Category: Education
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16.2 Look-up Tables
Having learned about digital memory devices in the last chapter, we know that it is possible to store binary data within solid-state devices. Those storage “cells” within solid-state memory devices are easily addressed by driving the “address” lines of the device with the proper binary value(s). Suppose we had a ROM memory circuit written, or…
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16.1 A Binary Adder
Suppose we wanted to build a device that could add two binary bits together. Such a device is known as a half-adder, and its gate circuit looks like this: The Σ symbol represents the “sum” output of the half-adder, the sum’s least significant bit (LSB). Cout represents the “carry” output of the half-adder, the sum’s…
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15.6 Memory with moving parts: Drives
The earliest forms of digital data storage involving moving parts was that of the punched paper card. Joseph Marie Jacquard invented a weaving loom in 1780 which automatically followed weaving instructions set by carefully placed holes in paper cards. This same technology was adapted to electronic computers in the 1950’s, with the cards being read…
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15.5 Read-Only Memory (ROM)
Read-only memory (ROM) is similar in design to static or dynamic RAM circuits, except that the “latching” mechanism is made for one-time (or limited) operation. The simplest type of ROM is that which uses tiny “fuses” which can be selectively blown or left alone to represent the two binary states. Obviously, once one of the…
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15.4 Historical, Nonmechanical Memory Technologies
Perhaps the most ingenious technique was that of the delay line. A delay line is any kind of device which delays the propagation of a pulse or wave signal. If you’ve ever heard a sound echo back and forth through a canyon or cave, you’ve experienced an audio delay line: the noise wave travels at…
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15.3 Modern Nonmechanical Memory
Now we can proceed to studying specific types of digital storage devices. To start, I want to explore some of the technologies which do not require any moving parts. These are not necessarily the newest technologies, as one might suspect, although they will most likely replace moving-part technologies in the future. A very simple type…
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15.2 Digital Memory Terms and Concepts
When we store information in some kind of circuit or device, we not only need some way to store and retrieve it, but also to locate precisely where in the device that it is. Most, if not all, memory devices can be thought of as a series of mail boxes, folders in a file cabinet,…
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15.1 Why digital?
Although many textbooks provide good introductions to digital memory technology, I intend to make this chapter unique in presenting both past and present technologies to some degree of detail. While many of these memory designs are obsolete, their foundational principles are still quite interesting and educational, and may even find re-application in the memory technologies…
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14.8 Practical considerations – Digital Communication
A principal consideration for industrial control networks, where the monitoring and control of real-life processes must often occur quickly and at set times, is the guaranteed maximum communication time from one node to another. If you’re controlling the position of a nuclear reactor coolant valve with a digital network, you need to be able to…
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14.7 Network Protocols
Aside from the issues of the physical network (signal types and voltage levels, connector pinouts, cabling, topology, etc.), there needs to be a standardized way in which communication is arbitrated between multiple nodes in a network, even if its as simple as a two-node, point-to-point system. When a node “talks” on the network, it is…
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14.6 Network Topology
If we want to connect two digital devices with a network, we would have a kind of network known as “point-to-point:” For the sake of simplicity, the network wiring is symbolized as a single line between the two devices. In actuality, it may be a twisted pair of wires, a coaxial cable, an optical fiber,…
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14.5 Optical Data Communication
A modern alternative to sending (binary) digital information via electric voltage signals is to use optical (light) signals. Electrical signals from digital circuits (high/low voltages) may be converted into discrete optical signals (light or no light) with LEDs or solid-state lasers. Likewise, light signals can be translated back into electrical form through the use of…
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14.4 Electrical Signal Types
With BogusBus, our signals were very simple and straightforward: each signal wire (1 through 5) carried a single bit of digital data, 0 Volts representing “off” and 24 Volts DC representing “on.” Because all the bits arrived at their destination simultaneously, we would call BogusBus a parallel network technology. If we were to improve the…
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14.3 Data Flow
Buses and networks are designed to allow communication to occur between individual devices that are interconnected. The flow of information, or data, between nodes, can take a variety of forms: With simplex communication, all data flow is unidirectional: from the designated transmitter to the designated receiver. BogusBus is an example of simplex communication, where the…
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14.2 Networks and Busses
This collection of wires that I keep referring to between the tank and the monitoring location can be called a bus or a network. The distinction between these two terms is more semantic than technical, and the two may be used interchangeably for all practical purposes. In my experience, the term “bus” is usually used…