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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?

There isn't much you can do to ensure that every manufacturer, everywhere, takes the USB specification seriously. But you can resolve the problems associated with under-powered USB ports by installing devices like powered USB isolators or powered USB hubs. They'll boost the USB signal up to the full 500 mA.

Ruggedizing USB for ground loops
It's very useful to be able to connect a remote device via USB. But the greater the distance between the connected devices, the more likely it becomes that communicating devices will be getting their power from different building ground references. When they do, the USB cable's ground wire can create a ground loop path.

In home/office environments the ground loop problem is rarely an issue. The connected devices will normally be quite close, usually sharing a wall outlet and a common ground. But industrial installations will be far more complex. Imagine a situation in which a process control system is powered from a source in one building location, but the front panel is powered somewhere else. When you connect a PC to the front panel via USB, powering the PC at the same location, you can create a ground loop with the process control system, which may be hundreds of meters away. Magnetic flux from motors or other high-power devices can use this ground loop to induce current noise on the ground. If you're lucky the only result will be data transmission errors. More extreme events, like surges and voltage overloads, will burn out integrated circuits and connectors.

In industrial environments, computers and connected devices should be protected with USB isolators. Ordinary surge suppression tries to limit spikes between the signal and ground line. But if the ground line rises, as it does in ground loop situations, then surge suppression won't stop it. Isolation allows the lines to float while keeping the local side at the proper ground and signal level. An isolator controls electrical flows while simultaneously ensuring that your data stream is not interrupted. You can deploy dedicated inline USB isolators as well as USB devices that perform additional functions while including isolation, like isolated hubs or serial converters (figure).

Figure: Inline isolators protect equipment from spikes, surges and ESD.

Isolation works by changing the nature of both the data signal and the 5 VDC power carried by the USB cable. The data enters the isolator as an electrical signal, is converted to either pulses of light or an electrical field, then back to an electrical signal again. The data flow is uninterrupted, but power surges and ESD are stopped cold at the isolation zone. The isolator protects against surges and ESD on the power line by briefly transforming the 5 VDC USB power to AC, through an isolation transformer, then back to DC again.

Ruggedizing USB cables
USB cables have some nice features. For example, the specification deliberately made it very hard to attach a USB connector incorrectly. The host devices that supply power use a type A connector and the devices that receive power use a type B connector. They can't be reversed. Users can't accidentally connect two USB power supplies and create a risk for fire or damage to expensive circuitry.

The specification also required that the power connections in the type A connector should be on the outside, and that the data connections (D+ and D-) would be on the inside, and recessed. This meant that the power connection would be established first, thus preventing data errors.

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