COMPUTER EXTERNAL DEVICES

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I / O operations are achieved through a variety of external devices that provide the means to exchange data between the external environment and the computer's Internet. An external device is connected to the computer with a link to the I / O module. This link is used to exchange controls, status, and data between I / O modules and external devices. External devices connected to the I / O module are often referred to as peripheral devices or, simply, peripherals. 
Broadly classified external devices into three categories: 
■ Human-readable: Suitable for communicating with computer users; 
■ Machine readable: Suitable for communicating with equipment; 
■ Communication: Suitable for communicating with remote devices. Examples of devices that can be read by humans are video display terminals (VDT) and printers. Examples of machine-readable devices are magnetic disks and tape systems, as well as sensors and actuators, such as those used in robotics applications. Disk and tape system as an I / O device In this discussion, as a memory device. 

From a functional point of view, these devices are part of the memory hierarchy. From a structural point of view, these tools are controlled by the I / O Module and will therefore be considered in this discussion. Module I / O Link to peripheral devices Control line Data path Address line Bus system, Generic Model Module I / O. Communication devices allow computers to exchange data with remote devices, which can be devices that can be read by humans, such as terminals, devices that are can be driven with a machine, or even another computer. 

In very general terms, the nature of the external device shown in the Interface to the I / O module is in the form of control signals, data, and status. The control signal determines the functions the device will perform, such as sending I / O module data (INPUT or READ), receiving data from the I / O module (OUTPUT or WRITING), reporting status, or performing some special control functions for the device (e.g. disk head position). Data in the form of a set of bits to be sent or received by the I / O module. Status signals indicate the state of the device. An example is READY / NO-READY to indicate whether a device is ready for data transfer. 

The control logic associated with the device controls the operation of the device in response to the direction of the I / O module. Transducers convert data from electricity to other forms of energy during output and from other forms to electricity during input. Usually, the buffer is associated with the transducer to temporarily store data transferred between the I / O module and the external environment. A buffer size of 8 to 16 bits is common for serial devices, whereas block-oriented devices such as disk drive controllers may have a much larger buffer. The interface between the I / O module and the external device will be checked. The interface between external devices and the environment is beyond the scope of this book, but a few brief examples are given here. Keyboard / Monitor:


The most common way of computer / user interaction is keyboard / monitor settings. Users provide input through the keyboard, the input is then sent to the computer and can also be displayed on the monitor. In addition, the monitor displays data provided by the computer. Buffer Transducer Logic Control Signal control from Module I / O Signal status to Module I / O Data bits to and from Module I / O Data (unique devices) to and from the Living environment. Related to each character is a code, usually 7 or 8 bits in length. The most commonly used text code is the International Alphabet Reference Reference (IRA) .1 Each character in this code is represented by a unique 7-bit binary code; thus, 128 different characters can be represented. Characters are of two types: print and control. Printable characters are alphabetic, numeric, and special characters that can be printed on paper or displayed on the screen. Some control characters are related to controlling printing or character display; an example is carriage return. Other control characters are related to communication procedures. See Appendix H for details. 

For keyboard input, when the user presses a button, this produces an electronic signal which is interpreted by the transducer on the keyboard and translated into the corresponding IRA code bit pattern. This bit pattern is then transmitted to the I / O module on the computer. On a computer, text can be stored in the same IRA code. At the output, IRA code characters are transmitted to an external device from the I / O module. Transducers on the device interpret this code and send the required electronic signals to the output device either Disk Drive: Disk drive contains electronics to exchange data, control, and status signals with the  module Electronic I / O plus to control disk read / write mechanism. In fixed-head disks, transducers are able to convert between magnets 


patterns on the surface of the moving disk and bits in the device buffer. A  head-moving disk must also be able to cause the disk arm to move radially in and  out of the disk surface.

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