PLC (Programmable Logic Controller)
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Last download on: Sun Mar 19, 2017 08:48:34 PM
A short chapter overviewing the types of memory devices available and their applications. An introduction to special purpose memory devices for implementing digital circuits (PROM, EPROM, EEPROM) and Boolean logic (Programmable Array Logic (PAL), Programmable Logic Array (PLA), etc.).
In Chapter 4, we examined a number of techniques that could be used to create a reckoning machine, or computer, through simple digital circuits. In that chapter, we were able to instil a crude form of human reasoning into the machine through Boolean logic circuits and to carry out simple manipulation of numbers via numerical circuits designed from digital circuits. Clearly, these circuits are not sufficient to create the sort of computer systems with which we have become familiar. Another important trait of computer systems is their ability to store data on both a short and long term basis. In Chapter 4, we examined the flip-flop and the register (collection of flip-flops) as a mechanism for short-term data storage. In this chapter, we need to delve further into more practical memory storage devices that can be built with a higher density and lower cost. Traditionally, magnetic (tape, floppy and hard disk) storage media have been used for the long term storage of data. The major reason for this is that these media are considered to be "non-volatile". This means that the contents remain in tact even when power is removed from the circuits responsible for storing and retrieving data from the media. More recently, optical devices such as compact disks have been introduced as an alternative format, again offering a relatively non-volatile storage of data at a very high density. Another reason for using all these types of storage systems is that they have provided relatively low cost data storage formats at times when other alternatives proved considerably more expensive. The problem with all these storage techniques is that they are based upon the movement of mechanical components that scan the surface of the medium and hence are very slow relative to the processing abilities provided by microprocessors and other digital circuits. For several decades now, short term data storage has been facilitated by semiconductor memory. Digital memory circuits are orders of magnitude faster than any of the mechanically driven, long-term data storage formats described above. However, in the past, they have taken up more physical space than the equivalent mechanical formats (particularly because of IC packaging and pin-out) and have generally suffered from volatility problems - in other words, most semiconductor memory storage devices lose their contents when power is removed. The last two decades have seen dramatic increases in the density of memory storage devices, thanks largely to the introduction of CMOS based circuits and improvements in semiconductor fabrication technology. The increases in density have been coupled to dramatic decreases in storage costs, to the extent where the costs of semiconductor memory are approaching those of mechanical storage systems. At this point however, the question of volatility has still not been resolved satisfactorily. Although it is possible to purchase memory devices which do not lose their contents when power is removed, the sorts of circuits that have this characteristic, and can be written to and read from, are still relatively costly and inefficient for general computing.
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