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03.16.2010, 11:32 AM
Brian, It has been so long since I have though about USB power and bandwidth negotiation with Windows and I am a little rusty. But yes, what you said it true. Some computers just don't allow the port to power both the link (about 40+ mA) and the ESC processor (40+ mA). You can see, the amount of current required is right at the 100 mA limit before negotiations start. The link is not very aggressive in its negotiations to make it a good Windows citizen. Because the USB host controller, the actual hardware that runs USB ports, can only control 4 ports, I have found that many times, with a desktop, simply going from front to back, or the other way around can get the link to work. This is because this changes the host controller hence software that runs the different ports. This is partially Windows stuff, specifically how Windows deals with specific hardware. The other part is the USB specification and how it allocates bandwidth and power. Getting in bed with Microsoft is necessary, at the same time . . . .
Another interesting note: this explains why you have to apply battery power if you are trying to connect a CC-BEC that is connected to an ESC. The USB port simple cannot power the clink, BEC and ESC's microprocessor and support components by itself and need help in the form of extra power.
Bernie
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