HEIDENHAIN MANUALplus 620 Technical Manual page 1764

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Topology of the USB interface
Functionality and
signal designations
1764
The USB interface connects the USB peripheral devices with the USB host.
The topology of a USB connection may consist of several levels arranged in a
star configuration. Every level consists of a USB hub to which other USB
devices or hubs are connected in a star configuration. A maximum of 127 USB
devices can be connected to a USB host in this way.
USB uses packet-based communication over two differential data lines. This
reduces radiation and increases transmission reliability. USB provides
significantly higher data transfer rates than other external interfaces (parallel /
Centronics, serial / RS-232, RS-422):
USB full speed of up to 12 Mbps
USB low speed of up to 1.5 Mbps
Conventional interfaces, such as the RS-232, are more suitable for time-critical
applications because they are not based on packets, which reduce the transfer
rate (in case of packets with only a few bytes) or delay transmission (when
collecting bytes for filling a packet).
Only four wires are required in a USB cable. Two for a power supply of 5 V
(with max. 500 mA / 2.5 W) and two for data transmission:
HEIDENHAIN Technical Manual MANUALplus 620