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CONTROL UPDATE: Definition of Some Common Computer Control Terms.

In the good old days, control systems seemed a lot simpler than now, with computers, PLCs and microprocessors taking over from switches and levers. This Control Update will attempt to make sense out of the new terminology by defining some of the more commonly used terms.

Bit -- If you were to look at a stream of data from a computer, what you would see is a series of zeroes and ones. 001100101101100101101100010, etc. The reason for this is that the computers are Digital, meaning they can only recognize two conditions: zero and one. To create information from these zeroes and ones, computers must put them together in different combinations, more or less like Samuel Morse did when he invented his Morse Code. The most basic piece of information is a bit, which can be either a zero or a one. Therefore, an eight bit word might be 01100111.

Byte -- Just to make things more confusing, some smart guy about 50 years ago decided to call an eight bit word a byte. This has been used by computer designers as the basis for their hardware and by software designers in their software structure ever since. Data is usually organized and transmitted in bytes. Older computers transmitted a byte at a time, while today's Pentium computers work with 32 bit, or 4 byte words.

Cycle Time -- Real time control computers seem to be doing things continuously because they do them so fast. In reality, control computers gather data from sensors and other inputs, they do some calculations and logic, send commands to control elements, such as valves, pumps, etc. The time it takes to accomplish all of the above is called cycle time, because when it is done with a cycle, the computer starts over again gathering data, analyzing it and taking action again. The cycle time needed for control is a function of the process being controlled. Jet fighter control cycle times might be only a millionth of a second. Refrigeration control systems usually have cycle times of a tenth of a second or so.

Analog -- Analog signals (both inputs and outputs) are those which can vary continuously from their low to high values. A room temperature is an analog value. The computer sees a varying voltage input from the sensor which represents the temperature.

Digital -- Digital signals are those which only have two values: on or off; true or false; zero or one. An example of a digital output is a signal which opens a valve or starts a motor. An example of a digital input is a signal from a float switch. Digital signals are usually 120VAC, but can be any voltage.

Analog to Digital Converter -- Sometimes called an A/D board, this component in a control system converts an analog signal of varying voltage into a digital word. A 12-bit A/D converter uses 12 bits to describe the input voltage for the computer, in 4096 steps from 000000000000 at the bottom to 111111111111 at the top of the scale. Twelve ones (111111111111) is seen by the computer as the number 4095. Since a digital computer controller only recognizes zeroes and ones, it needs an A/D board to convert the analog signals into this digital form so it can use analog signals in its control scheme.

We will continue with more of these definitions in the next issue.
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No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1998 Gale, Cengage Learning. All rights reserved.

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Publication:Frozen Food Digest
Geographic Code:1USA
Date:Oct 1, 1998
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