Oxford Butterworth-Heinemann - Principles and practices of aircraft maintenance ix, p.: , English, Book, Online. Aircraft digital electronic and computer. Aircraft Digital Electronic and. Computer Systems. Mike Tooley. R Routledge. Taylor & Francis Group. LONDON AND NEW YORK. Aircraft Digital Electronic and Computer Systems by Mike Tooley, , available at Book Depository with free delivery worldwide.
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lesforgesdessalles.infoom Aircraft Digital Electronic and Computer Systems www. ebookcom A next-generation Boeing aircraft with seating for up to . Downloads PDF Aircraft Digital Electronic and Computer Systems, 2nd ed, PDF Downloads Aircraft Digital Electronic and Computer Systems. An introduction to the principles of aircraft digital and electronic systems by Mike Tooley, this book is written for anyone pursuing a career in aircraft maintenance.
The optical transmitter consists ray of light at a boundary between two optical of an infrared light emitting diode LED or low- media. Introduction to Aircraft Electronic Systems 2. The prefetch queue is fed to the instruction decode unit which translates the instructions into 7. Give rea- c it avoids having a large number of different sons for your answers. We will explain how bus control logic.
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Figure 2. The binary numbers that are equivalent to the decimal numbers 0 to 9 are shown in Table 2. Notice how the most significant digit MSD is shown on the 2. Table 2. What is the binary weight of the MSB in the number ? Binary Decimal 0 1 2. For example, to convert the 5 binary number , we take each digit and multiply 6 it by the binary weight of the digit position 8, 4, 2 and 1 and add the result, as shown in Figure 2.
Convert the following binary numbers respective digit position, as shown in Figure 2. Convert the following decimal numbers to binary: Note how the binary number is built up in reverse order, i. Another form of binary number commonly used in 2. In the case of Figure 2. Convert the following decimal numbers to BCD: Convert the following BCD numbers to decimal: Similarly, the 2.
Because octal consists of eight digits 0 to 7 , a single octal digit can replace three binary digits. Put another way, by arranging a binary number into groups of three binary digits or bits we can replace each group by a single 2.
The other method of decimal to octal conversion involves successive division by eight, retaining the remainder as a digit with a value between 0 and 7 2. Note how the octal In order to convert a binary number to a decimal number is built up in reverse order, i. For example, the octal number is converted by taking each digit and then multiplying it by the octal weight of the digit position and adding the result, as shown in Figure 2.
The hexadecimal base 16 numbering system offers a practical compromise acceptable to both humans and machines. One hexadecimal digit can represent four binary digits, thus an 8-bit binary number can be expressed using two hexadecimal digits. For example, binary is the same as 83 when expressed in hexadecimal. The correspondence between a hexadecimal hex digit and the four binary digits it represents is quite straightforward and easy to learn see Table 2.
Note that in hexadecimal the decimal numbers from 10 to 15 are represented by the letters A to F, respectively. Furthermore, conversion between binary Table 2. In this case you simply need to arrange the binary number in groups of three binary 2 2 2 digits from right to left and then convert each group 3 3 3 to its equivalent octal number, as shown in Figure 2. Hexadecimal notation is much more compact than binary notation and easier to work with than decimal notation. For example, the hexa- decimal number of A7 is converted by taking each digit and then multiplying it by the weight of the digit position, as shown in Figure 2.
Note how, in the case of the example shown in Figure 2. In this case you hexadecimal number to its corresponding four-bit simply need to arrange the binary number in groups binary value, as shown in Figure 2. The method is of four binary digits, working from right to left, similar to that which you used earlier to convert octal before converting each group to its hexadecimal numbers to their binary equivalents.
ASCII is the means by which computers are able to store and exchange alphanu- meric data with other computers and peripheral devices such as character-based displays and printers.
Standard ASCII is based on a 7-bit binary code and caters for a total of 27 alphanumeric characters both upper and lower case , punctuation and special control characters.
Extended ASCII employs an eighth leading bit to provide an additional characters from a single 8-bit byte. The first 32 ASCII characters corresponding to hexadecimal codes 00 to 1F are non-printing control characters used to facilitate data transfer and control peripherals such as character-based displays and 2. The ASCII printable characters corresponding to method is similar to that which you used earlier to hexadecimal codes 20 to 7F represent letters, num- convert binary numbers to their octal equivalents.
Find the decimal equivalent of the octal number Find the octal equivalent of the decimal 1. Find the binary equivalent of the octal meric text. Convert the hexadecimal number 3F to: Convert the binary number to: The binary coded decimal BCD number is equivalent to the decimal number: The binary number is equivalent to the decimal number: The decimal number 37 is equivalent to the binary coded decimal BCD number: The decimal number 29 is equivalent to the binary number: Which one of the following numbers could not be an octal number: The octal number 73 is equivalent to the decimal The binary number is equivalent to number: The binary number is equivalent to The hexadecimal number AD is equivalent to the the octal number: The hexadecimal number is equivalent to the The number is equivalent to: The hexadecimal number C9 is equivalent to the decimal number: Because of this, there is a need for devices that can convert signals in analogue form to their equivalent in digital form, and vice versa.
This chapter introduces digital to analogue and analogue to digital conversion. Later we will show how data conversion devices are used in some practical aircraft systems. We shall begin by looking at the essential characteristics of analogue and digital signals and the principle of quantisation. The analogue signal shown in Figure 3. The actual voltages used to represent the logic digital converter using a four-bit binary code are levels are determined by the type of logic circuitry shown in Figure 3.
Thus, any voltage represented by a digital code in In order to represent an analogue signal using which the MSB is logic 1 will be negative. Figure 3.
The basic digital to analogue converter DAC has a number of digital inputs often 8, 10, 12 or 16 and a single analogue output, as shown in Figure 3. Note that shown in Figure 3. However, a further inverting amplifier stage can be added at the output in order to change the polarity if required. The voltage gain of the inputs to the operational amplifier determined by the ratio of feedback to input resistance and taking into account the inverting configuration is shown in Table 3.
For example, when the output of the latch takes the binary value the output voltage can be calculated from the relationship: An improved binary-weighted DAC is shown in Figure 3.
This circuit operates on a similar principle to that shown in Figure 3. The analogue switches are controlled by the logic inputs so that the respective output is connected to the reference volt- age Vref when the respective logic input is at logic 1, and to 0 V when the corresponding logic input is at Table 3.
A more practical 1 0 0 1 —5.
Note that only two resistance values are 1 1 0 0 —7. When compared with the previous arrange- The accuracy of a DAC depends not only on the values ment, this circuit offers the advantage that the of the resistance used, but also on the reference reference voltage is considerably more accurate and voltage used to define the voltage levels. Special band- stable than using the logic level to define the analogue gap references similar to precision zener diodes are output voltage. A further advantage arises from the normally used to provide reference voltages that are fact that the reference voltage can be made negative, closely maintained over a wide range of temperature in which case the analogue output voltage will become and supply voltages.
Typical accuracies of between 1 positive. Unfortunately, by virtue of the range of resistance The resolution of a DAC is an indication of values required, the binary-weighted DAC becomes the number of increments in output voltage that increasingly impractical for higher-resolution appli- it can produce, and it is directly related to the num- cations. Taking a bit circuit as an example, and ber of binary digits used in the conversion.
This can be achieved by adding further binary inputs.
For example, a DAC with eight inputs i. A bit device, on the other hand, will produce 1, i. The resolution of a DAC is generally stated in terms of the number of binary digits i. Typical DACs have resolutions of 8, 10 or 12 bits. The basic analogue to digital converter ADC has a single analogue input and a number of digital outputs often 8, 10, 12 or 16 lines , as shown in Figure 3. The presence of The simplest form of ADC is the flash converter these levels on the output signal can be undesirable shown in Figure 3.
This can be easily reference voltages using a number of operational accomplished by passing the output signal through a amplifiers IC1 to IC7 in Figure 3. When the low-pass filter, as shown in Figure 3. The filter is designed so that the residual sampling frequency components i. This shows an 8-bit converter that uses a DAC produces a binary output code that indicates the value usually based on an R—2R ladder together with a of the most significant logic 1 received on one of single operational amplifier comparator IC1 and a its inputs.
The 8-bit output most significant logic 1 and hence the binary output from the SAR is applied to the DAC and to an 8-bit code generated will be , as shown in Figure 3. For example, an 8-bit flash ADC is ready for use.
At the cations and this type of ADC is relatively simple and point at which the output from the comparator available at low cost. This type same as the analogue output from the DAC and, at this of ADC uses a ramp generator and a single operational point, the conversion is complete.
The output of the com- then generated and the 8-bit code from the SAR is parator is either a 1 or a 0, depending on whether the read as a digital output code. The output of the com- slower than flash types and typical conversion times parator is used to control a logic gate IC2 , which i.
Despite this, conversion frequency to the input of a pulse counter whenever times are fast enough for most non-critical appli- the input voltage is greater than the output from the 3. The number of clock pulses counted will analogue to digital converter see Figures 3. Typical waveforms for the ramp-type waveform are shown in Figure 3. The important 3 thing to note about this type of ADC is that, while the slope of the positive ramp depends on the input 4 voltage, the negative ramp falls at a fixed rate.
Hence this type of ADC can provide a very high degree of 5 accuracy and can also be made so that it rejects noise 6 and random variations present on the input signal. Give rea- c it avoids having a large number of different sons for your answers. Sensing and recording the strain in a beam 6. A bit DAC is capable of producing: Converting a high-quality audio signal into c 1, different output levels. The process of sampling approximating an 3.
Measuring DC voltages that may be analogue signal to a series of discrete levels is accompanied by supply-borne hum and referred to as: In a binary-weighted DAC the voltage gain for each digital input is determined by: A DAC can produce different output voltages.
This DAC has a resolution of: The resolution of a DAC is stated in terms of: The conversion time of a flash ADC is typically c positive input voltage. The DAC used in a successive approximation c successive approximation type. ADC is usually: Which one of the following ADC types uses a a ramp type large number of comparators: In a successive approximation ADC, the time An advantage of a dual-ramp ADC is: In this section we shall take a brief look at the principles of aircraft data bus systems before introducing some of the systems that are commonly used in modern aircraft.
With such a large number of avionic systems, a 4. Furthermore, some of the cabling runs in a Bus systems can be either unidirectional one way large aircraft can be quite lengthy, as shown in Figure or bidirectional two way , as shown in Figure 4.
Aircraft cabling amounts to a significant pro- They can also be serial one bit of data transmitted portion of the unladen weight of an aircraft and so at a time or parallel where often 8, 16 or 32 bits of minimising the amount of cabling and wiring present data appear as a group on a number of data lines at the is an important consideration in the design of modern same time.
Because of the constraints imposed by aircraft, both civil and military. Bus systems provide an efficient means of exchang- ing data between the diverse avionic systems found in a modern aircraft see Figure 4. These bus systems use serial data transfer because it minimises the size and weight of aircraft cabling. Imagine for a moment that you are faced with the problem of organising a discussion between a large number of people sitting around a table, all of whom are blindfolded and therefore cannot see one another.
These and other consider- ations would form an agreed protocol between the delegates for conducting the discussion. The debate should proceed without too many problems provided everybody in the room understands and is willing to accept the protocol you have established. In com- puters and digital systems communications protocols are established to enable the efficient exchange of data between multiple devices connected to the same bus.
Individual line replaceable units LRUs , such as the 4. The architecture Within the LRU, the dedicated digital logic and is often described in the form of a block schematic microprocessor systems that process data locally each diagram showing how the various system elements are make use of their own local bus system. These local bus interconnected and also how the data flow is organised systems invariably use parallel data transfer, which is between the elements.
The architecture of a system ideal for moving large amounts of data very quickly, based on the use of a unidirectional serial bus system but only over short distances. Protocols consist of a set of rules and specifications governing, among other things, data format and physical connections. Within the LRU, data are transferred using an internal parallel data bus either 8, 16, 32 or 64 bits wide.
The link between the two LRUs is made using a simple serial cable often with only two, four or six conductors. The 4. Within be asynchronous i. This can be appropriate voltage- and current-level shifting the achieved by the bus system illustrated in Figure 4. Each In order to transmit data using the serial data bus, coupler panel allows a number of avionic units to be information must be presented in a standard format.
In order to A typical format for serial data would use a word optimise the speed of data transfer and minimise length of 32 bits. This word comprises several discrete problems associated with reflection and mismatch, the fields, including: The ARINC specification defines the electrical and data characteristics and protocols that are used.
More modern aircraft e. Boeing and Airbus A Word size 32 bits use significantly enhanced bus specifications see page Bit-rate high kbps 49 in order to reduce the weight and size of cabling and to facilitate higher data rates than are possible with Bit-rate low One receivers or sinks.
Two speeds are available, Transmission of The modulation employed is bipolar return to zero sequential words is separated by at least four bit times BPRZ modulation see Figure 4. The composite of NULL zero voltage.
This eliminates the need for signal state may be at one of the following three levels a separate clock signal and it makes the system self- measured between the conductors: More length and the number of receivers connected to the information on ARINC and aircraft standards bus. Since each bus is unidirectional, a system needs to have its own transmit bus if it is required to respond to or send messages.
Hence, to achieve bidirectional data transfer it is The ARINC electrical characteristics are necessary to have two separate bus connections. Note that although there may only be one receiver on a particular bus cable, the ARINC specification supports up to The data shown in Table 4. Explain the difference between serial and parallel methods of data transfer. Parameter High speed Low speed 2. Explain why serial data transfer is used for Bit rate kbps Explain the function of each of the fol- per cent lowing bus system components: Rise time 1.
State the voltage levels present on the Fall time 1. The principal data formats defined in the specification In most cases, an ARINC message consists of a single are binary coded decimal BCD , which uses groups bit data word see Figure 4.
The 8-bit label of four bits to contain a single decimal digit and BNR field defines the type of data that is contained in the see below which is binary coding. For both of these rest of the word. ARINC data words are always 32 bits data types, the specification defines the units, reso- and typically include five primary fields, namely parity, lution, range, number of bits used and how frequently SSM, data, SDI and label.
ARINC convention numbers the label should be sent. A number of dif- The SSM field is used for information that assists ferent data formats is possible. The P field is the parity bit. ARINC uses odd parity. The parity bit is the last bit transmitted within the data word. In Figure 4. This binary format is known odd number of 1s in each transmitted word.
It is as BNR. The zero in the bit position indi- type of data e.
The remaining bits are padded with zeros. In this case each expressed in BCD format of 25, It is important to note that these Flight management often describe enhancements to existing standards. Some standards were based on the use of a six- Microwave landing system wire system, while others used a shielded two-wire twisted pair like ARINC or a coaxial cable. The bus supports connected devices employed with logic 1 represented by 12 V. The remaining fields include an 8-bit bidirectional bus system in other words, connected label and six BCD fields, five of four bits and one of two devices can transmit, receive or do both.
Another bits. This system was widely used in aircraft manufac- advantage of ARINC is that it achieves bidirec- tured prior to about The physical bus medium is shielded twisted pair STP.
Each frame comprises four encoded data at a data rate of 1 Mbps. Data words are sub-frames. A unique synchronising word appears at 1, bits long and are composed of one bit status the start of each sub-frame.
ARINC supersedes word and 3-bit data words. ARINC and caters for a number of different bit- rates and frame sizes. The standard uses a bus controller that can Similar to ARINC , this standard is a low-speed support up to 31 devices, which are referred to as bus system that is based on a single twisted pair of remote terminals.
The standard supports a bit-rate of wires. Due to the low data rate supported, this bus 1 Mbps. These systems are often used in small business and ARINC is a software protocol that can be layered on private general aviation aircraft. ARINC tional bus that permits the connection of up to ten supports high-speed data transfer to and from on- receivers and one transmitter.
The standard supports board digital systems permitting, for example, reading data rates of ASCB is a centrally and writing of 3. A bus that supports the transfer of data in both directions is referred to as: Data in each ring 2.
The main advantage of using a serial bus in an flows in opposite directions. The data rate is Mbps aircraft is: CDDI copper dis- tributed data interface and SDDI shielded twisted a there is no need for data conversion pair distributed data interface are similar network bus b it supports the highest possible data rates standards based on copper and shielded twisted pair c reduction in the size and weight of cabling. Which one of the following is used to minimise line voltage level indicates a logic 1 and no change reflections present in a bus cable: For reasons of cost and in order to reduce the number and complexity of network a coupler panels standards used in its aircraft, Boeing now plans to b bus terminators replace the system on the with a less expensive c shielded twisted pair cables.
Give reasons for your answers. In order to represent negative data values, BNR data uses: A low-cost bus system for the simple avionics fitted to a small business aircraft.
Connecting a weather radar receiver to a b BCD data and a binary sign bit radar display. A bus system for linking the various avionics systems of a modern passenger aircraft to 6.
A bus system for the avionics of a military a fibre-optic aircraft fitted with multiple radars and elec- b coaxial cable tronic counter measures ECMs. A bus that is self-clocking is referred to as: ARINC is designed for use with: The maximum data rate supported by the FDDI bus is: It is therefore not surprising that form: We then show how these gates can be used in simple combinational logic circuits before moving on For example: The chapter concludes with a brief introduc- If hungry or thirsty then go to the cafe tion to the two principal technologies used in modern or digital logic circuits,TTL and CMOS.
If cold and dark then go to bed 5. For example: The sort of decisions that we make are invariably conditional i. These words describe If thirsty then go to the bar logical conditions; next we look at ways in which electronic circuits can emulate these logic functions. Inverters also provide extra current drive and, symbols are invariably used and the logic elements like buffers, are used in interfacing applications where they represent operate in exactly the same way as they provide a means of regularising logic levels those used in non-aircraft applications.
Any other input combination results in a logic 0 output. Buffers do not affect the logical state of a digital signal i. Buffers are 5. Putting this another way, an OR gate will only produce a logic 0 output whenever all of its inputs are simultaneously at logic 0. Any other input combination will produce a logic 1 output.
The circle shown at the output of the gate denotes this inversion. NOT-OR gates will only produce a logic 1 output when all inputs are simultaneously at logic 0. Any other input combination will produce a logic 0 output. A circle is again used to indicate inversion. Exclusive-OR gates produce a logic 0 output whenever both inputs have the same logical state i. Exclusive-OR gates produce a logic 1 output whenever both inputs have the same logical state i.
In other words, they are the inputs are inverted. In Figure 5. This is equivalent to their outputs passed through an inverter or NOT an inverter NOT gate connected to one input of the gate , as shown in Figure 5.
AND gate, as shown. Two further circuits with inverted inputs are shown in Figure 5. This arrangement is equivalent to the two-input NOR gate shown. This arrangement is equivalent to the two- input NAND gate shown. The basic Boolean logic expressions are: With a little practice, the use of Boolean algebra should become second nature! This circuit is referred to as a majority vote circuit and its truth table is shown Note that it is important to avoid confusing the in Figure 5.
Figure 5. For example, in con- each node in the circuit. Using Boolean algebra this is shown as: This is a 28 V DC bus which is maintained in the event of an aircraft generator failure. Note also that the indicators are 5.
The inputs to this logic system consist of and logic 1 states through the logic diagram. Figure five switches that detect whether or not the respective 5. The output from the logic gear doors are closed this is the normal in-flight con- system is used to drive six warning indicators.
Four dition. Note how the primary door warning indicator of these are located on the overhead display panel and shows the pilot that the system is active. A switch is is the output from A7.
This, in turn, results in logic 1 also provided in order to enable or disable the five inputs to the indicators, which remain in the off non- door warning indicators.
The landing gear warning logic primary module In Figure 5. At the same time, the nose-door- A2 Regulated power supply for A7 and A11 open warning becomes illuminated. In this condition the output of A7 goes to landing gear door warning logic system is fitted. This time, however, both the nose-door- and provides a back-up in case the primary system open and left-wing-door-open warnings become fails. Primary or secondary system operation can be illuminated.
Draw the truth table and state the Boolean circuit i. Boolean algebra. Show how a four-input AND gate can be 5. Show how a four-input OR gate can be 2.
State the Boolean logic expression for the made using three two-input OR gates. Devise a logic gate arrangement that pro- vides the output described by the truth table shown in Figure 5. This high-impedance state permits the output of several tri-state devices to be connected directly together. Such arrangements are commonly used when a bus system has to be driven by several logic gates. The output state whether a logic level or high impedance is controlled by means of an enable EN input.
This EN input may be either active-high or active-low, as shown in Figure 5. Figures 5. Note that the state of the EN input determines 5. In this case, the state of the EN input means of generating precise time delays. Such delays determines whether the output takes the opposite become important in many sequential logic applica- logical state to the input or whether the output is tions where logic states are not constant but subject taken to its high-impedance state again shown as X in to change with time.
The action of a monostable is quite simple — its output is initially logic 0 until a change of state occurs at its trigger input. Immediately the trigger pulse arrives, the 4.
We need to provide a signal that the pilot can use output of the monostable changes state to logic 1. Monostable circuits the event of a fault condition. An example of the use of a monostable is shown in the auxiliary power unit APU starter logic shown in Figure 5. The output of the APU starter motor control logic goes to logic 1 in order to apply power to the starter motor via a large relay.
There are a few things to note about the logic arrangement shown in Figure 5. When the APU runs on its own we need to dis- engage the starter motor. We need to avoid the situation that might occur if the APU does not start but the starter motor runs continuously as this will drain the aircraft batteries.
Instead, we should run the starter motor for a reasonable time say, 60 seconds before disengaging the starter motor. The 60 seconds timing is provided by means of a positive edge triggered monostable device. Since the pilot is only required to momentarily press the APU START switch, we need to hold the condition until such time as the engine starts or times out i.
The monostable remains triggered and continues to pro- duce a logic 1 output for its second period. This results in the output of 5. The starter b is based on cross-coupled two-input NOR gates. Two the second monostable period. In this timed-out simple forms of R-S bistable based on cross-coupled condition the output of the AND gate goes to logic 0 logic gates are shown in Figure 5. The system then waits for the pilot to operate gate bistable has a number of serious shortcomings the APU START button for a further attempt at start- consider what would happen if a logic 1 was simulta- ing!
The D-type bistable has two inputs: Once set in one or other of these The data input logic 0 or logic 1 is clocked into the states, the output of a bistable will remain at a par- bistable such that the output state only changes when ticular logic level for an indefinite period until reset.
Operation is thus said to be A bistable thus forms a simple form of memory as it synchronous. These are usually called The simplest form of bistable is the R-S bistable. A logic 1 applied and as binary dividers. In either case, the bistable will J-K bistables see Figure 5. As with R-S bistables, the two outputs are complementary i. J-K bistables are the most sophisticated and flexible of the bistable types and they can be configured in various ways for use in binary dividers, shift registers and latches.
The timing shift register based on J-K bistables. The timing dia- diagram for this circuit is shown in Figure 5. Each gram for this circuit is shown in Figure 5. Note that stage successively divides the clock input signal by a each stage successively feeds data via the Q-output factor of two. Note that a logic 1 input is transferred to the next stage and that all data transfer occurs on to the respective Q-output on the falling edge of the the falling edge of the clock pulse.
The two basic logic families are com- plementary metal oxide semiconductor CMOS and 5.
TTL Std. Each of these fam- 5. Variants within the family are identified Representative circuits for a basic two-input NAND by the suffix letters, for example: Devices from this family are coded with the compatible with the but which prefix number 74 e. Sub-families based on has buffered outputs the use of variations in the technology are distinguished by letters which follow the initial prefix, for example: These two families have quite different char- where VOH MIN is the minimum value of high state acteristics, supply requirements and logic levels.
TTL logic devices, on the other hand, tend to be faster but have a much lower noise margin and are therefore more susceptible to noise and interference. The two main logic families are further divided into a number of different sub-families based on variations in the parent technology.
Which of the devices shown is a two-input OR gate? What voltage would you expect to measure on pin 14 of a device? Sketch a circuit including pin numbers showing how a device could be used as a dual R-S bistable. Sketch circuits including pin numbers showing how a could be used as: The most appropriate logic family for use in a portable item of test equipment is: The logic device shown in Figure 5.
The logic gate arrangement shown in Figure 5. The normal supply voltage for a TTL logic device is: A two-input NAND gate will produce a logic 0 output when: The noise margin for standard TTL devices is: In a binary counter, the clock input of each 9. If a voltage of 3 V is measured at the input to the gate, this would be considered equivalent to: The device shown in Figure 5.
The truth table shown in Figure 5. This chip hardware and software and are capable of processing is equivalent to many thousands of individual tran- large amounts of data in a very short time. This sistors. The main components are: Three different buses are present; these are: The clock usually consists of a high- trol signals throughout the system. The number of individual lines present within the address bus and data bus depends upon the particular microprocessor employed.
Signals on all lines, no 6. Data and addresses are repre- — particularly large ones — are not very convenient. This format Some basic microprocessors designed for control is easier for mere humans to comprehend and also and instrumentation applications have an 8-bit offers the significant advantage over base 10 numbers data bus and a bit address bus.
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