Hindenburg-Level USB Meltdown
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 (12 Mbps) until they are able to negotiate a Hi-Speed (Up to 480 Mbps) 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.
Isolation can be added to your network in many different ways, and in many different places. You can use in-line isolators that protect just a single piece of equipment. You can use isolated USB hubs that protect many devices at the same time. You can install isolated Ethernet servers, isolated repeaters, isolated expansion cards, and heavy-duty isolators for DIN-rail mounting. You can even get USB “key” isolators that you can tuck into your laptop bag.
Solutions—High-Retention USB Ports
Manufacturers have strengthened USB’s physical connections by introducing high retention USB ports. High retention ports look much like the USB ports installed in office-grade equipment, and they’ll work with any USB cable. Connecting and disconnecting cables feels pretty much the same, yet they grip cables much more firmly than a standard port. A typical high retention port can resist 3.4 lbs. of force. You won’t dislodge the cable with vibration, or by brushing up against it.
Some of USB’s features may seem to be design flaws when USB is used in harsh environments. But they’re all easily addressed via conversion, isolation and the use of high retention USB ports. Just remember the laws of physics, think about electrical potentials and plan ahead. There’s no reason that your expensive equipment should ever have to share the costly, untimely and unnecessary fate of the Hindenburg.
Hindenburg-Level USB mishaps
Whether the proximate cause was a ground loop, ESD, EMI or even a lightning strike, current surges can destroy electronic circuitry. The following photographs demonstrate what can happen in industrial environments when USB connections are used without appropriate precautions. Isolate. Convert. Extend. Connect.
About the Author
Brian Foster is the product manager for the serial and USB product lines at B&B Electronics and an expert in network reliability at the physical layer. Before joining B&B Electronics he held U.S. Navy staff command positions in Japan and Washington State, where he was responsible for submarine communications throughout the Pacific and Indian Ocean regions, including satellite-based Internet Protocol systems, LF and VLF command and control networks. Foster’s career in data communications began in the Navy’s submarine service, where he served in three different nuclear boats managing their internal networks as well as external communications.