Marc, Opto isolating USB is generally not normally practiced. I think this is because the highish speeds (full speed) and the bi-directional comms make it difficult. Opto-isolating high speed USB is not practiacl, although there are alternative isolators that will handle the speed USB is designed to be used over short distances (and generally light-duty equipment) anyway, so earth currents are not much of a problem. (Mostly).. My recommendation would be not to attempt it unless there was a real need. If there is a real need, can you isolate at another point instead? - e.g an SPI bus is a lot easier to isolate as it is uni-directional. . Same with RS232.. If you do have a problem with an earth loop, try placing a low resistance (heavy duty) bond wire between the earths of the equipment concerned. Most of the current will flow through the bond wire and less through your USB earth. Just my 2c RP On 23 March 2010 07:55, Marc Nicholas wrote: > On Mon, Mar 22, 2010 at 2:21 PM, wrote: > >> >> I am asking this because I've always seen RS-232 interfaces isolated, and >> my configuration is odd (normally a stand-alone unit with a stand-alone >> power supply, and an optional USB data download/control interface.) Is >> there any harm from isolating an interface which doesn't requite it? >> >> Any help appreciated. Thanks! >> >> > My worry would be slew issues from not have an opto-isolator receiver that > could deal with 12Mhz? > > AFAIK, most implementations are just fine without opto-isolation. > > -marc > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist