Ok, I basically have a circuit like this:
__________
          |
      Vdd |------+---- 5V
          |      /
          | 4.7k \
          |      /
       D+ |------+---- to USB D+
          |
       D- |----------- to USB D-
          |
   PIC    |
 18F4550  |         -- to USB V+
          |
     Vusb |-------+
          | 470n ---
          |  cap ---
      GND |-------+--- to USB GND
__________|     __|__
                 ___
                  _

Now, USB V+ is not connected; it is power from the computer (or host) at 5
volts. USB GND is connected to my chip's ground as well as the ground of
the power supply for the chip. I wonder however, should I opto-isolate my
D+ and D- connections (to avoid ground loops, voltage spikes and other
nasties), and use the USB V+ and USB GND wires to power the opto-isolator?

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!



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