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Ruggedize USB connections for harsh environments

Posted: 19 Apr 2012 ?? ?Print Version ?Bookmark and Share

Keywords:USB? port? adapter? electromagnetic interference? isolators?

When a technician connected his laptop to a customer's piece of industrial machinery he got a nasty surprise. There was a quick puff of smoke, and he lost his hard drive, his USB port, and the USB adapter that he'd connected to the machine.

The problem, he soon discovered, was that while his laptop was plugged into the wall, the machine he was servicing was connected to the shop's single-phase power supply and its leads had been reversed to keep it from tripping a GFI switch. As the laptop and the connected device now had different ground potentials, the USB cable became the path to a lower ground state. It was an expensive lesson.

A kiosk manufacturer plugged a card reader into the kiosk's built-in computer. Everything was powered up, and all of the connections looked fine, yet the two devices wouldn't communicate. The manufacturer's natural inclination was to assume that either the card reader or the cable was faulty. But that wasn't the issue. The problem lay in the kiosk's built-in computer. It was a low cost single-board device, and its USB ports were not providing full power.

An injection molding machine manufacturer found a way to enhance part quality and cut material costs by regulating the temperatures of multiple heating elements. The process created a powerful electromagnetic field. The controlling computer could be kept at a safe distance, but the USB hub and cables were inside the field, and the resulting electromagnetic interference (EMI) was a communications nightmare.

"You can expect all kinds of problems when you're working with USB," says Mike Fahrion, director of product management for B&B Electronics, and a specialist in data communications. "It was originally designed as a standard bus for connecting computer peripherals in safe home and office desktop environments. But its cross-platform, hot-swappable interface turned out to be so useful that USB now appears in everything from emergency medical equipment to precision agricultural guidance systems; environments that it was never designed to handle."

The initial intent and the result
USB was intended to connect mild-mannered home and office devices like printers, keyboards, media players, cameras and external disk drives. And it wouldn't just connect them; it would carry 5 VDC power as well, in many cases eliminating the requirement that the peripheral devices include their own power supplies. In fact, rechargeable devices would be able to use USB to restore their internal batteries to full power.

Devices would be able to draw up to 5 unit loads from a USB port, with a unit load in USB 2.0 being defined as 100mA. "Low-power" devices would draw one unit load or less (under 100 mA). "High-power" devices could draw the full 500 mA. Additionally, devices that required more than 500 mA could be equipped with a Y-shaped cable that would enable them to draw power from two USB ports at the same time. Devices with internal power, like printers, would register as "low-power" devices, requiring only 100 mA. Devices would initially function at low power, but would be able to request high power if needed. That was the specification, at any rate.

Ruggedizing USB power
In reality, unfortunately, it turns out that designers and manufacturers don't always rigidly adhere to the USB specification. That single-board computer in the shopping mall kiosk, for example, wasn't providing 500 mA for high-power USB devices to the USB port. It was providing 100 mA, just one unit load. A connected device with internal power would function properly, but a high-power device requiring 500 mA would not. The same weakness appears in a lot of devices; common examples would include some of the cheaper netbooks. As the number of applications calling for USB continues to increase, so will the number of instances in which devices fail to function correctly due to the lack of full 500 mA power at the USB port.

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