CN100382522C - A Scheduling Method for Realizing Ethernet Deterministic Communication - Google Patents

Abstract

A scheduling method for realizing Ethernet deterministic communication, which divides the information in the network into periodic information and non-periodic information. For periodic information, the control method of time slot access is adopted to perform periodic data exchange in a fixed time slice ; For non-periodic information, adopt centralized scheduling mode based on token, and carry out in the interval of periodic information communication, that is, the stage of non-periodic information communication; The scheduling method of the present invention defines at least one master device and one or more slave devices on the network , the slave device can only send aperiodic information after obtaining the token from the master device; at the same time, there is at least one clock server on the network for clock synchronization between each device (including the master device and the slave device). The invention reasonably dispatches periodic and non-periodic information in Ethernet, realizes real-time and deterministic communication on the industrial control network based on Ethernet+UDP/IP, and satisfies the real-time and deterministic communication of industrial control network requirements.

Description

A Scheduling Method for Realizing Ethernet Deterministic Communication

technical field

The invention relates to a scheduling method for Ethernet communication, in particular to a scheduling method for deterministic communication between multiple network nodes in an industrial control network based on Ethernet+UDP/IP.

Background technique

As a mature network technology, Ethernet has many advantages such as low cost, stability and reliability, etc. It has been widely used in the field of office automation and industrial control, and has become one of the most popular communication networks. The currently used Ethernet standards mostly use the CSMA/CD protocol with collision detection in the MAC (Media Access Control) layer. The working process is: when a certain node on a network needs to send data, It first monitors the channel, and if the channel is busy, it continues to wait until it detects that the channel is free, and then sends the data. If two or more nodes are listening and waiting to send data, when the channel is detected to be idle, each node immediately (almost at the same time) starts sending data, and a conflict occurs at this time. If a node detects a collision during transmission, it immediately stops transmission and sends a "congestion" signal to the channel to ensure that all other nodes on the network also see the collision. In an Ethernet-based communication system, in order to avoid conflicts, each node uses a 1-persistent Binary Exponential Back-Off (BEB, Binary Exponential Back-Off) algorithm to deal with conflicts. It is effectively applied in real-time networks such as control networks.

The industrial control network is a typical real-time application system, in which the tasks (such as the execution of function blocks) are usually triggered according to a certain time interval, and there is a deadline requirement for the execution time of the tasks. This task is called a periodic task. There is also a task in the real-time application system, which only occurs when a specific event is triggered, such as device configuration, fault diagnosis, program upload/download, running record, alarm processing, etc. Such tasks are called aperiodic Tasks, non-periodic tasks are triggered randomly. These two tasks are reflected in the communication of the industrial control network, which are two types of communication information: periodic communication information and non-periodic communication information. Periodic information is real-time information, non-periodic information is non-real-time information, and periodic communication information and non-periodic communication information have different time characteristics. Once the system configuration is complete, the periodic communication messages are sent with time determinism. Non-periodic communication information is often burst information, which is uncertain in time.

In order to improve the real-time and certainty of Ethernet communication, people have done a lot of research and put forward various methods to improve CSMA/CD. According to the improvement methods of CSMA/CD, they mainly include: improving the Ethernet MAC protocol, The network layer increases the transmission control mechanism, etc. CSMA/DCR (DeterministicCollision Resolution) is the most representative MAC protocol improvement method. When a conflict occurs, the deterministic binary tree addressing method is adopted, and CSMA/DCR adopts a preorder traversal method according to the node address to resolve the conflict. When a collision occurs, nodes with low priority stop competing for the channel, while nodes with high priority continue to compete for the channel until successful transmission. Although this method of modifying the Ethernet MAC protocol can greatly improve the determinism of Ethernet communication, it is at the expense of changing the Ethernet firmware (Ethernet controller hardware and software), and it is difficult to integrate with standard commercial Ethernet. Network compatibility, and increased development costs.

The most typical method of adding transmission control mechanism on the upper layer of Ethernet is Time Division Multiple Access (TDMA) strategy. TDMA allocates a certain bandwidth for each node, and each node sends information within a fixed time slice to ensure that each The information of each node has a definite sending time. Because TDMA does not need to transmit additional control information, the utilization rate of network bandwidth is relatively high. However, TDMA is a node-based method, which cannot reflect the actual bandwidth requirements of each node, and cannot guarantee timely transmission of aperiodic information. Another way to increase transmission control on the upper layer of the Ethernet is the master-slave transmission control mode, which is a centralized transmission control mode. There is at least one master device and multiple slave devices in the network. The slave device can only send data after receiving the control message from the master device. The advantage of this master-slave transmission control method is that it can ensure that each slave device has the opportunity to send data, but the disadvantage is that it cannot handle sudden communications in the industrial control network, such as alarm information in the industrial control network.

Therefore, although the above methods have improved the real-time and certainty of Ethernet transmission to a certain extent, they are either at the expense of changing the Ethernet structure (such as CSMA/DCR), or at a lower network level (such as the MAC layer). The biggest disadvantage of these methods is that they are difficult to implement, often involve hardware, and cannot be compatible with traditional standard Ethernet, and cannot meet the requirements of two types of information in industrial control networks—periodic information (that is, real-time information) Special requirements for simultaneous processing with non-periodic information (usually non-real-time information).

Contents of the invention

The purpose of the present invention is to provide a scheduling method for deterministic communication at the user layer above UDP without changing the original structure of the Ethernet, so as to reasonably schedule periodic and aperiodic information in the industrial control network. The real-time and deterministic communication is realized on the industrial control network based on Ethernet + UDP/IP to meet the real-time and deterministic requirements of the industrial control network based on the traditional Ethernet standard.

In order to achieve the above object, the present invention adopts following technical scheme:

A scheduling method for realizing Ethernet deterministic communication, characterized in that,

defining at least one master device and one or more slave devices on the Ethernet, and having at least one clock server on the Ethernet;

The information in the Ethernet is divided into periodic information and non-periodic information, and the control mode of time slot access is adopted for the periodic information, and periodic data exchange is performed in a fixed time slice; Based on the token-based centralized scheduling method, data exchange is performed during the interval of the periodic information communication, that is, the aperiodic information communication phase;

Include the following steps:

a. Before the system starts, the master device is responsible for configuring the entire system, including specifying the size of the transmission cycle; specifying the time length for each slave device to send cycle information in a transmission cycle; setting each slave device in the transmission cycle The time deviation between the start moment of the transmission cycle information in the transmission cycle relative to the start moment of the transmission cycle; and download the information to each slave device;

b. After the system is powered on, all master devices and all slave devices first synchronize clocks with the clock server;

c. After the transmission cycle starts, the master device adopts a time slot access control method for the periodic information, and each slave device sends the periodic information at a predetermined time, and each slave device transmits the periodic information in the next period of time after sending the periodic information , send a statement message to the master device, informing the master device whether it has aperiodic information to send, if there is aperiodic information to send, the master device will correspond to the IP address of the slave device, information priority and expected information The sending time is stored in an aperiodic information sending queue, and the slave device waits for the aperiodic information scheduling token in the next aperiodic information communication phase;

d. In the non-periodic information communication phase of a transmission cycle, the master device performs the following operations:

d1. Scan the non-periodic message sending queue to determine whether the queue is empty, if the queue is not empty, go to d2; otherwise, end;

d2. According to the information in the queue, send the aperiodic information scheduling token to the slave device with the highest priority aperiodic information, and start a timer at the same time, the timer size is the token holding time;

d3. Waiting for the token to be released from the device;

d4. Determine whether the confirmation from the slave device is received, if so, go to d1; otherwise, go to d5;

d5. Determine whether the timer overflows, if not, go to d3; otherwise, go to d1;

After the aperiodic information communication phase ends, the master device terminates the above operations, and simultaneously saves unprocessed information in the aperiodic information sending queue;

e. In the non-periodic information communication phase of a transmission cycle, the slave device performs the following operations:

e1. Waiting for the aperiodic information scheduling token;

e2. Judging whether the aperiodic information scheduling token is received, if not received, go to e1; otherwise, go to e3;

e3. Determine whether the priority of the information is not less than the priority of the token, if true, go to e4; otherwise, go to e6;

e4. Determine whether the token holding time is not less than the information sending time, if true, go to e5; otherwise, go to e6;

e5. Send aperiodic information;

e6. Send confirmation information to the master device, release the aperiodic information scheduling token, and end.

The entire network transmission time is divided into an infinite number of transmission periods of equal length, each transmission period includes a periodical information communication phase and an aperiodic information communication phase, and all slave devices send and receive the transmission period in each transmission period Periodic information and the non-periodic information.

In the step c, the master device adopts a time slot access control method for periodic information communication: each device on the network, including the master device and the slave device, maintains clock synchronization with the clock server, and the master device in step a. During system configuration, determine the length of time for each slave device to send periodic information in a transmission cycle and the offset of the start time of slave device sending cycle information relative to the start time of the transmission cycle.

In the period information communication phase of step c, after each slave device sends the period information at a predetermined time, there is still a period of time for sending a statement message to the master device, notifying the master device that it is in the next Whether there is any non-periodic information to be sent within the time; if so, indicate the sending time required for the aperiodic information and the priority of the information. After the master device receives the message, there will be aperiodic information to be sent The IP address of the device, the priority of the information, and the expected sending time of the information are stored in a queue; if the master device does not receive a statement message from a slave device within three consecutive cycles, it is considered that the slave device The device has failed.

The aperiodic information communication in steps d and e is based on token-based centralized scheduling, which is to divide the aperiodic information in the network into different priorities, and the master device decides which slave device should be the first according to the priority of the aperiodic information. Send, high-priority information will get the token first, and have the priority to send.

The clock synchronization method is realized by the simple network time protocol (SNTP, Simple Network Time Protocol) between the device and the clock server.

Due to the adoption of the above technical solution, the scheduling method of the present invention reasonably schedules periodic and non-periodic information in the industrial control network, and realizes real-time and deterministic communication on the industrial control network based on Ethernet+UDP/IP.

Description of drawings

FIG. 1 is a schematic diagram of the division of the transmission period of the network transmission time;

FIG. 2 is a schematic diagram of an information communication process in a transmission cycle;

Fig. 3 is a kind of format of the aperiodic information scheduling token used in an embodiment of the scheduling method of the present invention;

FIG. 4 is a processing flow of the master device for aperiodic information in one embodiment of the scheduling method of the present invention;

Fig. 5 is the processing flow of the aperiodic information from the device in one embodiment of the scheduling method of the present invention;

Fig. 6 is a schematic diagram of realizing clock synchronization by SNTP.

Detailed ways

The technical solutions of the present invention will be further described below in conjunction with the drawings and embodiments.

The scheduling method of the present invention defines at least one master device and one or more slave devices on the Ethernet, and is provided with at least one clock server on the Ethernet;

And the information in the Ethernet is divided into periodic information and non-periodic information, the control method of time slot access is adopted for periodic information, and periodic data exchange is carried out in a fixed time slice; for non-periodic information, token-based centralized Scheduling mode, data exchange is performed in the interval of periodic information communication, that is, in the non-periodic information communication stage;

Include the following steps:

a. Before the system starts, the master device is responsible for configuring the entire system, including specifying the size of the transmission cycle; specifying the time length for each slave device to send cycle information in a transmission cycle; setting each slave device to transmit in a transmission cycle The time deviation of the start moment of the cycle information relative to the start moment of the transmission cycle; and download the information to each slave device;

FIG. 1 is a schematic diagram of division of transmission periods of network transmission time. As shown in FIG. 1 , the network transmission time is divided into an infinite number of transmission periods of equal length. Each transmission cycle is composed of a periodic information transmission phase and an aperiodic information transmission phase, as shown in FIG. 2 .

Fig. 2 is a schematic diagram of an information communication process in a transmission cycle, and there are 6 slave devices communicating in the example network in Fig. 2 . The length of time for each slave device to send cycle information is determined by the master device during system configuration; the offsets of the start time of each slave device sending cycle information relative to the start time of the transmission cycle are different, so as to avoid It eliminates the possibility of conflicts when multiple devices access network resources at the same time. At the same time, each device sends periodic information with certainty in time, that is, once a device sends a periodic information at a certain moment, the next time the device sends the same periodic information can be calculated by the following method:

NextSendTime=CurrentTime+transmission period

Wherein, NextSendTime is the time when the device transmits the same cycle information next time, and CurrentTime is the moment when the current cycle information is transmitted.

b. After the system is powered on, all master devices and all slave devices first synchronize the clock with the clock server;

Fig. 6 is a schematic diagram of realizing clock synchronization by SNTP. Keeping strict clock synchronization between devices described in the present invention is realized through the UDP-based SNTP protocol, as shown in FIG. 6 . Using SNTP to achieve clock synchronization between devices is actually to allow each device to periodically exchange SNTP messages with the clock server to calculate the time difference between the device and the clock server, thereby adjusting the local clock so that the local clock of the device is consistent with the clock server The time difference between them is kept within the allowable range. Four timestamps are used when calculating the time difference between the device and the clock server: T ₁ , T ₂ , T ₃ and T ₄ , and their meanings are as follows:

T ₁ : the local time stamp when the device sends the clock synchronization request;

T ₂ : the time stamp (standard time) when the clock server receives the clock synchronization request;

T ₃ : the time stamp (standard time) when the clock server sends the clock synchronization response;

T ₄ : The local time stamp when the device receives the clock synchronization reply.

Simple Network Time Protocol implements clock synchronization based on an assumption that the transmission delay between the device to the clock server and the clock server to the field device is equal. Based on this assumption, we calculate the time deviation T _d between the field device and the clock server through the following algorithm.

T ₂ -(T ₁ +T _d )=(T ₄ +T _d )-T ₃

According to the above formula, the time deviation T _d between the device and the clock server can be calculated:

Td=((T2-T1)+(T3-T4))/2

The device can adjust the local clock according to the time deviation T _d , so as to achieve synchronization with the clock server. After all master devices and all slave devices on the network are synchronized with the clock server in the same way, it means that all master devices and all slave devices on the network are also synchronized in time.

c. After the transmission cycle starts, the master device adopts a time slot access control method for the periodic information, and each slave device sends the periodic information at a predetermined time, and each slave device transmits the periodic information in the next period of time after sending the periodic information , send a statement message to the master device, informing the master device whether it has aperiodic information to send, if there is aperiodic information to send, the master device will correspond to the IP address of the slave device, information priority and expected information The sending time is stored in an aperiodic information sending queue, and the slave device waits for the aperiodic information scheduling token in the next aperiodic information communication phase;

In the periodic information communication stage, after each device sends the periodic information at a predetermined time, there is still a period of time, as shown in the dashed box in Figure 2 . During this time, the slave device sends a declaration message to the master device, informing the master device whether there is any aperiodic information to be sent in the next time. If yes, also indicate the sending time required for the aperiodic information and the priority of the information. After the master device receives the message, it will have the IP address of the device to send the aperiodic information, the priority of the information and the information priority. The expected send times are kept in a queue. In this way, the master device can know whether each slave device has aperiodic information to send within a transmission cycle, and at the same time, the master device can also monitor the status of each slave device. If the master device does not receive a declaration message from a slave device within three consecutive cycles, the device is considered to have failed.

d. In the aperiodic information communication phase of a transmission cycle, the master device performs the following operations, and FIG. 4 is a processing flow of the master device for aperiodic information in one embodiment of the scheduling method of the present invention:

d1. Scan the non-periodic information sending queue to determine whether the queue is empty, if the queue is not empty, go to

d2; otherwise, end;

d2. According to the information in the queue, send the aperiodic information scheduling token to the slave device with the highest priority aperiodic information, and start the timer simultaneously (the timer size is the token holding time);

d3. Waiting for the token to be released from the device;

d4. Determine whether the confirmation from the slave device is received, if so, go to d1; otherwise, go to d5;

d5. Determine whether the timer overflows, if not, go to d3; otherwise, go to d1;

After the aperiodic information communication phase ends, the master device terminates the above operations, and simultaneously saves unprocessed information in the aperiodic information sending queue;

In the phase of aperiodic information communication, the master device will be sent by the master according to the information of the slave device (including the IP address of the device to send aperiodic information, the priority of the information, and the expected sending time of the information, etc.) obtained in the phase of periodic information communication. The scheduler in the device implements the sending of aperiodic information scheduling tokens through special control messages.

Fig. 3 is an implementation format of the aperiodic information scheduling token in this embodiment. The 1-byte message type field is used to identify the message type, that is, whether it is an aperiodic information scheduling token, for example, 0 indicates an aperiodic information scheduling token, and other values indicate other types of ordinary messages. The 1-byte priority field identifies the priority of the token. The last 4 bytes are used to indicate the token holding time.

The duration of the token is determined according to the aperiodic information sending time provided by the device to send aperiodic information, generally not less than the aperiodic information sending duration to ensure the aperiodic information is sent completely. The duration of sending a message can be estimated by:

Information sending duration (s) = information length (bit) / network bandwidth (Mbps)

Wherein, the information length is composed of the following parts: the length of valid data, the length of the UDP message header, the length of the IP message header and the length of the Ethernet frame header.

After the slave device receives the aperiodic information scheduling token from the master device, it gets the right to send aperiodic information within a certain period of time, and the time length is specified in the token. In order to ensure the normal transmission of periodic information, the token holding time (token holding time refers to the time from obtaining the token to releasing the token) cannot exceed the total time occupied by the non-periodic information communication phase. If the slave device completes sending or reaches the maximum allowable sending time (that is, exceeds the total time occupied by the non-periodic information communication phase), it sends an acknowledgment message to the master device and releases the token at the same time.

e. In the aperiodic information communication phase of a transmission cycle, the slave device performs the following operations, and Fig. 5 is a processing flow of the slave device to aperiodic information in one embodiment of the scheduling method of the present invention:

e1. Waiting for the aperiodic information scheduling token;

e2. Judging whether the aperiodic information scheduling token is received, if not received, go to e1; otherwise, go to e3;

e3. Determine whether the priority of the information is not less than the priority of the token, if true, go to e4; otherwise, go to e6;

e4. Determine whether the token holding time is not less than the information sending time, if true, go to e5; otherwise, go to e6;

e5. Send aperiodic information;

e6. Send confirmation information to the master device, release the aperiodic information scheduling token, and end.

In order to ensure that important and urgent aperiodic information (such as alarm information) can be sent in time, each aperiodic information also has different priorities. Taking the three levels of priority as an example, they are respectively level 0, level 1, and level 2, where priority level 0 is the highest priority, priority level 2 is the lowest priority, and priority level 1 is in between. Aperiodic information scheduling tokens are also classified into the same priority classes as aperiodic information. After receiving the token from the device, compare the aperiodic information in the sending buffer with the priority of the token, only if the priority of the aperiodic information is not lower than the priority of the token, and the token holding time is not less than The aperiodic information is sent only when the information is sent, otherwise, the token is released directly to the master device.

Due to the adoption of the above technical solution, the present invention provides a scheduling method for deterministic communication at the user layer above UDP, to reasonably schedule periodic and non-periodic information in the industrial control network, using Ethernet+UDP/IP as the The real-time and deterministic communication is realized on the basic industrial control network to meet the real-time and deterministic requirements of the industrial control network based on the traditional Ethernet standard.

Claims (6)

1. A scheduling method for realizing Ethernet deterministic communication, characterized in that, defining at least one master device and one or more slave devices on the Ethernet, and having at least one clock server on the Ethernet; The information in the Ethernet is divided into periodic information and non-periodic information, and the control mode of time slot access is adopted for the periodic information, and periodic data exchange is performed in a fixed time slice; Based on the token-based centralized scheduling method, data exchange is performed during the interval of the periodic information communication, that is, the aperiodic information communication stage; Include the following steps: a. Before the system starts, the master device is responsible for configuring the entire system, including specifying the size of the transmission cycle; specifying the time length for each slave device to send cycle information in a transmission cycle; setting each slave device in the transmission cycle The time deviation between the start moment of the transmission cycle information in the transmission cycle relative to the start moment of the transmission cycle; and download the information to each slave device; b. After the system is powered on, all master devices and all slave devices first synchronize clocks with the clock server; c. After the transmission cycle starts, the master device adopts a time slot access control method for the periodic information, and each slave device sends the periodic information at a predetermined time, and each slave device transmits the periodic information in the next period of time after sending the periodic information , send a statement message to the master device, informing the master device whether it has aperiodic information to send, if there is aperiodic information to send, the master device will correspond to the IP address of the slave device, information priority and expected information The sending time is stored in an aperiodic information sending queue, and the slave device waits for the aperiodic information scheduling token in the next aperiodic information communication phase; d. In the non-periodic information communication phase of a transmission cycle, the master device performs the following operations: d1. Scan the non-periodic information sending queue to determine whether the queue is empty, if the queue is not empty, go to d2; otherwise, end; d2. According to the information in the queue, send the aperiodic information scheduling token to the slave device with the highest priority aperiodic information, and start a timer at the same time, the timer size is the token holding time; d3. Waiting for the token to be released from the device; d4. Determine whether the confirmation from the slave device is received, if so, go to d1; otherwise, go to d5; d5. Determine whether the timer overflows, if not, go to d3; otherwise, go to d1; After the aperiodic information communication phase ends, the master device terminates the above operations, and simultaneously saves unprocessed information in the aperiodic information sending queue; e. In the non-periodic information communication phase of a transmission cycle, the slave device performs the following operations: e1. Waiting for the aperiodic information scheduling token; e2. Judging whether the aperiodic information scheduling token is received, if not received, go to e1; otherwise, go to e3; e3. Determine whether the priority of the information is not less than the priority of the token, if true, go to e4; otherwise, go to e6; e4. Determine whether the token holding time is not less than the information sending time, if true, go to e5; otherwise, go to e6; e5. Send aperiodic information; e6. Send confirmation information to the master device, release the aperiodic information scheduling token, and end. 2. A scheduling method for realizing Ethernet deterministic communication as claimed in claim 1, wherein the entire network transmission time is divided into an infinite number of transmission cycles of equal length, and each transmission cycle includes cycle information In the communication phase and the aperiodic information communication phase, all slave devices send and receive the periodic information and the aperiodic information in each transmission cycle. 3. A scheduling method for realizing Ethernet deterministic communication according to claim 1, characterized in that, in the step c, the master device adopts a time slot access control method for periodical information communication: every Each device, including the master device and the slave device, maintains clock synchronization with the clock server, and the master device determines the time length for each slave device to send periodic information in a transmission cycle and the time length for the slave device to send The offset of the start time of the cycle information relative to the start time of the transmission cycle. 4. A scheduling method for realizing Ethernet deterministic communication according to claim 1, characterized in that, in the phase of periodic information communication in the step c, after each slave device sends the periodic information at a predetermined time, there is still For a period of time, it is used to send a declaration message to the master device, informing the master device whether it still has aperiodic information to send in the next time; if so, it also indicates the required sending of the aperiodic information time, and the priority of the information, after the master device receives the message, it will save the IP address of the device to send the non-periodic information, the priority of the information and the expected sending time of the information in a queue; if the master If the device does not receive a statement message from a slave device within three consecutive cycles, it considers the slave device to be invalid. 5. A kind of dispatching method that realizes Ethernet deterministic communication as claimed in claim 1, is characterized in that, described step d, e aperiodic information communication is based on the centralized dispatching mode of token, is the aperiodic in the network The information is divided into different priorities, and the master device decides which slave device to send first according to the priority of the aperiodic information, and the information with high priority will get the token first, and has the priority to send. 6. A scheduling method for realizing Ethernet deterministic communication according to claim 1, characterized in that, the clock synchronization method is realized by the simple network time protocol between the device and the clock server.

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