Information Technology Reference
In-Depth Information
tem, capacitance and impedance of cables, cross-talk (the amount of interference be-
tween local signal transmissions), and so on.
•
Ease-of-connection
. This includes the availability of cables and connectors, and how
easy it is to add and remove devices from the bus. Some busses allow devices to be
added or removed while the bus is in operation (hot pluggable). A good example of a
hot-pluggable bus, which is easy to connect to, is the USB.
•
Communications overhead
. This is a measure of the amount of data that is added to the
original data, so that it can be sent in a reliable way. Local, fast busses normally have a
minimum of overhead, whereas remote, networked busses have a relatively large over-
head on the transmitted data.
•
Bus controller topology
. This relates to the method that is used to control the flow of
data around the bus. Some busses, such as SCSI, require a dedicated bus controller
which is involved in all of the data transfers, whereas the PCI bus can operate with one
or more bus controller devices taking control of the bus. Other busses, such as Ethernet,
have a distributed topology where any device can take control of the bus.
•
Software interfacing
. This defines how easy it is to interface to the bus with software,
especially when using standard interface protocols, such as TCP/IP or MODBUS.
•
Cable and connectors
. This defines the range of cables and connectors that can be used
with the bus. There is a wide range of cables available, such as ribbon cables (which are
light and are useful inside computer systems), twisted-pair cables (which are easy to con-
nect to and are useful in minimising cross-talk between transmitted signals) and fibre op-
tic cables (which provide a high capacity communications link and minimise cross talk
between transmitted signals). For example, Ethernet can use BNC connectors with coax-
ial cables, RJ-45 connectors with twisted-pair cables and SNA connectors with fibre op-
tic cables.
•
Standardisation of the bus
. Most busses must comply with a given international stan-
dard, which allows hardware and software to interconnect in a standard form. There are
normally standards for the electrical/mechanical interface, the logical operation of the
bus, and its interface to software. For example, the IEEE has defined most of the
Ethernet standard (especially IEEE 802.3), and the EIA have defined the RS-232 stan-
dard. International standard agencies, such as the IEEE, ISO, ANSI and EIA, provide a
more secure standard than a vendor-led standard.
•
Power supply modes
. Some busses allow power saving modes, where devices can
power themselves down and be powered up by an event on the bus. This is particularly
useful with devices that have a limited power supply, such as being battery supplied.
2.1.2 Bus components
Devices connect to each other within a computer using a bus. The bus can either be an inter-
nal bus (such as the IDE bus which connects to hard disks and CD-ROM drives within a PC)
or an external bus (such as the USB which can connect to a number of external devices, typi-
cally to scanners, joypads and printers). Busses typically have a number of basic compo-
nents: a data bus, an optional address bus, control lines and handshaking lines, as illustrated
in Figure 2.2. Other lines, such as clock rates and power supply lines are not normally dis-
played when discussing the logical operation of the bus. If there is no address bus, or no con-
trol and handshaking lines, then the data bus can be used to provide addressing, control and
handshaking. This is typical in serial communications, and helps to reduce the number of
connections in the bus, although will generally slow down the communications.
