Protocol analysis
R&S
®
RTE
817User Manual 1326.1032.02 ─ 20
Figure 13-92: USB Topology: mass storage device, printer, USB hub, and mouse (human interface
device) connected to a USB host
More information, including the USB specification, is available online within the web
domain http://usb.org.
This chapter provides an overview of protocol characteristics, data transfer, packet
structure, address and endpoint formats of USB and trigger possibilities.
USB characteristics
Main characteristics of USB are:
●
Four-wire design: USB requires a shielded cable containing four wires. Two of
them, called D+ and D-, form a twisted pair (for low speed, they may not be twis-
ted). These data lines transmit differential data signals and single-ended signal
states, both referenced to a third wire: the GND or ground. The fourth wire, called
VBUS (voltage bus), carries a nominal 5 V supply, which may be used to power a
device.
●
Host-to-device communication: in USBs "speak-when-spoken-to" protocol, commu-
nication is always initiated by the host. Consequently, there is no direct communi-
cation between USB devices, apart from few exceptions.
●
Addressing scheme: a maximum of 127 connected devices can be distinguished,
because a packet's address field length is limited to 7 bits. USB devices have up to
16 OUT endpoints (from host to device) and up to 16 IN endpoints (from device to
host).
●
USB transactions consist of two or three packets: token, data, and typically hand-
shake
●
Packet type: a packet identifier (PID) is sent as a first byte within the packet and
specifies the different packet types.
●
NRZI (Non Return to Zero Inverted): a zero (0) is encoded as a transition of the
physical level, whereas a one (1) has no transition, thus it is represented by a
steady level.
Figure 13-93: Example of an NRZI sequence
●
Bit stuffing: a zero (0) is inserted after every 6 consecutive ones (111111). This
ensures sufficient transitions to keep the phase-locked loop (PLL) synchronized
and locked onto the data stream.
USB (option R&S
RTE-K60)
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