TABLE OF CONTENTS In this chapter, we study PC host requests on the USB bus from the bottom up:
We look at the signaling on the wires, and discover an underlying real-time structure of frames and microframes.
Within these (micro)frames we see a packetized signaling scheme at one of three standard speeds.
We learn that sequences of packets are used to generate transactions.
We list the special packets that a PC host uses to communicate with hubs that support devices at different speeds.
We learn that control transactions are used during enumeration, and that three additional transaction types (interrupt, bulk, and isochronous) are used during run time.
We study the control requests that a PC host can use to interact with an I/O device and a hub device.
If you were to put an oscilloscope on the data wires of a USB you would see a pair of differential signals at one of three standard speeds. The two data wires, D + and D -, are driven at the same time and typically in anti-phase (there are a few single-ended signaling methods; these are described later). The USB data wires are connected point-to-point and the signaling is half-duplex, which means that only one end of the wire pair is driven at a time. The protocol, presented in the following paragraphs, defines how the two ends of the wire pair alternate in the transmission of information. In this discussion, the upstream device is the transmitter, and the downstream device is...
