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Android Tablet cannot find USB device via USB port, but via OTG cable

I have an Android tablet with a mini USB port and a USB port, and I want to write an application for communicating with a USB key. I wrote a demo to find out the USB key, but there is no answer.

My concern is that if I ran this demo on my other tablet, which only has a mini-USB port but supports USB OTG, it can find the key through the OTG cable. When I insert the USB key into the USB port, there is no answer. I want to know what the difference is.

Can't I use the USB API to find the key in the USB port? And what's the difference between communicating with a USB device in a mini USB port using an OTG cable and on a USB port directly?

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OTG cables have well-defined contacts to tell the device that it should switch to USB Host mode (the tablet provides power). The USB port may be OTG compatible, but only if you have an OTG cable. Usually, the USB ports on tablets are installed as additional ports, which means that they consume energy rather than provide it.

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