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12 USB Configuration Guide
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.
The host will give network bandwidth priority to periodic devices and will share any remaining bandwidth
equally among any active bulk transfer devices.
USB supports many types of devices and three data transfer rates, low speed, full speed, and high speed.
(A keyboard does not need to transfer data to the system at the same speed as a cable modem.)
The transfer speed capabilities of a device are often referred to by the USB specification revision in which
the transfer rate was defined. Therefore a USB 2.0 Hub will support high, full and low speed, but a
USB 1.1 hub will only support full and low speed.
The USB specification was developed and is maintained by the USB organization. The USB organization
also conducts a certification program. The USB organization will issue the USB logo to a device,
which has passed a series of tests that verify the device has correctly implemented the USB interface,
as defined by the USB specification. There are many types of USB logos depending on which features
of the USB specification a device supports. A USB 2.0 logo-ed device correctly supports high, full,
and low speed data transfer.
DAT USB Tape Drives
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Troubleshooting
