The effects on the aircraft maintenance industry
By Jim Sparks
The term "digital" often brings about thoughts of consumer electronics with labels containing phrases like "All internal components are non-user serviceable" or "Removal of cover by non-authorized personnel voids warranty."
In fact the word digital relates to the way that a computer performs its functions, by counting digits. The computer concept dates back to around 1830 when Charles Babbage developed a very basic mechanical device for performing computations. It wasn't until the mid 1940s that the technology surfaced which provided the foundation for modern digital concepts. The first electronic computer required an entire room and employed vacuum tubes. By today's standards it could only accomplish about the same things as a very basic hand-held calculator.
Digital technology has all but taken over the aircraft maintenance industry. Aircraft systems on many current production aircraft utilize microprocessors to perform tasks originally left up to the pilots, navigators, communications officers, or flight engineers. A microprocessor is often referred to as a computer. It is in fact the most important part of the computer, the stored data.
We now have electronic engine controls (EEC) interfacing with digital automatic flight control systems (DAFCS), that are updated by flight management systems (FMS) that have the ability to choose and tune radio navigation stations. All of the above may then work with the auto landing system. Aircraft systems are not the only digital inroads. Aircraft documentation is now more often than not presented in an electronic format. Plus the tools needed to maintain these new machines often require some basic computer knowledge.
Technology has rapidly progressed from the days of the vacuum tube to the transistor and on to integrated circuits and depending on their function may incorporate more than a million transistors.
Analog and digital
Electronic circuits are broken down into two extensive groupings, analog and digital.
An analog signal may be described as one with continuous values, whereas a digital signal has a discrete or specific value. This can be easily realized by observing an analog voltmeter connected to a variable DC power supply. With the voltmeter selected on the appropriate range and then increasing the output of the power supply slowly from 0 to 10 volts, the sweep of the meter movement will look and display each and every value produced. A digital voltmeter connected to the same power supply will only read specific values. That is if the range is set to read in tenths of a volt, we will observe 1.1, 1.2, 1.3, etc. as voltage is increased. If we reset the meter range we can read in one hundredths of a volt 1.11, 1.12, 1.13, etc. In any case the meter will not display all the values like its analog counterpart.
The first thing to understand is that "digital" is like a language or a means of communication. In other words, we will not find a digital signal itself producing work. Digital data in itself can not light a light bulb or operate an electric motor. It can command some type of electronic switch often called a driver to perform the above-mentioned tasks plus a myriad of others. Digital is nothing but a form of electronic communication.
A distinct digital advantage is that digital data can also be processed and transmitted very efficiently. Storage capacity also gives digital the lead as compact memories can hold incredible amounts of data and then reproduce it with tremendous accuracy. A compact disc (CD) is a good example of a storage device for digital media. At some point in a sound studio somewhere Elvis, Credence Clearwater Revival, or maybe even Dean Martin began to produce sound waves. These were captured by microphones and converted to analog electrical signals. Now those signals can go to an analog to digital (A to D) converter and become digitized. This data can then be stored on the CD. Once the CD is installed in a player, the digital data is read by a laser beam and sent to a digital to analog (D to A) converter, which will allow the digitized stored frequencies to be exactly reproduced, amplified, and then sent to either a headset or speaker system for reproduction.
A matter of logic
Digital data is represented in logic levels with the voltage represented as either a 0 or 1. These also make up the two digits of the binary numbering system. In fact combining the words binary and digit result in BIT, short for binary digit. In most digital circuits a 1 represents a higher voltage and a 0 represents a lower voltage level. In electronic terms a 1 is a high logic level and a 0 is a low logic level. The interpretation of a 1 and 0 can be altered depending on the application.
The term logic is applied to digital circuits and is used to imply that a certain set of conditions or propositions have been imposed. Logic in its most basic form is human reasoning. This can be used to declare certain statements true providing a specific set of rules or circumstances exist. In other words any device or circumstance that will influence the end result has to be considered. All statements should use two state characteristics such as true or false, yes or no, on or off, high or low and of course 1 or 0.
In the 1850s George Boole, an Irish mathematician, created a system for expressing logic statements using symbols so that problems can be solved using principles of basic algebra. This system is widely used today in the design and fault resolution of digital circuits.
The building blocks for any microprocessor consist of logic circuits and although there are numerous types dealing with many specialized circumstances, the foundation is based on three distinct types, the YES gate, AND gate, and OR gate. Devices of this type will supply either a high or low output based on the nature of the inputs. In other words, by understanding the function of the gate and by knowing the state of the inputs a determination or conclusion can be reached regarding the output. Inputs are all located on one side of the logic symbol while the lone output is confined to the opposite side of the representation.
Digital waves are used as the means of transmitting the varied logic states and consist of rapidly changing voltage levels representing the highs and lows. On this waveform the pulse or bit has a leading edge that occurs first and is considered a rising voltage while the trailing edge occurs last and has a declining voltage. The time for completion of the leading edge phase is called the "rise" time while the trailing edge is referred to as the "fall" time. An "ideal pulse" would be one where the rise and fall time would be instantaneous, however in reality time is a factor. The pulse width is a measure of the duration or time. A non-ideal pulse is generally measured at 50 percent of the rise and fall points. This practice makes it possible to conclude that all pulses are ideal for most digital work.
Transfer or communication of digital data is another essential part of any computerized system. Once the bits are formatted they form a "digital word" called a "BYTE." This is comprised of eight bits. Digital waves are transmitted using either a serial or a parallel form. The serial connection uses a single conductor with all data being sent one bit after the other taking into account specific time intervals for each piece of binary data.
Data sent in a parallel form will send all bits at the same time on different conductors. That is each bit has its own data line. In this case only one time interval is needed. In short, data can be transferred faster using a parallel connection but more data lines are required.
Microprocessors often use metal oxide semiconductor (MOS) technology, which require less area on a microchip and tend to consume less power than Transistor Transistor Logic (TTL).
Due to the makeup of MOS devices electro static discharge (ESD) is the leading cause of failure. Precautions when testing or even handling computerized devices are essential and include:
- Shipping and storage devices should all be ESD compliant.
- All equipment to contact the device including the technician should be grounded.
- Never remove a MOS device while the circuit is powered.
Digital circuit's heart and soul
Numbering systems are the heart and soul of digital circuits. Not only is this the principle of computer operation, it is often the digital systems' means of reporting malfunctions. The decimal version is what most all of us have grown up with and implies units of 10. We have zero through nine but we are not limited to that. By placing an additional digit either to the left or right we have the capability to count endless quantities. Binary numbering is an alternative means of representing quantities and is less complicated than the decimal system because it only uses two digits. As with the decimal system the position of the digit starts with the smallest value on the right progressing to higher values on the left.
The table on page 36 illustrates the format of the binary system with the column on the far right representing either 1 or 0. The second column from the right represents decimal number 2 with column three representing 4 and the fourth column being 8. By using this chart it is easy to determine that decimal number 15 is represented as 8 + 4 + 2 + 1.
Ten numeric digits and six alphabet letters are used for the hexadecimal numbering system (Page 37). This system is used as a compact way of displaying binary numbers and in many cases avionics system manufacturers use this as a method of fault identification.
The oscilloscope is one of the most used instruments for testing and troubleshooting digital circuits. Information display is in a graphic form enabling the operator to look at an exact representation of a digital signal. This includes observation of the voltages and time periods. One of the primary reasons to use digital systems is the self-diagnostics or "built-in test equipment."
In reality the term digital will cover more topics than can be discussed here. What is important is the realization that the digital age is upon us all and expanding prospects face each and every aviation maintenance technician. It used to be easy to tell where the airframe stopped and either the engine or avionics systems started. This is not the case today. Even though many electronic devices require very little attention or line maintenance, understanding the concepts and mastering the tools involved will be the only way to keep today's and tomorrow's aircraft in an airworthy and flight-ready condition.