SERIAL DATA TRANSMISSION The simplest, most economical, and easiest to use method of transferring digital information from one point to another is serial data transmission. In a serial system, the digital data is sent one bit at a time. This means only one pair of transmission wires is required. The serial transmission of data is far slower than parallel transmission. In most computer systems, the low speed is not a disadvantage. Data rates achievable in serial data systems are sufficiently high to make them very practical. Serial data transmission is preferred because it is inexpensive. It is especially beneficial in transferring data over long distances. For long distances, you can see that multiple parallel lines are far more expensive than a single cable. Low-speed serial data transmission also offers another benefit. The data rate is slow enough to permit the transmission of data over telephone lines. In this case the digital data is converted into a pair of audio tones representing binary 1s and 0s. These can be transmitted economically for long distances over standard telephone lines.    In addition, low cost tape recorders can be used to record the serial data. This provides a low cost means of mass data storage and retrieval. Standard audio cassette recorders are widely used in this application. Serial data transmission also permits transmission of data by radio. A radio communications path represents only a single interconnecting link similar to a transmission line pair. Therefore, for data to be transmitted by radio, it must be in serial format. Serial digital data is used to modulate a radio carrier in various ways. In digital computer systems, you will find that both serial and parallel data transmission methods are used. Where high speeds and short distances are required, the parallel method is used. The serial method is used where low cost, simplicity, low speed, and long distances are necessary. INPUT/OUTPUT (I/O) DEVICES I/O devices are similar in operation but perform opposite functions. It is through the use of these devices that the computer can communicate with devices external to the computer itself (peripheral devices). The I/O section of a computer provides the necessary lines of communication and generates such signals necessary for the computer to communicate with, and when necessary, control the operation of the I/O devices. The I/O section, once it has been initiated by the control section, usually operates independently of the control section, except when it must time-share memory with the control section. Input Devices Input data may be in any one of three forms: . Manual inputs from a man-machine interface (MMI), such as a keyboard or console . Analog and/or digital inputs from instruments or sensors l Inputs from a source on or in which data has been previously stored in a form intelligible to the computer Computers can process hundreds of thousands of computer words per second. Thus, a study of the first method (manual input) reflects the inability of human-operated keyboards or keypunches to supply data at a speed that matches the speed of digital computers. A high average speed for keyboard operation is two or three characters per second. This actually translates to a data input rate of less than one computer word per second due to coding time. Since the computer can read several thousand times this amount of information per second, it is clear that manual inputs should be minimized to make more efficient use of computer time. Instruments used as input sensors are capable of supplying several thousand samples regarding pressure, temperature, speed, and other measurements per second. This is equivalent to 10,000 to 20,000 bits or binary digits per second. Digital computers that use these devices must be equipped with analog-to-digital (A/D) converters (assuming the input is in an analog format) to convert physical change to specific increments. Input data that has been previously recorded on punched cards, perforated tapes, magnetic tapes, or magnetic drums or disks in a form understood by the program may also be entered into the computer. This method is much faster than entering data manually from a keyboard. The most commonly used input devices in this category are magnetic tape readers and paper tape (perforated tape) readers. One of the main features of computers is their ability to process large amounts of data quickly. In 8-11