USB and ground loops

USB was originally designed for desktop use, but it is increasingly used for industrial applications. For USB to survive in those environments, it needs to be made more rugged. It means that ruggedizing USB for ground loops and using better (more protection and better locking) connectors are often needed.

Ruggedize USB connections for tough environments article tells that when you connect a remote device via USB, 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, because 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 and you can get considerable noise through ground loop: ground potentials can vary and magnetic flux from motors or other high-power devices can use this ground loop to induce current noise. 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.

Traditionally, to avoid this problem many types of industrial communication standards, like RS-485, use differential signal transmission with no ground connection to avoid potential ground loops minimizing induced noise in the system and potential for data errors. However, with standard USB cables the ground connection cannot be avoided.

Ruggedize USB connections for tough environments article recommends that in industrial environments, computers and connected devices should be protected with USB isolators Isolation allows the lines to float while keeping the local side at the proper ground and signal level.

Technically implementing such USB isolator has some difficulties because it is not easily isolated using discrete digital isolators. Isolating a USB port to eliminate the cable ground connection is inherently difficult because there are no control signals to indicate whether data is being transmitted downstream (to the peripheral) or upstream (to the host). There are several possible approaches to isolating USB. Generally it is a good idea to use ready made USB isolators. And before rushing to buy one, check carefully when technical characteristics of them, for example the USB version and communication speeds they support (some isolators can only support USB 1.1 and lower communication speeds, not the full speed).

You need to think also the electromechanical characteristics of USB interface on industrial environment. The same easy connection and extraction that makes USB so useful in an office environment can be a problem when you take USB off the desktop to industrial applications. Manufacturers have started to address the problem with “high-retention” USB ports (requires more force to dislodge them), ruggedized USB cables, sealed industrial USB connectors and ports with thumbscrews.

1 Comment

  1. Tomi Engdahl says:

    What’s Inside a USB Isolator?
    http://hackaday.com/2014/04/21/whats-inside-a-usb-isolator/

    [Lindsay] decapped a USB isolator to take a look at how the isolation worked. The decapped part is an Analog Devices ADUM4160.

    Reply

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