Information Technology Reference
In-Depth Information
Table 3.4
External PC busses
Bus
Description
Typical devices
connected
SCSI
The SCSI (Small Computer System Interface) bus is used to connect to
a wide range of device, and is typically used in workstations and Apple
computers. It allows devices to connect using cables which connect
from one to the next (a daisy chain). In its standard form, it allows for
up to seven devices to be connected (SCSI-I), but new standards
(SCSI-II/III) allow up to 15 devices to connect. It can also be used as
an internal bus system or as an external one. In Apple Macs and work-
stations, SCSI is used to connect hard disk drives.
Max: 5 MB/s (SCSI-I), 20 MB/s (SCSI-II), 40 MB/s (SCSI-III).
Hard disk drive
CD-ROM drive
Scanner
Back-up device
RS-232
RS-232 is a standard interface on most computer systems. It uses serial
communications, to send data one bit at a time. The speed of the trans-
fer is set by the bit rate. Typical bit rates are 9600 bps (bits per second),
14 400 bps, 28 800 bps and 56 000 bps. It is typically used to transfer
files from one computer to another, and to connect to a modem. In the
past, it was also used to connect to a serial mouse, but mice typically
connect using the PS/2 mouse connector. Typically PCs have one or
two serial port, which are given the names: COM1: and COM2:.
Max: 7 KB/s (56 000 bps)
Modem
Mouse
File Transfer (with
Null Modem cable)
Parallel
port
The parallel port transfers 8 bits of data at a time. In its standard form,
it only supports a maximum rate of 150kbps, with only one connected
device. It also slows down the processor, as it must involve itself with
the transfer of the data. In its standard form, it uses a 25-pin D-type
connector to connect to the PC. As technology has improved a new
standard named ECP (Extended Capabilities Port Protocol)/EPP (En-
hanced Parallel Port) was been developed to increase the data rate, and
also to connect multiple devices to it (as the SCSI bus). These allow the
transfer of data to be automatically controlled by the system, and not
by the processor. Typically, now with ECP/EPP, several devices can
connect to the port, such as a printer, external CD-ROM drive, scanner,
and so on. Its main advantage is that it is standard on most PCs, but
suffers from many disadvantages. Typically PCs have a single parallel
port, which is given the name LPT1:. In many cases, it is being re-
placed by USB.
Max: 150 kbps (Standard), 1.5 Mbps (ECP/EPP)
Printer
CD-ROM drive
Scanner
File Transfer (with
Parallel Port cable)
USB
USB (Universal Serial Bus) allows for the connection of medium
bandwidth peripherals such as keyboards, mice, tablets, modems, tele-
phones, CD-ROM drives, printers and other low to moderate speed
external peripherals in a tiered-star topology. It is typically used to
connect to printers, scanners, external CD-ROM drives, digital speak-
ers, and so on. It is likely to replace the printer port and the serial port
for connecting external devices.
Max: 12 Mbps.
Digital speakers
Scanner
Printer
Video camera
Modem
Joystick
Monitor
PS/2
Port/
Key-
board
Initially, on PCs, the serial port was used to connect a mouse to, which
reduced the number of connections to the serial port. Typically, these
days, a mouse connects to the PS/2 mouse port, which has a small 5-
pin DIN-like connector. This is the same connector that is used to con-
nect to the keyboard.
Keyboard
Mouse
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