DC2004, Chapter 1 Errata and Remarks

This discussion really belongs in Chapter 9, but the text has addressed this topic under "High Tech Talk" in Chapter 1.

The following are excerpts from
Thomas D. Truitt and A. E. Rogers, Basics of Analog Computers, John F. Rider Publisher, Inc. (December 1960)

The word "analog" refers to a thing which maintains a "relation of likeness with another, consisting in the resemblance not of the things themselves, but of two or more attributes, or effects".

There are three categories:

1. scale models
2. automata
3. analog models

The adjectives "digital" and "analog" are in common use in 1960 to reference electronic computers.  Specifically, they refer to the general purpose digital and the general purpose d-c electronic analog computers.  It is important to recognize that while analog computer refers mostly to this one specific type of analog computer, it can just as well refer to certain mechanical and hydraulic devices, to general purpose a-c electronic computers, and to a variety of special purpose computers.  All of these have one characteristic in common - that the components of each computer or device are assembled to permit the computer to perform as a model, or in a manner analogous to some other physical system.  Thus all analog computers fall in the third category.

A computer does not process data into information.  Information is a property of data.  Information is the meaning represented by data.  Processing can make information easier to detect or understand.  Meaningless data does not become information because it is processed by a computer, or processed by anything else.  It is still garbage.

Input Device. An input device is a device from which input is received. In addition to devices common to a microcomputer in an office, a device such as a laser that provides input signals to a computer used in an industrial setting is an input device. The input device provides a stream of discrete or continuous input, and does not care how the computer interprets and uses the input. It is possible that the input data can be used as instructions as well as data upon which instructions operate. All input is data. Some data are instructions. This is an important concept for programmers and computer science majors.

Output Device. An output device is a device to which output is sent. In addition to devices that display data intended for humans to see or hear, a device such as a control actuator on a numerically controlled lathe is an output device. The output does not have to be meaningful to a human. The data sent to an output device must be in a format expected by that device, which depends upon the design of that particular device. Some output devices accept discrete data, other output devices accept continuous data.

System Unit. Another name for the box containing the components is chassis. The term chassis is used more by people working with computers that are not personal computers.

Passive Back Plane is an alternative to the Motherboard. The passive back plane is used in industrial or military applications where the dominant hardware requirement is to reduce the Mean Time To Repair (MTTR). A passive back plane has only connectors and buses between connectors. The goal of the passive back plane approach is to permit a technician to merely swap printed circuit cards until the computer is fixed. For a microcomputer, such a design permits restoring the unit to working condition within 5 minutes of a failure. The motherboard approach emphasizes reducing the cost of manufacture. Motherboards take much longer to remove from a chassis. Due to their size, motherboards are also more prone to failure through flexure and individual component failure than individual circuit cards such as used in the passive back plane approach. Motherboards are good for use in an office environment, but not in a harsh setting.

Internet: A world wide collection of interconnected networks that use TCP/IP. There are other world wide digital networks. Use of TCP\IP distinguishes Internet from other networks.

Analog computers also exist and are important. While the description below is old, it is still valid. Digital methods are not fast enough for some computing tasks. Often, this is limited by the speed and resolution of analog to digital converters. When this is the case, analog computers are used. A critical issue with analog computers is keeping it calibrated properly, and detecting when it is out of calibration. 

The following is from  From:  T. D. Truitt and A. E. Rogers, Basics of Analog Computers, John F. Rider Publisher, Inc. (1960)

Page 1-23

'The characteristic most commonly associated with analog devices is the continuous nature of their behavior.  That is, the time variations of their several physical features are smooth changes.  Between any two positions of a physical variable there are an infinite number of intermediate positions.  Unfortunately, some people use the adjectives "analog" and "continuous" interchangeably.  This usage is not correct, for although the two terms are loosely related, their association lies only in the fact that the majority of primary systems which we want to simulate by analog means are continuous devices.'

"In contrast to systems with continuous behavior there are systems whose variables can assume a certain number of discrete states or levels.  These systems change from one state to another in an abrupt or discontinuous manner.  Such systems are called discrete-state or digital systems.  It would be possible to simulate a discrete-state system by analog means by building a model from discrete state devices, and these then would be called analog devices.  We can still say that the model simulates the primary system in a continuous manner, for the behavior of each is the same for every instant in time.  Thus the analog characteristic on "continuousness" might be best expressed as: maintaining correct correspondence between analog and primary system for every instant of time. "

Page 1-24

"Although a continuous device may assume an infinite number of positions between any two given positions, in any practical analog device it is not always certain which position is assumed. This is due to the ever-present random variables, mechanical vibration and electrical noise.  Conversely, the condition of a discrete-state device is always known and its resolution is often adjustable to provide better and better approximations to the continuous variable (at some cost in speed or equipment). "

Page 1-36

The building blocks of a general purpose analog computer include devices such as

Notice that the definitions are given in terms of representing values rather than taking on values.  It is the usual case that digital signals are not represented as square wave signals when being transmitted for any distance other than a short distance.  Also, the signals used to magnetically record digital data are not square wave signals.  A lot of work goes into encoding digital signals to make them more recoverable from storage media or after transmission.