Aircraft Analog Computers. Aircraft Digital Computers

Early aircraft computers were used to take continuous streams of inputs to provide flight assistance. Examples of aircraft analog inputs are fuel gauge readings, throttle settings, and altitude indicators. Landau (1) defines an analog computer as a computer for processing data represented by a continuous physical variable, such as electric current. Analog computers monitor these inputs and implement a predetermined service when some set of inputs calls for a flight control adjustment.

For example, when fuel levels are below a certain point, the analog computer would read a low fuel level in the aircraft’s main fuel tanks and would initiate the pumping of fuel from reserve tanks or the balancing of fuel between wing fuel tanks. Some of the first applications of analog computers to aircraft applications were for automatic pilot applications, where these analog machines took flight control inputs to hold altitude and course. The analog computers use operational amplifiers to build the functionality of summers, adders, subtracters, and integrators on the electric signals.

Aircraft Digital Computers. As the technologies used to build digital computers evolved, digital computers became smaller, lighter, and less power- hungry, and produced less heat. This improvement made them increasingly acceptable for aircraft applications. Digital computers are synonymous with stored-program computers. A stored-program computer has the flexibility of being able to accomplish multiple different tasks simply by changing the stored program.

Analog computers are hard-wired to perform one and only one function. Analog computers’ data, as defined earlier, are continuous physical variables. Analog computers may be able to recognize and process numerous physical variables, but each variable has its unique characteristics that must be handled during processing by the analog computer. The range of output values for the analog computer is bounded as a given voltage range; if they exceed this range, they saturate. Digital computers are not constrained by physical variables.

All the inputs and outputs of the digital computer are in a digital representation. The processing logic and algorithms performed by the computer work in a single representation of the cumulative data. It is not uncommon to see aircraft applications that have analog-to-digital and digital-to-analog signal converters.

This method is more efficient than having the conversions done within the computers. Analog signals to the digital computer are converted to digital format, where they are quickly processed digitally and returned to the analog device through a digital-to-analog converter as an analog output for that device to act upon. These digital computers are smaller, more powerful, and easier to integrate into multiple areas of aircraft applications.

Landau (1) defines a digital computer as a computer for processing data represented by discrete, localized physical signals, such as the presence or absence of an electric current. These signals are represented as a series of bits with word lengths of 16, 32, and 64 bits. See microcomputers for further discussion.

Wakerly (2) shows number systems and codes used to process binary digits in digital computers. Some important number systems used in digital computers are binary, octal, and hexadecimal numbers. He also shows conversion between these andbase-10 numbers, as well as simple mathematical operations such as addition, subtraction, division, and multiplication.

The American Standard Code for Information Interchange (ASCII) of the American National Standard Institute (ANSI) is also presented, which is Standard No. X3.4-1968 for numerals, symbols, characters, and control codes used in automatic data processing machines, including computers. Figure 1 shows a typical aircraft central computer.

Figure 1. Typical aircraft central computer

Microcomputers. The improvements in size, speed, and cost through computer technologies continually implement new computer consumer products. Many of these products were unavailable to the average consumer until recently. These same breakthroughs provide enormous functional improvements in aircraft computing. Landau (1) defines microcomputers as very small, relatively inexpensive computers whose central processing unit (CPU) is a microprocessor.

A microprocessor (also called MPU or central processing unit) communicates with other devices in the system through wires (or fiber optics) called lines. Each device has a unique address, represented in binary format, which the MPU recognizes. The number of lines is also the address size in bits. Early MPU machines had 8-bit addresses. Machines of 1970 to 1980 typically had 16-bit addresses; modern MPU machines have 256 bits.

Common terminology for an MPU is random access memory (RAM), read only memory (ROM), input-output, clock, and interrupts. RAM is volatile storage. It holds both data and instructions for the MPU. ROM may hold both instructions and data. The key point of ROM is that it is nonvolatile. Typically, in an MPU, there is no operational difference between RAM and ROM other than its volatility.

Input-output is how data are transferred to and from the microcomputer. Output may be from the MPU, ROM, or RAM. Input may be from the MPU or the RAM. The clock of an MPU synchronizes the execution of the MPU instructions. Interrupts are inputs to the MPU that cause it to (temporarily) suspend one activity in order to perform a more important activity.

An important family of MPUs that greatly improved the performance of aircraft computers is the Motorola M6800 family of microcomputers. This family offered a series of improvements in memory size, clock speeds, functionality, and overall computer performance.

Personal Computers. Landau (1) defines personal computers as electronic machines that can be owned and operated by individuals for home and business applications such as word processing, games, finance, and electronic communications. Hamacher et al. (3) explain that rapidly advancing very large-scale integrated circuit (VLSI) technology has resulted in dramatic reductions in the cost of computer hardware. The greatest impact has been in the area of small computing machines, where it has led to an expanding market for personal computers.

The idea of a personally owned computer is fairly new. The computational power available in handheld toys today was only available through large, costly computers in the late 1950s and early 1960s. Vendors such as Atari, Commodore, and Compaq made simple computer games household items. Performance improvements in memory, throughput, and processing power by companies such as IBM, Intel, and Apple made facilities such as spreadsheets for home budgets, automated tax programs, word processing, and three-dimensional virtual games common household items.

The introduction of Microsoft’s Disk Operating System (DOS) and Windows has also added to the acceptance of the personal computers through access to software applications. Improvements in computer technology offer continual improvements, often multiple times a year. The durability and portability of these computers is beginning to allow them to replace specialized aircraft computers that had strict weight, size, power, and functionality requirements.