Are All USB Cables Created Equal?

Are All USB Cables Created Equal?

December 2010 - Are All USB Cables Created Equal? Recently we found out that they are not! The USB-A to USB-B cable is one of the most common cables you will find in the lab or attached to your computer at home. Typically five to six feet long, they may be used to connect your home printer to your desktop computer. In fact Contemporary Controls includes one with the BAS Portable Router. The BAS Portable Router allows BACnet MS/TP messages to be sent to a BACnet/IP Ethernet device. It is a popular BACnet commissioning tool because a laptop computer running BACnet/IP client software can access a BACnet MS/TP field network with ease.

However, the BAS Portable Router requires power to operate and uses the USB port on the laptop computer for this purpose. There is no communication over USB – communication is over Ethernet – only power is derived over USB making the USB port very handy.

Recently in the lab, our engineering intern noticed that several BAS Portable Routers failed to work. We checked what he was doing and everything appeared correct. We tried different computers with the same result. We then lifted the cover on the BAS Portable Router and did some rudimentary voltage checks and noticed the output of the 3.3 volt regulator was below 3 volts! The CPU in the product was not coming out of reset because it was starved for voltage. Checking the input power we noticed we were only receiving 3 volts from the connected cable. We replaced the intern's cable with the cable that is normally shipped with the product and the product came to life.

We measured the intern's cable and noticed that each conductor was measuring 3 ohms while we were expecting a fraction of an ohm! We then speculated that instead of copper or even aluminum conductors, steel conductors were being used. We then took a speaker magnet to the cable itself and noticed the magnetic attraction which we feel confirms our suspicion. From the outside, the cable looks like a perfectly good USB cable but not the type we use in production. Its source is a mystery but from the color of the cable jacket it appears to have come from a discount computer store for it was packaged with a store-bought peripheral. Even our intern does not know where he found the cable. However, it demonstrates that what appears universal is not always the case.

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