Information Technology Reference
In-Depth Information
DID (device identification register)
Address offset
02-03h
Default value
7112h
Attribute
Read only
The DID register contains the device identification number. This register, along with the VID
register, defines the USB host controller.
9.3 Firewire
The main competitor to USB is the Firewire standard (IEEE
1394-1995) which is a high-speed serial bus for desktop peripheral devices, typically
for video transfers. It supports rates of approximately 100, 200 and 400 Mbps, known as
S100, S200 and S400 respectively. Future standards promise higher data rates, and ultimately
it is envisaged that rates of 3.2Gbps will be achieved when optical fibre is introduced into the
system. It is generally more expensive than USB to implement for both the host computer
and peripherals.
Its main features are:
100/200/400 Mbps transfer rate.
Point-to-point interconnect with a tree topology; 1000 buses with 64 nodes gives 64 000
nodes.
Automatic configuration and hot plugging.
Isochronous data transfer, where a fixed bandwidth is dedicated to a particular periph-
eral.
Maximum cable length of 4.5 m.
Firewire also complements USB in that it supports high-speed peripherals, whereas USB
supports low-to-medium speed peripherals. It is an attractive alternative to technologies such
as SCSI and it may provide a universal connection to replace many of the older connectors
normally found at the back of a standard PC. This should subsequently reduce the costs of
production of computer interfaces and peripheral connectors, as well as simplifying the re-
quirements placed on users when setting up their devices. This is made possible by the fol-
lowing features of the IEEE-1394 bus:
Hot pluggable - devices can be added or removed while the bus is still active.
Easy to use - there are no terminators, device addressing or elaborate configuration of-
ten associated with technologies like SCSI.
Flexible topology - devices can be connected together in many configurations, thus the
user need not consider logical locations on the network.
Fast - suitable for high bandwidth applications.
Rate mixing - a single cable medium can carry a mix of different speed capabilities at
the same time
Inexpensive - targeted at consumer devices.
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