12 Usb Configuration Guide; Usb In Hp Storageworks Dat Devices; Usb Terminology - HP 157770-001 - DAT Drive 20/40 Tape User Manual

12 USB Configuration Guide

In this chapter:
• "USB in HP StorageWorks DAT
• "USB terminology" on page 69
• "Setting up a USB
• "USB cables" on page 70

USB in HP StorageWorks DAT devices

The HP StorageWorks DAT tape drive is a USB 2.0 logo certified device. It supports the high-speed
USB transfer rate with a theoretical maximum transfer of over 50 MB/s. (The actual backup rate will
be less. This is defined by the transfer rate of the tape drive and the compressibility of the data.)
To benefit from the high-speed performance of the USB 2.0 interface, the USB StorageWorks DAT
tape drive should be connected to its host system via a USB 2.0 port. Any USB hubs between the
tape drive and the system port should also be USB 2.0 compliant.
To maximize tape drive performance, minimize the use of other USB devices when using the USB
tape drive.

USB terminology

The Universal Serial Bus, USB, is a communication interface where an intelligent host controls all the
network activity. A command from the host to a particular device is broadcast to all devices on the
network. A device waits for a command addressed to itself and then responds as requested. The host
determines how much of the overall USB network bandwidth a given device can use at any time. Any
given USB network has only one host.
Hubs act as command repeaters and multipliers. This allows many devices to be supported by one
host. A host command can arrive at the upstream port of the hub and be transmitted via the downstream
ports to many other device and hubs. (Likewise, device commands arriving at a downstream port are
also repeated upstream.) There is a special hub called a root hub. The root hub's upstream port is
connected directly to the host. All USB systems have a root hub. This is why a system typically has
more than one USB port. All the USB ports on the system are connected to the host system via the root
hub.
USB devices are not treated equally by the host.
• Periodic devices, which consist of interrupt and isochronous devices, are given a higher priority
than bulk transfer devices.
• Interrupt devices, such as keyboards and mice, typically send brief but important input information
to the system.
• Isochronous devices, such as web cameras, typically require timely transmission of data, such as
a video stream.
• Bulk transfer, such as printers and CD-ROM, must receive or transmit 100% accurate data. But
how fast or often the data is transmitted is not as important as the accuracy of the data.
devices" on page 69
network" on page 70
DAT USB Tape Drives 69