The modern aeroplane is highly dependent on the digital computer, and this piece of equipment governs almost every facet of its operation.
Analogue Computers
Analogue computers are non programmable and deal with infinite continuous values rather than discrete ones. It uses digits from 0 to 9 and operates as a mechanical computer using a rotating gear or wheel to represent different values, eg. if the wheel is between 0° and 10° it represents 0 or between 11° and 20° it represents 1. The analogue computer thus suffers from friction between the moving parts and mechanical wear. The speedometer in a car is an everyday example of an analogue computer, since it is attached to a sensor that counts the revolutions of the road wheels and, using an assumed wheel radius, calculates the distance covered since the last reset. It adds this to the distance at the start of the run and indicates the total distance the car has covered since new. It also uses the distance per unit time to provide an indication of speed. The speedometer is thus a calculating machine, which uses a data input and by carrying out a calculation it converts the input into another form of information; speed via a moving needle and distance as a digital read out.
Analogue computers are still widely and effectively used although they suffer from the following limitations and shortcomings:-
¾ They are specific to a particular role and a separate computer is required for different applications.
¾ They use moving parts.
¾ They tend to be bulky and heavy.
Digital Computers
A Digital computer is also a calculating machine, but instead of using synchro and gears different voltages are used to represent the digits from 0 to 9. For example 0 – 0.9 volts would represent the digit 0 and a voltage from 1.0 – 1.9 volts would represent the digit 1 etc. This machine uses actual high-speed arithmetic to do the necessary calculations typically using a decimal number system. It is also possible to convert decimal values into digital values, or to convert analogue values into binary code. Everything that a digital computer does is based on one operation, which is represented by the ability to determine if a ‘switch’,
or ‘gate’ is open or closed. That is, the computer can recognise only two states in any of its
microscopic circuits, ie. an on/off, high voltage or low voltage, or in the case of numbers, 0 or 1. It is equally valid to reverse the process and produce an analogue value from a digital process using binary arithmetic.
The speed at which the computer performs this simple act, however, is what makes it such an essential element of the modem technology aeroplane. Computer speeds are measured in megahertz, or millions of cycles per second. A computer with a "clock speed" of 133 MHz is capable of executing 133 million discrete operations every second.
Digital computers are also normally integrated with other systems on an aeroplane, via signal-interfacing devices such as ‘Analogue-to-Digital (A/D) Converters’ and ‘Digital-to- Analogue (D/A) Converters’. The ‘Input Interface’ converts analogue data into a digital
format and the ‘Output Interface’ converts the digital data into an analogue format.
The processing speed of a Digital computer and its calculating power are further enhanced by the amount of data, which is handled during each cycle. If a computer checks only one switch at a time, that switch will only represent two commands or numbers. For example ‘ON’ would symbolise one operation, and ‘OFF’ would symbolise another. By checking groups of switches linked within a single unit simultaneously, the computer is able to increase the number of operations it can recognise during each cycle. For example, a computer that checks two switches at one time can represent four numbers (0 to 3) or can execute one of four instructions at each cycle, one for each of the following switch patterns: OFF-OFF (0); OFF-ON (1); ON-OFF (2); or ON-ON (3).
When digital computers were first introduced they were capable of checking eight switches (binary digits) or ‘Bits’ of data during every cycle, or a ‘Byte’, which contains 256 possible
patterns of ‘ONs’ and ‘OFFs’ (or 1's and 0's). A computer uses a standard information
format that consists of a group of bits, or a ‘Word’, which equates to:-
¾ an instruction. ¾ part of an instruction.
¾ a particular type of datum, eg. a number, a character or a graphics symbol. The pattern 11010010, for example, might be binary data (in this case, the decimal number 210) or it might tell the computer to compare data stored in its switches to data stored in a certain memory chip location.
The total list of recognisable operations or patterns, which a computer is capable of, is called its ‘Instruction Set’.
Computer Architecture
The physical components of a computer are known as ‘Hardware’ and a digital computer is
not a single component machine, but is made up of the five distinct elements, as shown in the following diagram.
The programmes used in a computer are alternatively known as ‘Software’. Input Devices
Input devices are the means by which a computer is fed with the information required for problem solving, and consist of the following typical hardware:-
¾ Keyboard ¾ Scanner
¾ Touch sensitive screen ¾ Speech recognition ¾ Mouse
¾ Joy stick
¾ Data from sensors
¾ As long as the data is identifiable the computer’s processor will be able to recognise it, and will accordingly route it along the appropriate internal ‘Buses or Data Lines’.
These form a network of communication lines that connect the internal elements of the processor, and also leads to external connectors linking the processor to the other elements of the computer system. The following types of CPU buses exist:-
¾ A ‘Control Bus’ consists of a line that senses input signals and another line
that generates control signals from within the CPU.
¾ The ‘Address Bus’, is a one-way line from the processor that handles the
location of data in memory addresses.
¾ The ‘Data Bus’, a two-way transfer line that both reads data from memory
and writes new data into memory.
Central Processing Unit
The Central Processing Unit (CPU) may consist of a single chip, or a series of chips that are able to perform arithmetic and logical calculations, and can also control the operations of the other system elements. A ‘Microprocessor’ is a miniature CPU chip, which incorporates
additional circuitry and memory. CPU chips and microprocessors consist of the functional sections, shown below.
The CPU receives input data and uses that data to carry out specific instructions, from which an output is derived. Typical ‘Input’ data might be wind velocity and direction, or even the
distance to run to a destination. The CPU then carries out calculations on this data using the following parts to give ‘Output’ data, such as TAS, or time to run to the next waypoint.
Central Control Unit. This unit coordinates the functions being carried out in each section
of the computer via a ‘Communication Link’ or ‘Data Transfer Bus’. The Control Unit
decodes or reads the patterns of data being held in a designated ‘Register’, or temporary
storage area, and keeps track of any instructions. The register also holds the location and results of these operations, and the control unit translates the pattern into an activity, such as adding or comparing. It also indicates the order in which individual operations use the CPU, and regulates the amount of CPU time that each operation may consume.
Memory. This is normally divided into either ‘Volatile Memory’, which is lost whenever the
computer loses power, and ‘Non-Volatile Memory’, which remains in the system until it is
over-written with new data. The main types of internal memory are:-
RAM (Random Access Memory). This is Volatile memory and the data deposited
in it is thus lost whenever the power is turned off, or alternative states are written in.
ROM (Read only Memory). This is Non-Volatile memory and normally contains
data that has been inserted on the chip during its manufacture. The ROM typically contains start-up details and mathematical formulae, which will be maintained even after the power has been switched off. Replacing the entire chip is the only way to change the instructions on a ROM.
PROM (Programmable Read Only Memory). This form of memory is non-volatile,
but unlike the ROM can be reprogrammed once only, with the chip still fitted in the aeroplane’s computer.
EPROM (Erasable Programmable Read Only Memory). This type of memory is
also non-volatile and can be reused indefinitely. It can be totally erased and then reprogrammed with the chip still fitted in the computer.
Arithmetic and Logic Unit (ALU). This chip gives the computer its calculating capability,
allowing both arithmetical and logical calculations using a combination of digital logic circuits. These circuits are used to make specific true-false decisions based on the presence of multiple true-false signals at the inputs, and the signals may be generated by either mechanical switches, or by solid-state transducers, which are combined together to form an
‘Integrated Circuit (IC)’ Output Devices
The output devices enable the user to see the results of the computer's calculations or data manipulations. The most common output device is the video display screen, which is a monitor that displays characters and graphics on a ‘Cathode-Ray Tube (CRT)’, or
television-like screen. A screen is usually small, and portable computers commonly use liquid crystal displays (LCD) or other forms of screen.
Examples of such screens are the EFIS and ECAM displays on modern aeroplanes.
The standard output devices include printers and modems. A modem links two or more computers by translating digital signals into analogue signals so that data can be transmitted via telecommunications.
Outputs may also be in the form of signals that are sent to the operating devices, and are typically used to control the engines or Automatic Flight Control System on the aeroplane.
Storage Devices
Computer systems can store data internally (in memory) and externally (on storage devices). External storage devices, may physically reside within the computer's main processing unit, or external to the main circuit board. These devices store data as electrical charges on a magnetically sensitive medium such as an audiotape, or a disk coated with a fine layer of metallic particles, or alternatively as an imprint on a ‘Laser Readable Disk’. The most
common external storage devices are called ‘Floppy’ and ‘Hard Disks’. Floppy disks can
contain from several hundred thousand bytes to well over a million bytes of data, depending on the system. ‘Hard, or Fixed’,' disks cannot be removed from their disk-drive cabinets,
which contain the electronics to read and write data onto the magnetic disk surfaces. Hard disks can store from several million bytes to a few hundred million bytes. ‘CD-ROM’
technologies, which use the same laser techniques that are used to create audio compact disks (CDs), also provide storage capacities in the range of several gigabytes (billion bytes) of data.
Operating Systems
An operating system is a master control program, which is permanently stored in the memory. They interpret user commands and request various kinds of services, such as display, print, or copy a data file; list all files in a directory; or execute a particular program. Different types of peripheral devices, such as disk drives, printers, communications networks and so on, handle and store data differently from the way the computer handles and stores it. Internal operating systems are usually stored in ROM memory, and are developed primarily to co-ordinate and translate data flows from dissimilar sources, such as disk drives or co- processors (processing chips that perform simultaneous but different operations from the central unit
Programming
A program is a sequence of instructions that tells the hardware of a computer which operations to perform on the data. Programs can be built into the ‘Hardware’ itself, or they
may exist independently as ‘Software’. In some specialised computers, the operating
instructions are embedded in their circuitry; as in the Flight Management System (FMS). Once a computer has been programmed, it can only do as much, or as little as the software controlling. Software in widespread use includes a wide range of applications programmes and instructions to the computer on how to perform various tasks.