From zero foundation to simple entry, detailed introduction of bus technology

“The bus is called “BUS” in English, which is our Chinese “public car”, which is a very vivid metaphor. The route taken by the public bus is certain, and any of us can take the public bus to any stop on the public bus route. If we humans are compared to Electronic signals, this is the true meaning of calling it “BUS” instead of “CAR” in English.

“

The bus is called “BUS” in English, which is our Chinese “public car”, which is a very vivid metaphor. The route taken by the public bus is certain, and any of us can take the public bus to any stop on the public bus route. If we humans are compared to electronic signals, this is the true meaning of calling it “BUS” instead of “CAR” in English.

Of course, professionally speaking, a bus is a structural form that describes electronic signal transmission lines, a collection of signal lines, and a common channel for transmitting information between subsystems. Through the bus, the information between the components in the entire system can be transmitted, exchanged, shared, and logically controlled. For example, in a computer system, it is a common channel for CPU, memory, input, and output devices to transmit information. The various components of the host are connected through the host, and the external devices are connected to the bus through the corresponding interface circuit.

With the development of modern network information, especially in terms of cost and space, bus transmission instead of point-to-point transmission is a hot spot for current development. Its emergence will provide the greatest convenience and the most effective technical solutions for information transmission.

The basic composition of the system bus:
Data bus, transfer data information.

Address bus, transfer address information.

Control the bus and transmit control information (to complete the bus operation function).

The power cord provides a power signal for the system.

1
Function of the bus

Data transfer function

The data transmission function is the basic function of the bus, which is expressed by the bus transmission rate, that is, the number of bytes transmitted per second, and the unit is Mbps (megabytes per second).

Multi-device support function

If multiple devices use one bus, the first is the question of the right to occupy the bus. Which master device applies to occupy the bus is determined by the bus arbiter.

Interrupt

Interruption is the mechanism by which the computer responds to emergency affairs. When a service agreement is made between the external device and the main device, the interruption is a contact signal to realize the service agreement.

Error handling

Error handling includes error detection processing such as parity error, system error, battery failure, etc., as well as providing corresponding protection countermeasures.

2
Bus data transmission process

1. Apply to occupy the bus

The bus master device (such as CPU, DMA controller, etc.) that needs to use the bus makes a request to the bus arbitration organization to occupy the bus. After the bus arbitration organization determines that the response condition is met, a response signal is sent, and the next bus transfer cycle The bus control right is granted to the applicant.

2. Addressing

The bus master device that has obtained the bus control right sends out the address of the memory and I/O port to be accessed this time through the address bus, selects the accessed module through address decoding and starts data conversion.

3. Transfer data

The bus master is also called the master module, and the accessed device is called the slave module. The operation between the master module and the slave module is controlled by the master module to transmit data through the data bus between the two slave modules.

4. End

The information of the master and slave modules are removed from the bus, and the bus is released for use by other master modules.

3
Types of microcomputer bus

On-chip bus

It is a bus located between the unit circuits inside a large-scale and ultra-large-scale integrated chip as an information channel between these unit circuits. Such as the bus between CPU internal ALU, register group, controller and other components.

Local bus (also called internal bus)

Usually refers to the information channel between the components on the microcomputer motherboard. Because it is a bus inside a circuit board, it is also called an on-board local bus. The more typical local buses are: IBM-PC bus, ISA bus, EISA bus, VL and PCI bus, etc.

System bus (also called external bus)

It refers to the bus on the backplane of the microcomputer, which is used to form the channel between the plug-in boards of the microcomputer system and the CPU modules of the multiprocessor system. Typical system buses include STD-BUS, MULTI-BUS, VME, etc.

Communication bus

It is the information channel between the microcomputer system and the system, between the microcomputer system and other instruments or equipment. This kind of bus is often not computer-specific, but is formed by borrowing and applying existing bus standards in other fields of the electronics industry. Popular communication buses such as: EIA-RS-232C, RS-422A, RS-485, IEEE-488, VXI and other bus standards.

The relationship between the buses

4
Advantages of using bus technology

1. Simplify the design of software and hardware: As the bus definition is very strict, any manufacturer or individual must make plug-in boards according to its standards. With the specifications, it brings great convenience to users in hardware design and simplifies the design process.

2. Simplify the system structure: using a standard bus, as long as each functional module (board) is hung on the bus, the hardware system of the microcomputer can be easily formed.

3. Facilitate system expansion: For a microcomputer system composed of a standard bus, as long as it is designed in accordance with the bus standard and user expansion requirements or directly purchases a plug-in board and plugs it into the bus slot to achieve the purpose of expansion.

4. Facilitate the update of the system: With the continuous development of electronic technology, new devices continue to emerge, and the microcomputer system must also be continuously updated. It can be easily improved by replacing the original device with a new device on a plug-in board using a standard bus. System performance without having to make major changes.

5
Classification of bus technology

There are many ways to classify buses, such as external and internal buses, system buses and non-system buses, and so on.

1. Classified by function
The most common is to divide the data bus in terms of function, which can be divided into an address bus, a data bus, and a control bus. In some systems, the data bus and the address bus can be shared under the control of the address latch, that is, multiplexed.

The address bus is dedicated to transmitting addresses. In the design process, the most common choice is to select the storage address of the external memory from the CPU address bus. The number of bits of the address bus often determines the size of the memory storage space. For example, if the address bus is 16 bits, the maximum storage space is 216 (64KB).

The data bus is used to transmit data information. It is divided into one-way transmission and two-way transmission data bus. The two-way transmission data bus usually adopts a two-way three-state bus. The number of bits of the data bus is usually the same as the word length of the microprocessor. For example, the word length of the Intel 8086 microprocessor is 16 bits, and its data bus width is also 16 bits. In actual work, what is transmitted on the data bus is not necessarily the data in the full sense.

The control bus is used to transmit control signals and timing signals. For example, sometimes the microprocessor must first send out read/write signals, chip select signals, and read interrupt response signals through the control bus when operating on the external memory. The control bus is generally bidirectional, and its transmission direction is determined by the specific control signal, and its number of bits must also be determined according to the actual control needs of the system.

2. Classified by transmission method
According to the method of data transmission, the bus can be divided into serial bus and parallel bus (collection of circuit diagrams based on various bus technologies). From the principle point of view, the parallel transmission method is actually better than the serial transmission method, but its cost will increase. In layman’s terms, the parallel transmission path is like a multi-lane highway, while the serial transmission allows only one car to pass through a single-lane highway. At present, common serial buses are SPI, I2C, USB, IEEE1394, RS232, CAN, etc.; while parallel buses are relatively few in types, such as IEEE1284, ISA, PCI, etc.

3. Classified by clock signal mode
According to whether the clock signal is independent, it can be divided into a synchronous bus and an asynchronous bus. The clock signal of the synchronous bus is independent of the data, that is to say, a separate line is used as the clock signal line; the clock signal of the asynchronous bus is extracted from the data, usually using the edge of the data signal as the clock synchronization signal .

The basic principle of bus transmission

According to the previous definition of the bus, it can be seen that the basic function of the bus is to transmit signals, so that the information of each subsystem can be transmitted effectively and timely, in order not to interfere with each other’s signals and avoid overcrowding in the physical space. A good way is to use multiplexing technology, which means that the basic principle of bus transmission is multiplexing technology. The so-called multiplexing refers to a mechanism in which multiple users share a common channel. At present, the most common methods include time division multiplexing, frequency division multiplexing, and code division multiplexing.

Time Division Multiplexing (TDMA)

Time division multiplexing is to divide the channel into multiple time periods according to time. Signals from different sources will require responses in different time periods, and the transmission time of each other’s signals will not overlap on the time axis.

Frequency Division Multiplexing (FDMA)

Frequency division multiplexing is to divide the available frequency band of the channel into several non-overlapping frequency bands, and the frequency spectrum of each signal after frequency modulation occupies one of the frequency bands, so as to realize the transmission of multiple signals of different frequencies in the same channel . When the receiving end receives the signal, an appropriate band-pass filter and frequency demodulator will be used to restore the original signal.

Code Division Multiplexing (CDMA)

Code division multiplexing means that the transmitted signal will have its own specific identification code or address code. The receiving end will distinguish the transmission information on the common channel according to different identification codes or address codes. Only the identification code or address code The transmitted information will be received only when it is exactly the same.

7
Main technical indicators of the bus
The main technical indicators for evaluating the bus are the bus bandwidth (that is, the transmission rate), the width of the data bit (bit width), the operating frequency and the reliability and stability of the transmitted data.

Bandwidth (transmission rate), bit width and operating frequency

The bandwidth of the bus refers to the amount of data transferred on the bus per unit time, that is, the maximum data transfer rate of MB per banknote. The bit width of the bus refers to the number of bits of binary data that the bus can transmit at the same time, or the number of bits of the data bus, that is, the concept of 32-bit, 64-bit and other bus widths; the wider the bus, the greater the data transmission rate. The wider the bandwidth of the bus. The working clock frequency of the bus is in MHz, which is related to the transmission medium, signal amplitude and transmission distance. Under the same hardware conditions, the frequency when we use differential signal transmission is often much higher than that of unilateral signals. This is because the amplitude of differential signals is only half of that of unilateral signals.

The bandwidth, bit width and operating frequency of the bus are closely related to each other, and the relationship between them:

Reliability of transmitted data

Reliability is the most critical parameter for evaluating the bus. Without reliability, the transmitted data is all wrong information, and the actual meaning of the bus is lost.

In order to improve the reliability of the bus, the usual measures are as follows:

Before the data frame is sent, the transmitter listens to the bus, and only when the bus is in an idle state can the data frame be transmitted to the bus, thus avoiding data conflicts between different nodes.

The twisted pair differential signal is used to transmit data to reduce the voltage rise and fall of the single wire and reduce the high-order harmonics generated by the edge of the signal.

Properly let the edge of the data have a certain slope.

Increase the matching resistance and capacitance to reduce the signal transmission on the bus and balance the distributed capacitance on the bus.

Use appropriate network topology and shielding technology to reduce interference from other signals.

8
Several typical bus technologies

1. STD system bus
Modular small board structure, open flexible configuration:
The STD bus makes the microcomputer system divided into several modules and made into standard function templates (plug-in cards). Users can choose function templates to compose their own microcomputers according to their needs. The plug-in cards and peripherals can be connected in other ways, so they can flexibly and conveniently form a microcomputer system that meets different requirements.

High reliability, high anti-interference ability and high signal quality:
The excellent physical characteristics of the STD bus make it resistant to harsh environments. Its modular and small size structure makes it resistant to shock and vibration, and can also reduce problems caused by self-heating. Since the STD bus uses the edge of the printed circuit board as the connector, it can prevent the plug-in card from being inserted backwards and the pins from bending or breaking. At the same time, the structure of the STD bus allows the signal flow to flow from the bus interface to the user interface in an orderly manner, which improves the quality of the signal.

Compatible structure, supporting products and complete functions:
The compatible structure of the STD bus allows 8-bit STD products to work with new standard 16-bit or 32-bit STD products. The STD bus also supports multi-processor systems. With the development of technology and the promotion and application of STD products, the functions of its standard plug-in boards are continuously enhanced, and the supporting products are becoming more and more abundant, which brings great convenience to use.

2. RS-232C communication bus
RS-232C is a serial communication bus standard and an interface standard between data terminal equipment (DTE) and data communication equipment (DCE). It was derived from the CCITT remote communication standard by the Electronic Industries Association (EIA) in 1969 One of the standards. The purpose of this standard was to enable the equipment produced by different manufacturers to achieve plug compatibility, that is, no matter which manufacturer’s equipment, as long as it has an RS-232C standard interface, it can be plugged into each other without any conversion circuit. However, this standard only guarantees hardware compatibility but not software compatibility.

The RS-232C standard includes mechanical indexes and electrical indexes. Among them, the mechanical indexes stipulate: The connector (pins and sockets) from the RS-232C standard interface to the outside is a 25-pin plug with a “D”-shaped protective shell.

The main features of this bus:

There are few signal lines. The RS-232C bus has a total of 25 wires, which includes two channels, the main and the sub-channel, which can be used for duplex communication. In practical applications, most only use the main signal channel (ie the first channel), and only use a few of them (usually 3-9 lines).

Long transmission distance (as opposed to parallel). Because RS-232C adopts serial transmission mode and converts TTL level to RS-232C level, the distance can reach 30m in baseband transmission. If the optically isolated 20A current loop is used for transmission, the transmission distance can reach 1000m. Of course, if you add a modem to the serial interface and use wired, wireless or optical fiber for transmission, the distance will be farther.

There are many transmission rates to choose from. The standard transmission rates specified by RS-232C are: 50, 75, 110, 150, 300, 600, 1200, 2400, 4800, 9600, 19200 baud. It can be flexibly used in devices of different speeds.

Strong anti-interference ability. RS-232C adopts negative logic. When there is no load, any voltage between +3~+25V represents logic “0”, and any voltage between -3~-25V represents logic “1”, and it has no interval and does not return to zero. Level transmission, thereby greatly improving the anti-interference ability.

3. RS-422A bus

RS-422A uses a balanced output transmitter and a differential input receiver. The transmitter has two output lines. When one line jumps to a high level, the other output line jumps to a low level, so the voltage polarity between the lines is reversed. In the RS-422A line, two wires are used to send signals, and two wires are used to receive signals. For duplex communication, there are at least 4 wires. Since the RS-422A line is fully balanced, under normal circumstances, the RS-422A line does not use a common ground wire. This minimizes the interference to the communication line due to the difference in ground potential between the two communication parties. The signals generated by the two ground potentials become common mode interference and will be filtered by the differential receiver, and this interference can cause errors in the RS-232C circuit.

4. RS-485 bus

The RS-485 bus uses an interface circuit for full-duplex communication, which requires two pairs of wires or 4 wires, which increases the line cost. RS-485 is suitable for both sending and receiving parties to share a pair of lines for communication, and it is also suitable for sharing a pair of lines between multiple points for bus networking. The communication is only half-duplex.

Due to the sharing of a line, only one transmitter is allowed to send data at any time, and the other transmitters must be in the off (high impedance) state, which is controlled by the transmit enable terminal on the transmitter chip. For example, when the terminal is high, the transmitter can send data, and when it is low, both output terminals of the transmitter are in a high-impedance state, as if they were disconnected from the line.

5. IEEE 488 bus

IEEE 488 is a parallel external bus, which was formulated by HP in the 1970s. In 1975, the IEEE recommended the IEEE-488 standard bus. In 1977, the International Electrotechnical Commission (IEC) also approved and recommended the bus and named it IEC-IB. Therefore, this kind of bus also uses multiple names of IEEE-448, IEC-IB (IEC interface bus), HP-IB (HP interface bus) or GP-IB (general interface bus). Due to the introduction of the IEEE-448 bus, when using the IEEE-448 standard to build a computer-controlled test system, there is no need to add a lot of complicated control circuits. The IEEE-488 system uses rack-mounted intelligent instruments as the main device , Constitutes an open building block test system, so IEEE-488 bus is currently one of the most widely used communication buses in the industry.

Conventions used by IEEE-488 bus:

The data transmission rate is less than or equal to 1MB/S.

The number of devices connected to the bus (including the microcomputer as the master) is ≤15.

The maximum distance between devices is ≤20M.

The total cable length of the entire system is less than or equal to 220M. If the cable length exceeds 220M, the timing relationship will be changed due to the delay, resulting in unreliable work. In this case, a modem should be attached.

All digital exchanges must be digital.

The bus stipulates the use of a 24-wire combination plug socket and adopts negative logic, that is, a level less than +0.8V is used to represent a logic “1”; a level greater than 2V is used to represent a logic “0”.

How the devices on the system work:

The “listener” approach. This is a receiver that receives data on the data bus. A system can have more than two “listeners” working at the same time.

“Speaker” approach. This is a type of transmitter. A system can have more than two “speakers” but only one speaker can work at any one time.

The “controller” approach. This is a device that issues commands to other devices, such as addressing other devices, or allowing the “speaker” to use the bus. There can only be one controller at any one time.

IEEE-488 bus transmission data timing: The data transmission on the IEEE-488 bus adopts asynchronous mode, that is, each byte of data is transmitted through the three signal lines of DAV, NRFD and NDAC for handshake communication.