isolator stops power surges and ESDs at the isolation zone. The isolator controls surges and ESD on the power line by transforming the 5 VDC USB power to AC, then back to DC.

Isolation has a minor disadvantage. USB devices default to Full-Speed (12Mbit/s) until they are able to negotiate a Hi-Speed (Up to 480Mbit/s) connection rate with the USB hub. The USB device initiates the negotiation by driving 17.78mA into the D- data line for at least a millisecond. The connected hub responds by alternately injecting 17.78 mA into the D- and the D+ lines. If the USB device detects at least three of these "chirp pairs" it will decide that the hub is Hi-Speed capable, and it will establish a Hi-Speed connection. Isolators, however, interfere with this negotiation when they convert the DC signal to AC at the isolation zone. That makes the negotiation fail, and the USB devices will default to Full Speed. That's fast enough for most industrial applications, of course. Leaving your devices unprotected is unwise, yet until things go wrong, unprotected devices can establish some very fast connections.

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