KURZUS: Járművillamosság és elektronika 2.

MODUL: IV. modul: Gépjárművek kommunikációs rendszerei

Lesson 9: Digital data transmission, number systems

| The purpose of learning this lesson is: Understanding the methods and operation of digital data transmission. In order to understand the operation of bus systems, basic concepts of digital technology are also explained.
Necessary to learn: After learning the lesson, students should know the basic operation and methods of digital data transmission. You should be familiar with number systems, conversions between number systems, and basic digital concepts at the level necessary to understand the following lesson.
Dr. Fodor Dénes, Dr. Szalay Zsolt: Autóipari kommunikációs rendszerek Dr. Bokor József, Dr. Gáspár Péter: Járműfedélzeti kommunikáció Dr. Kováts Miklós, Dr. Szalay Zsolt: Gépjárművek buszhálózatai (Maróti kiadó)
Duration of learning a lesson: To learn the lesson, approx. 90 minutes required.
Introduction
In recent years, a paradigm shift has been observed in the field of industrial communication and control networks. Today, microcontrollers have become more efficient and cheaper, which has allowed manufacturers to embed them in remote I/O devices, push buttons, sensors, and other components, creating "smart devices" that can perform their control tasks independently. Thus, instead of the centralized control systems that dominated in the 1970s and 1980s, the so-called distributed control systems became more and more widespread.
The majority of today's vehicles contain a large number of electronic control systems and the number of devices is constantly increasing. This is due to the safety and comfort needs of the user on the one hand, and environmental protection considerations on the other.
The complexity of the functions of the systems makes the exchange of data between the elements of the systems inevitable. In traditional systems, data is exchanged via dedicated data lines, but as the control functions become more complex, this is increasingly difficult and expensive to implement. In complex control systems, the number of connections could no longer be increased after a while. More and more systems were developed for vehicles that required the cooperation of several control devices. (Engine control, stability control, automatic transmission, dashboard buttons.)
Centralized control system
Figure 1
The units that make up the system are connected to a central control unit whose task is to coordinate the entire system. The central controller (master) cyclically queries the messages of the other devices (slaves). Thus, although the maximum time a device has to wait for the bus is deterministic, such a model has several significant drawbacks. The different units have different types of connectors, so a large number of wires are required to connect them to the center, and as the complexity of the system increases, the number of wires and the size of the connectors also increases. Detecting errors in such a master-slave system is complicated, and when the central unit stops working, the entire system becomes inoperable.
Distributed control system
Figure 2
The basic principle of such systems is that all units are connected to a common communication line (bus). Units connected to a network in this way now communicate with each other independently. The use of the network resulted in a new control concept, the so-called distributed control.
In distributed systems, only one bundle of wires is needed, which often already provides the power supply to the components, thereby reducing the number of physical connectors. Fewer wires not only make the system more reliable, but also simpler and, above all, cheaper.
With this solution, continuous expansion of the system is possible, since only one type of standardized connector is needed, so it is even possible to integrate devices from different manufacturers into a common system.
Electronic control units
With the advent of digital technology, new possibilities have opened up for the control and regulation of the electronic systems of vehicles. The control unit receives the signals of the sensors through its inputs, processes and evaluates them, and then, based on its internal program, calculates the control signals necessary to control the intervening elements. Its program (software) is located on an internal storage, and its execution is carried out by a microcomputer.
Parts of the electronic control unit:
Analog and digital inputs Power supply Microprocessor RAM Flash EPROM EEPROM Diagnostic module Outputs for operating actuators Communication and diagnostic interfaces (K-Line, LIN, CAN, Flexray stb.)
Structure of a Bosch Motronic control unit (example)
Figure 3
Digital data transfer
A quantity is called digital if it can take only a finite number of discrete values. In contrast, analog quantities can take on any value.
In the case of digital data transmission, the data is typically transmitted in binary format in packages corresponding to the data storage units. Based on these, we can talk about data groups of 4, 8, 12 or 16 bits, 1...254 bytes, or possibly even larger, the bits of which can be sent one after the other in time (serial data transmission) or simultaneously in time (parallel data transmission) to the receiving station.
Number systems
Binary number system
In digital systems, information is represented by the two binary digits 0 and 1. These digits in the binary number system - the smallest basic units of information - are called bits.
One bit can represent two pieces of information. If we want to describe a physical quantity in a finer resolution, we have to use more bits. In this case, the resolution increases according to powers of two. The eight-bit information unit is called a byte, which can be used to map 28 = 256 different pieces of information. Place values increase from right to left according to powers of two.
Decimal number system
The decimal number system got its name from the fact that numbers are written with 10 characters (0-9). Place values increase from right to left according to powers of 10.
Hexadecimal number system
The hexadecimal number system gets its name from the fact that it uses 16 characters (0-9; A-F) to describe numbers. Adding the characters (0-9) of the decimal number system, we can enter the individual values with the characters A (10), B (11), C (12), D (13), E (14) and F (15). Place values increase from right to left according to powers of 16.
Binary, hexadecimal and decimal number representation
Figure 4
To understand the operation of analog-to-digital and digital-to-analog converters, as well as to understand the structure of the message frame of communication bus systems, it is necessary to understand the conversion between number systems. Here are two examples:
Conversion between binary-decimal number systems
Figure 5
Conversion between hexadecimal-binary-decimal number systems
Figure 6
Serial/parallel data transfer
In the conventional parallel transmission mode, the data is transferred in multi-bit groups at the same time, per data unit (e.g. 8 bits at a time). A separate wire is reserved for each bit, thus the number of connections (parallel wires) is large.
Characteristics of parallel information transfer:
Cable-intensive solution. In terms of operation, it is easy to review and inspect. High signal transmission security. Typical information transfer errors of IT systems do not appear.
On the other hand, in the case of serial data transmission, the bits carrying the information are transferred one by one, one after the other. The advantage of serial transmission is the small number of wires and simple connectors, which, in addition to reducing costs and dimensions, also have a good effect on reliability.
Characteristics of serial digital information transmission:
A single sensor system Simple, cheap, low-mass electrical network Communication between the control units takes place according to a uniform (standardized) protocol High data transfer rates can be used (e.g. 1 Mbit/s) The system can be easily expanded Different data transmission media can be used (copper wire, optical cable) It can be easily examined using the method of serial diagnostics
Serial/parallel communication
Figure 7
Synchronous/asynchronous data transmission
The transmitter and receiver units must work in sync to correctly interpret the data. This is possible with a separate line transmitting the sync signal, or the receiver can also synchronize at the signal transition of each bit (e.g. at the trailing edge). In asynchronous communication, the signal is sampled multiple times, possibly at programmable times.
Synchronous communication
Figure 8
Synchronous communication requires at least two communication lines, one for clock and one for data.
Asynchronous communication
Figure 9
If the data changes from 1 to 0 or from 0 to 1, an edge is generated in the communication line. At each edge, the receiving side's clock is reset.
Since a separate dedicated synchronous signal (clock signal) is available in the case of synchronous communication, synchronizing the transmitter and receiver does not a problem. In the case of asynchronous communication, on the other hand, only a wire that also serves to transmit data can create an opportunity for synchronization, namely by synchronizing the receiver's clock at the edges (when the data changes from 1 to 0 or from 0 to 1).
Since it cannot be guaranteed which bit sequence the data stream is made up of, it is not possible to know in advance when the next signal change will occur, an arbitrarily long, unchanged signal sequence can also occur. In this case, the data transmission would "fall out of sync" after a time determined by the time difference between the transmitter and the receiver, which is of course not permissible. There are many strategies to avoid this, e.g. Manchester coding, EFM, start-stop system, bit insertion, etc. which guarantee that there is definitely a signal change after a given number of bits. If the coding is such that the clock signal can be restored from bits that also carry data, then we are talking about an isochronous self-clocking signal (e.g. Manchester coding), while if the signal for synchronization is separated in time from the data bits (start-stop system), then the transmission is an anisochronous self-clock signal.
Common bit coding formats
Figure 10
It can be seen that, in addition to NRZ coding, all three others ensure that there are edges between several consecutive bits of the same value, which help interpretation and synchronization.
In a technical sense, we speak of multiplex communication when many conversations are carried out according to one rule. This can be achieved in several ways, for example:
Frequency division multiple access (FDMA) feature e.g. on cable TV networks Time Division Multiple Access (TDMA) Local data networks (e.g. Ethernet, CAN) operate in a carrier-sensing multiple access with collision detection (CSMA/CD) system
The essence of multiplex communication is to have a clear system of rules that defines who gets to "speak" during communication, even if by chance several people start "talking" at the same time.
Characteristics of wired data transmission
Bandwidth: Depending on the required transmission speed and transmission distance, the quality required from the line changes, the primary parameter of which is attenuation. At high frequencies, the attenuation of the cable per unit of length (dB/100m) increases due to the skin effect and dielectric loss. From an electrical point of view, the ideal wire would have a large surface area and its insulation would be vacuum Modulation speed, data transmission speed: In many places, the quantities bit/s and baud (=symbol/second) are used as alternatives to each other, but this is wrong in principle. Bit: the basic unit of information in a binary (two-state) system. The ratio of the data transfer rate (bit/s) to the modulation rate (baud) is called (channel) coding efficiency. Its value can be greater than or less than 1, since in some cases 1 symbol can carry several bits of information, but not all symbols carry actual information (e.g. stuff bit, inter-frame pause, acknowledge, synchronization bits, error correction and verification code). Termination: If the length of the transmission path (l) is comparable to the distance traveled by the signal during the bit time (t bit), then the wire must be terminated with a resistor corresponding to the wave impedance, at least at one end, to avoid multiple reflections. Interference protection: Signal transmission lines are exposed to external disturbing electric and magnetic fields, very often they run together with other lines, either along their entire length or only a short distance away. A change in the current flowing in the other wires or the voltage measured on them changes the useful signal through inductive or capacitive coupling and adds a disturbance signal. This interference signal can be common mode, when the interference signal has the same direction/polarity in the two conductors, or differential mode, when the directions are opposite (as in the case of the useful signal). Differential mode interference can be effectively combated by the symmetrical design of the cable (twisted pair). Another way to reduce the interference signal is shielding. The outer, continuous conductor prevents the external electric field from penetrating inside, and also slows down the changes in the penetrating magnetic field. This provides protection against differential mode interference signals, as they are created by changes in the electric or magnetic field penetrating between the two conductors. it conducts the lines of force of the electric field with a small resistance, "shorting" it towards the reference point to which it is connected.

| Questions
1. What were the reasons for the creation of the car bus networks? (several correct answers are possible)
| Use of a large number of electronic control units in motor vehicles.
| Analog signals can easily be transmitted via bus networks.
| Cabling has become expensive and complicated with the proliferation of electronic units in vehicles.
| Bus networks made it easier to diagnose electronic systems.
2. Which of the following are components of a general control unit? (several correct answers are possible)
| Analog and digital inputs
| Microprocessor
| LCD screen
| EPROM
3. What are the most important advantages of a distributed control system compared to a central control system?
| No need for digital data transfer.
| Communication is via bus networks, which makes cabling easier.
| Only one central control unit is needed, which makes the system cheaper.
| Later expansion of the system can be easily implemented.
4. Which statement is true about digital data transmission? (several correct answers are possible)
| You can easily transmit analog signals.
| Only serial data transmission is possible.
| Information is carried by bits, which can be 0 and 1.
| Information is represented in a binary number system.
5. Which statement is true about serial data transmission? (several correct answers are possible)
| The information takes place on a large number of wires, simultaneously in time.
| A separate wire is reserved for each bit.
| Cable-intensive solution.
| The bits containing the information are transferred one by one, one after the other.
6. The value of the decimal number 11 in binary and hexadecimal number systems:
| 1111; C
| 0101;D
| 1011;B
| 1001;A
7. The value of the hexadecimal number D in binary and decimal number systems:
| 1101; 13
| 0011; 12
| 1100; 15
| 1010; 11
8. The value of the binary number 1110 in decimal and hexadecimal number systems:
| 8; A
| 10; B
| 13; C
| 14; E
9. Which statement is true about asynchronous data transfer? (several correct answers are possible)
| During the data transfer, the receiver's clock is synchronized at the edge change.
| At least two communication lines are required, one for clock and one for data.
| No need for a separate synchronization clock.
| The NRZ encoding is suitable for asynchronous data transmission in itself.