PC expansion bus systems - PC Based Instrumentation and Control - page 94

Figure 2.20 USB cable (cross-sectional view)
Cables
USB cables comprise four conductors, two power conductors and two signal
conductors. Full-speed cable consists of a signalling twisted pair, V BUS , GND,
and an overall shield. Full-speed cable must be marked to indicate suitabillity for
USB usage. Full-speed cable may be used with either low- or full-speed devices.
When full-speed cable is used with low-speed devices, the cable must meet all
low-speed requirements. Low-speed cable does not require twisted signalling
conductors or the overall shield (since radiation of EMI is significantly reduced
with low-speed data transmission).
The current USB specification describes three USB cable assemblies;
detachable cable , full-speed captive cable , and low-speed captive cable . The
recommended colours for the cable assembly are white, grey, or black.
A cross-sectional diagram of a full-speed USB cable is shown in Figure 2.20.
USB connectors
To minimize end user termination problems, USB uses a 'keyed connector'
protocol. The physical difference in the Series 'A' and 'B' connectors insure
proper end user connectivity. The 'A' connector is the principle means of con-
necting USB devices. All USB devices must have an 'A' connector. The 'B'
connector allows device vendors to provide a standard detachable cable. It is
important to note that:
• Series 'A' plugs are always oriented upstream towards the host system
• Series 'B' plugs are always oriented downstream towards the USB device.
The following list explains how the plugs and receptacles can be mated:
• The Series 'A' receptacle mates with a Series 'A' plug. Electrically, Series
'A' receptacles function as outputs from host systems and/or hubs.
• The Series 'A' plug mates with a Series 'A' receptacle. The Series 'A' plug
always is oriented towards the host system.