SafetyNET p Protocol. Presented By Sharani Sankaran

Transcription

1 SafetyNET p Protocol Presented By Sharani Sankaran

2 Outline Introduction SafetyNETp Physical Layer Communication Model Precise Clock Synchronization Safety Communication Model

3 INTRODUCTION SafetyNET p is defined in the IEC standards series for industrial fieldbus systems and has emerged as the standard for Ethernet-based fieldbus communication in automation technology. Its objective was to enable fieldbus communication on Ethernet in real-time and to simultaneously provide data communication. Safety Net p is a multimaster bus system.

4 Outline Introduction SafetyNETp Physical Layer Communication Model Precise Clock Synchronization Safety Communication Model

5 Safety NET p Physical Layer SafetyNETp implements two communication models, known as realtime frame line (RTFL) and real-time frame network (RTFN). RTFL has been designed for fast intracell communications and RTFN. SafetyNET p communication variants are based on the well-known Standard Ethernet IEEE technology. This allows the use of standard components and makes compatibility easier.

6 RTFL Topology It is based on a linear topology, and the transport of data is supported through standard Ethernet frames. Each RTFL device is addressed individually by its predecessor in the line using the MAC address. During start-up, management frames are exchanged between the bus master and the OD's (i.e., ordinary devices). After entering the steady-state operating cycle, frames are sent along a linear structure with point-point transmission.

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8 RTFN Topology RTFN allows Safety NET p to be used within standard Ethernet networks without adoption of special hardware. Standard Ethernet topologies can be used for RTFN as long as they meet the real time requirements. Cables and Connectors: The ability to???control the connection technology? in an industrial environment is a key criterion in gaining aeptance for industrial communication systems. Cabling: Cabling of SafetyNET p devices is based on the guidelines published in the IAONA Industrial Ethernet Planning and Installation Guide document. SafetyNET p mainly uses ethernet cables.

9 Outline Introduction SafetyNETp Physical Layer Communication Model Precise Clock Synchronization Safety Communication Model

10 Communication Model SafetyNETp supports two distinct logical channels the cyclic data channel (CDC) and the message channel (MSC). CDC is used to transfer cyclic process data and an MSC to transfer non-time-critical data such as diagnostic information, device programming instructions, and configuration parameters. Every SafetyNET p frame is characterized by a 2-byte header field. SafetyNET p specifies 2 further acyclic services: Communication Management Service(CMS):To perform the initialization of Safety NET p network Time Synchronization Service(TSS): Used for node synchronization service

11 General Frame Structure???

12 In RTFL Communication model, the CDC (CDCL) is assigned the header values 0x02 and 0x03, to write or read the frame to/from the channel. The header values 0x00 and 0x01 identify the MSC for writing and reading messages. In RTFN, the CDC is identified by the header value 0x60, and 0x70 is used for communication on MSC. SafetyNET p frames can be transported as the payload of either standard Ethernet frames or UDP/IP datagrams, depending on the communication model used to build the network segment.

13 Cyclic data Channel The CDC is mainly used to transfer cyclic process data. In RTFL, the CDC is a reserved section of the Ethernet frames. In RTFN, the CDC is based on a cyclic sequence of point-to-point communications CDCN messages can be transported either at MAC or UDP level. In both communication models each cyclic data is identified by Packet Identifier(PID). CDCL control fields are the following: Cycle counter(2bytes) identifies the number of the current cycle. Frame counter (1 byte) identifies the frame number within the actual cycle. Length (2 bytes) is the length in octets of the CDC write pointer field plus the CDC payload section. CDC write pointer(2 bytes) identifies the write section for cyclic communication

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15 CDCN PDUs foresee two control fields only: 1. Version (1 byte) identifies the version of the CDCN protocol. 2. Size (2 bytes) is the size in octets of the CDC payload section. The CDC payload section is common to both communication models and consists of a sequence process data objects(pdo). 1. PID (3 bytes) uniquely identifies the PDO within the network. 2. Len(1 byte) is the length of the whole PDO including the PID and the Len fields. 3. DataLen-( 4 bytes )are the process data being transmitted.

16 Message Channel The MSC is used for acyclic communication to transmit data conerning diagnostics. Devices write data into MSC packets and the message is transferred by the means of Message Transfer protocol (MSC-MTP). MSC-MTP provides a confirmed service. When RTFL is used, MSC consists of exactly one frame of each communication cycle. The MSCL frame size is configurable, but in the case MSC were not able to accommodate all messages in a single cycle.

17 MSC-MTP communication session starts with either an init frame or init fast frame message sent to the destination. If the size of the data is small then the sender invokes init fast frame to avoid segmentation which embeds all the data to be transmitted. The destination device responds with an acknowledgment message. If the communication session started with an init_fast frame and the size of the transmitted data is smaller than MSS,the sender is automatically notified that all data has been successfully recieved. The meaning of the control fields is the following: Cycle counter (2 bytes) identifies the number of the current cycle. MSCL control (1 byte) contains control bits to set parameters for the communication. Bit 0 is used to reset previously assigned priorities, bit 1 shows if the line is running in diagnostic mode or not, bit 2 signals whether the PCS clocks are synchronized or a delay measurement is being carried out, and bit 3 is used to know if the local clock is controlled by means of time stamps.

18 The four most significant bits are reserved for future use. System time (8 bytes) stores the time when the packet was processed by the master clock. Reserved (2 bytes). Length (2 bytes) is the length in octets of the MSC write pointer header field plus the MSC data section. MSC write pointer (2 bytes) points to the next write position in the MSC data section.

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20 Communication Management The two most important messages of the network verification class are the RTFL network verification prepare and the RTFL network verification information. RTFL network verification information message is used by devices to transmit their status,configuration parameters, and hardware information. This message is identified by a SafetyNET p frame header value of 0x12, followed by a 2-byte sequence number and identification data section. The reception of both types of messages mentioned earlier is confirmed by means of a network verification acknowledgment message

21 The network configuration message class is used by RD to set up the communication line and configure the OD's The former allows RD to configure the ODs, while the latter is used by the ODs for correct reception of configuration parameters. RTFN CMS (communication management system) consists of 2 sets of messages: RTFN Scan Network : it is used by the protocol to retrieve information about the status of device. RTFN Connection management set: It is used to initialise and manage links between publisher and subscriber. RTFN scan network consists of two messages. The first one, RTFN scan network read request is broadcast to query the status of the network devices. The second message, RTFN scan network read response, is the reply of any device in the network to the broadcasted request.

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23 SafetyNET p Application Layer CANOpen Application Layer in SafetyNET p CANopen is a nonproprietary open fieldbus standard, specified and standardized by the CAN in Automation organization. The SafetyNET p base element inherited from CANopen is the object dictionary (ODIC). ODIC is a technique to group objects in a device and specify uniform communication/device and functions which are stored and retrieved using objects. Process data communication is handled by PDOs (process data object) in SafetyNET. PDOs are logical data containers transferred between SafetyNET p devices over either RTFN or RTFL. PDO communication is typically cyclic over the CDC logical channel.

24 SafetyNET p grants remote access to the ODIC by means of SDOs (service data objects). SDOs can be used to read or write PDO configurations. The SDO communication is based on the client server model and is implemented using a point-to-point link between two devices. The transmission of an SDO message is a confirmed service, and an abort service is also provided to notify SDO communication error. Devices can send diagnostic events using the emergency (EMCY) service, which broadcasts standardized messages.

25 Outline Introduction SafetyNETp Physical Layer Communication Model Precise Clock Synchronization Safety Communication Model

26 Precise Clock Synchronization SafetyNET p provides suitable configuration and control services to this purpose to be use with RTFL. The PCS protocol is based on master clock. The master clock then delivers a time base for synchronization to OD's.

27 Outline Introduction SafetyNETp Physical Layer Communication Model Precise Clock Synchronization Safety Communication Model

28 Safety Communication Model A safety system is in charge of detecting faults to prevent failures of critical processes that could cause injuries or damages either to people health, environment. The approach commonly adopted to set up safe communications over conventional networks is based on the concept of black channel. Networks and protocols play a key role because they are in charge of transferring information from safety sensors to safety controllers. The safety layer implements safety related transmission functions and also maintains the integrity of the communication channel.

29 CBlack Channel Approach in SafetyNET p The approach used by protocol to transmit safety related data on a conventional communication line is based on the concept of black channel. The safety layer is on top of the application layer. The CDC packet section of the SafetyNET p PDU contains either PDO's for standard cyclic communications or safety process data objects(spdo's). Acyclic messages contain either SDOs or safety service data objects(ssdos), for standard and safe transfers on MSC.

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31 CSafetyNET p Error Detection Mechanisms The IEC defines the communication errors that must be detected by the safety layer. SafetyNET p is able to cope up with following 9 types of communication errors: Corruption: messages may be corrupted due to errors in transrnmission medium. Unintended repetition: Due to errors on the communication channel. Incorrect Sequence:the natural sequence of messages transmitted from a particular source is altered. Loss: A message or acknowledgement is not recieved. Unacceptable delay: messages must be recieved with a predefined deadline but they can be delayed during transmission. Insertion : Mainly due to faults/errors in communication line. Masquerade: Mainly due to faults/errors in communication line, i.e., a non safety related message is treated as safety relevant by the reciever. Addressing: A safety related message is sent to wrong destination as valid message.

32 Detection Mechanisms: Safety NET p introduces a set of techniques that are used to detect the type of communication errors. Sequence Number: It is adopted for transmission of SPDO's. Timeout: The time elapsing between any 2 messages recieved is checked by safety related devices. Connection ID: It is introduced in SPDO transmissions to identify the sender of the frame without ambiguity. Data Integrity Assurane: A cyclic redundancy check is introduced to verify the integrity of SPDO's Different data integrity assurance: Different data integrity checks are aimed at detecting standard data which are received and modified to interpret them as safe data

33 Safety Communication Layer The safe channel is built on top of CDC since safe data are cyclic in nature. The general structure of a cyclical safety PDU is identical to standard CDC communications. The difference between standard and safe channels is in the format of CDC payload section. In standard communication, CDC is used only to transfet process data using PDO's. In safe communications, SPDO's or safety heartbeats are transmitted via CDC

34 Safety Process Data Objects The structure of SPDO's is more complex sine there are additional fields used for communication error detection. The consecutive number field implements sequence number safety mechanism while PID and SID identifiers is used to build a unique connection ID. The data integrity assurance and different data integrity assurance are granted by the 32-bit CRC, which is computed using the 0x polynomial. The adoption of the redundant data SID consecutive number and CRC fields is used by the safety layer to check the integrity of communication channel

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36 Safety HeartBeat Service SHB is a confirmed service and messages are exchanged in a cyclic way. The cycle time of the service mainly checks the delivery time of messages against maximum predefined threshold. Each device involved in a communication relationship is forced to monitor interlocutor by cyclically sending SHB request messages. SHB request PDUs, the PDO data section used to transport safety-related data is replaced with two fields: SCL state Safety AP state SCL state contains information about the state of the SCL The Safety AP state field contains information about the state of the safety application. SHB response PDU's do not carry any data and confirm that an SHB request has been recieved.

37 Safety Application Layer The safety application layer provides applications with the interface to access the safetynet p safe services. The safe address of a SafetyNET p device is its safety ID (SID). SID is one of the mandatory safety objects. SPDO's are specified as TxSPDO and RxSPDO on the producer and consumer sides. Communication Parameters define the SPDO transfer mode while the mapping parameters define the SPDO contents. The Failsafe Consumer Heartbeat list contains parameters needed to program the reception of SHB and SHB responses. SafetyNETp does not support master slave model but delegates the safety configuration management to system application level.