symbols, graphics, or drawings. Printers range from
electric typewriters to high-speed printers. Low-speed
line printers include the dot-matrix printer commonly
used with personal computers. High-speed line printers
are normally used with larger computers to print supply
requisitions, paychecks, inventory, or financial reports
at 10 lines per second and faster. Laser printers provide
high quality print and print a full page at one time. They
are used with all types of computers.
Plotters
Plotters are used with a computer to plot coordinate
points in the form of a graph. Digital incremental
plotters, in either online or offline operation with a
digital computer, provide a high-speed plotting system
of high versatility and reliability. For online operation, a
small adapter unit converts the computer output signals
to a form suitable for driving the plotter.
INPUT/OUTPUT (I/O) CHANNELS
The input/output (I/O) channels provide for
communications between the CPU and all peripheral
devices. This is accomplished by electrical cables that
carry both data and control information to and from the
computer and peripheral devices.
Signals are transmitted and received through a
cable connecting the CPU and its online devices. This
cable or line provides a path for the signal to travel and is
called a channel. Not only signals for monitoring but
also data are transmitted via channels. All channels
between the CPU and the peripheral devices are
designated as I/O channels. An I/O channel may be
used for data input, data output, or data input and output,
depending on whether the peripheral device handles
input only, output only, or both input and output.
Channel Types
simplex circuits are seldom used, because a return path
is generally needed to send acknowledgment, control
information, or some type of error signals.
DUPLEX CHANNELS.— A duplex channel
simply means that within each cable connection, there
are two paths (lines) for the transmission of data. One
path is for sending, and one is for receiving, similar to
your telephone. There are two types of duplex charnels,
half-duplex, and fill-duplex. A half-duplex channel is
capable of transmitting and receiving signals, but only
in one direction at a time, similar to citizens’ band (CB)
radio transmissions. Therefore, it is necessary to check
that the line is clear (idle) before starting a transmission.
A full-duplex channel provides for simultaneous
transmission in both directions, as in the use of the
telephone.
Data Transmission Methods
Data may be transmitted over a channel in one of
two ways, in either serial mode or parallel mode.
SERIAL MODE.— In serial transmission,
basically only three wires are needed: one to transmit
data, one to receive data, and one to ground. The data is
sent or received in the form of bits, one after another in
serial fashion, as shown in figure 1-7. This type of
transmission is highly desirable whenever the computer
system is linked to outside peripherals over a long
distance, such as remote terminals.
PARALLEL MODE.— Parallel transmission is a
little more difficult to hookup than serial. It requires all
the data bits in a byte to be transmitted at one time (batch
mode); so you usually have nine or ten wires going
between the computer and the peripheral devices. You
have seven or eight lines for the data bits (figure 1-7)
that make up a letter, number, or special character; one
line to ground; and one or two lines called handshake
lines.
The handshaking signals communicate
information back and forth between the peripheral
device and the computer. This information lets the
I/O channels may be simplex or duplex.
S I M P L E X C H A N N E L S . — In simplex
operations, communications are in one direction only,
such as a radio. If a device such as a terminal were to be
connected to such a circuit, it would only be capable of
sending or receiving data, but not both. For this reason,
Figure 1-7.—Serial and parallel data transmission.
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