Unformatted text preview: rance of a typical serial port is shown in Figure 9.39.
A serial port limits the speed at which it can transfer data because of the fact that it can
transfer only one bit at a time. Parallel ports overcome this limitation of serial ports by
allowing 8 or 16 bits of data to be transferred at a time. Accordingly, a parallel port has 8
or 16 parallel wires for sending and receiving data. Hence a parallel port with 8 wires can
transfer one byte (8 bits) at a time and one with 16 wires can transfer two bytes (16 bits)
at a time. The process of data transfer in case of a parallel I/O port is illustrated in Figure
9.40. Notice that unlike the case of serial I/O port, the conversion of bytes into bit
streams and vice-versa is not required in this case. This also leads to faster data transfer
than in case of serial I/O ports.
A parallel port can obviously handle a higher volume of data in the same amount of time
than a serial port. Hence they are used for connecting faster I/O devices such as printer,
analog-to-digital and digital-to-analog converters, etc. The physical appearance of a
typical parallel port is shown in Figure 9.41.
A computer's system bus has a finite number of slots (called expansion slots) to which
device controllers can be connected. If every I/O device comes with its own device
controller card, then a computer system will have the capability to connect only a very
limited number of I/O devices because it will soon run out of available expansion slots.
This problem is overcome by using a SCSI interface, which is an interesting and
important variation of the separate device controller idea for each device. As shown in
Figure 8.27 and discussed in Chapter 8, a SCSI interface uses a generic device controller
(called SCSI controller) and allows any device with an SCSI interface to be directly
connected to the SCgl bus of the SCSI controller. That is, instead of connecting device
controller cards into the computer's system bus via the expansion slots, SCSI extends the
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- Spring '14