Datasheet

Input and Peripheral Devices
61
Bits of Data Simultaneously Conveyed A serial cable carries only one bit at a time. A parallel
cable carries multiple bits at a time (usually eight).
Data Transmission Speed This is expressed in kilobits or megabits per second and refers
to the overall data throughput.
Type of Connector A wide variety of connectors are used in PCs today, including the
DB style (as with legacy parallel and serial ports and VGA monitors), Centronics style
(as with printers and some SCSI devices), and USB.
Parallel vs. Serial
A cable (and its port) can be either parallel or serial, and it isn’t always immediately obvious
from looking which is which. For example, both parallel and serial cables can use the DB-25
style of connector.
Both parallel and serial cables have multiple wires inside them, but they use them for dif-
ferent purposes. A parallel cable uses eight wires to carry bits of data in each direction, plus
extra wires for signaling and trafc control. A serial cable uses only one wire to carry data
in each direction; all the rest of its wires are for signaling and trafc control.
Transmission Speed
Neither parallel nor serial is intrinsically faster than the other. There are both fast and slow
parallel and serial connections. For example, a legacy serial port such as for an external
modem carries data fairly slowly (about 115Kbps), but a USB cable (also serial) carries data
very quickly (up to 12Mbps for USB 1.1, and even faster for USB 2.0480Mbps).
Connector Types
The following are common connector types:
DB A D-shaped connector with a metal ring around a set of pins. Named for the number
of pins/holes used: DB-25, DB-9, DB-15, and so on. Can be either parallel or serial. Common
uses: VGA video, legacy serial devices such as external modems, and parallel printer cables
(the connector on the PC only; the printer end uses Centronics).
RJ Registered jack; a plastic plug with small metal tabs, like a telephone cord plug. Num-
bering is used in the naming: RJ-11 has two metal tabs, and RJ-14 has four. Both are used
for telephone systems. RJ-45 has eight tabs and is used for Ethernet 10BaseT/100BaseT
networking. Always serial.
BNC Stands for Bayonet-Neill Connector or British Naval Connector. A metal wire
surrounded by shielding, like a cable television connector. Used for 10Base2 Ethernet
networking. Always serial.
Centronics A plastic block with metal tabs flat against it, surrounded by a D-shaped
metal ring. Used to connect a parallel printer cable to the printer, and also for some SCSI
devices. Always parallel.
Ribbon Connector A rectangular block consisting of a set of square holes that connect to
pins on a circuit board. Used to connect floppy drives, IDE drives, and some SCSI devices
to their controllers. Always parallel.
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