There is "what's legal" and "what you can get away with". The primary motivation for the 100mA and 500mA strategy, with respect to the USB rules, is based upon user plugging together a hardware setup and then ensuring a reasonable user experience without things going into crash and burn mode.

This is particularly important to keep in mind for USB lashups where there is an unpowered hub in the chain. An unpowered hub gets all its power from its input USB cable and then in turn fans that out to the down wind ports on the hub. Clearly it is not even at all technically feasible to expect that you could draw 500mA out of multiple ports of a 4-port unpowered hub.

On the other hand, an experimenter or engineer that takes a few minutes to think about what they are doing can setup a viable USB power utilization scheme that takes the 500mA off the port without abiding to all the emumeration rules and other USB design intentions. A good advice if you want to do this is to make sure you take your load power from a powered hub or from a port that is sourced directly from the computer motherboard. Almost all of the motherboards I looked at and worked with have the power fed to the USB connectors through simple poly fuse components. Even many Intel boards will enable power to multi port output USB connectors using a single polyfuse to service the multiple ports. Say for example a motherboard has a group of three USB ports that go to a front panel. All three of those USB ports often use a single >1.5 Amp polyfuse. Technically what this means that if a single port of the three were in use it would be possible to draw 1.5A from the single connector.

One would never want to build a product this way but for a DIY experiment or a home brew test setup you could very well decide to dive right in and take 500mA right at time of DC PWR on.

Michael Karas