Dear Group, Our Engineering Department thinks that USB is the worlds best interface. Though it has a lot going for it, I'm not sure if is all that, but never the less, I'm seeing it used to control and interface with commercial and industrial instrumentation requiring the more severe immunity testing for Europe; e.g. 10volt/meter Radiated Immunity, 10Vrms Conducted Immunity, and 2KV Fast Transient Testing (1KV using Clamp).
To these higher levels, we have failed almost every USB system we have ever encountered using USB cables longer than 2 meter in length. I understand that USB is suppose to be able to goto 5 meter. The failure shows itself as a communication error that usually requires resetting the hardware. Fairly recently we evaluated one of those "USB Hubs". Someone realized that if you run a 5 meter USB cable into a HUB you can run it out another 5 meters. Our Immunity testing failed the HUB configuration in the most miserable ways. A detailed examination of the manual that came with the USB Hub proudly displayed a DOC and the CE marking, but gave no special conditions or mention of cable lengths. At home I have a USB Flatbed scanner that came with a 2 meter USB cable with ferrite beads on BOTH ends. What might I expect if I went to Best Buy and bought a 5 meter USB cable and installed it on my scanner? I guess I would expect it NOT to comply with emissions requirements, who knows about immunity, but would it even function? In any case, I don't think it is right that I can purchase a scanner and only when I get it home and open the box I find out that I can only use the provided 2 meter cable. Being USB, I expected to be able to put any 5 meter cable on it. I hate surprises like that, don't you? We have tested dozens of different USB cables, looking for a solution to the Immunity Problems we are encountering (Even the Gold Plated 12MB/s versions which tested no better than the cheep ones). We discovered that we could get it to pass Immunity if the impedance of the USB cable was improved. We would simulate this by going over a standard USB cable with a better, heavier braided shield and then soldering the shield to the backshells (connecting a ground strap between the instrument and the computer would often give similar results). The problem with this approach is that no one makes a cable like this. I understand that USB cables are constructed according to the USB standard which includes a DC cable shield impedance requirement which in my opinion should be lower and include impedance requirements for the entire frequency band. So, to all you USB Experts out there, please education me in the finer points of USB. 1. Does the USB specification take into account any Immunity Requirements? If so, to what levels? 2. Any recommendations to improve a USB systems performance during Immunity Testing? 3. What are others doing? I expect that they are testing with short USB cables then specifying (or not) the length in the user's manual. 4. What changes can we expect to see in the future of USB? We know the interface is getting faster and faster. Is functionality, cable length, Emissions, and Immunity being considered? Brian Kunde LECO Corp. ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Ron Pickard: emc-p...@hypercom.com Dave Heald: davehe...@attbi.com For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: http://ieeepstc.mindcruiser.com/ Click on "browse" and then "emc-pstc mailing list"
