Protocol analysis
R&S
®
RTP
802User Manual 1337.9952.02 ─ 12
●
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
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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 13-22: Example of an NRZI sequence
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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.
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Little Endian scheme: both multiple bits and multiple bytes are transmitted with the
least significant bit/byte (LSB) sent first, while the most significant bit/byte (MSB) is
sent last.
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HSIC (High-Speed Inter-Chip): an industry standard for USB chip-to-chip intercon-
nection with a 2-signal (strobe, data) source synchronous serial interface, using
240 MHz DDR signaling to provide only high-speed (480 Mbps) data rate.
Table 13-28: Simplified symbolic representation from the USB standard
Bus State Protocol Levels
Differential "1"
D+ High, D- Low
Differential "0"
D+ Low, D- High
Data "J" State Low speed D+ Low, D- High (differential "0")
Full speed D+ High, D- Low (differential "1")
Data "K" State Low speed D+ High, D- Low (differential "1")
Full speed D+ Low, D- High (differential "0")
In the example in Table 13-28, High speed can be assumed to be like Full speed. For a
complete overview of bus states and for an electrical definition of High and Low speed,
refer to chapter 7 of the USB specification.
Data transfer
In contrast to a conventional bus, USB is more like a network protocol, using target
addresses and endpoints. However, USB features a bus master, called the host. It
transmits packets of data to all devices or hubs connected to the host (or connected to
a device or hub, which is in turn connected to the host). All connected devices receive
USB (option R&S
RTP-K60)
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