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Cathode-Ray Tube (CRT) Terminals

A cathode-ray tube (CRT) terminal looks like a television with a typewriter-like keyboard. It allows you, the operator, to enter programs and data directly to the computer. At the same time, it displays the program or data on the display screen of the CRT. It can also receive information directly from the computer and display it on the screen. You can add a printer, a modem, and adapters to the CRT terminal to provide for printed output and communications through telephone lines.


Printers are widely used output devices that express coded characters as hard (paper document) copy. They print computer results as numbers, letters, words, 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 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.


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

I/O channels may be simplex or duplex.

SIMPLEX CHANNELS.\ 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, 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

Figure 1-7.\Serial and parallel data transmission.

peripheral device know when the computer is ready to accept another character and vice versa. This type of transmission is used when you want to have fast data transfer. Its drawback is that the computer cannot be too far away from the peripheral device.


We said that both signals and data can be transmitted and received through cables (communications lines), which we refer to as I/O charnels. When we transmit data directly to the computer over long distances, it becomes necessary to add two other devices, one at each end of the communications line. These devices are called modems. The word modem is an acronym for MOdulator-DEModulator. A modem converts the digital signal produced by your terminal (or the computer) to an audio signal suitable for transmission over the communications line. The modem at the other end of the line reconverts the audio signal back to a digital signal before it is supplied to the computer (or your terminal). If this conversion were not carried out, the digital signal would degenerate and become garbled during transmission.

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