Realization of Hart Communication in DPV1
Realization of Hart Communication in DPV1
PROFIBUS is a field bus technology used for factory automation workshop level monitoring and field device layer data communication and control. It can realize decentralized digital control and field communication from field device layer to workshop level monitoring, thus providing a feasible solution for realizing comprehensive factory automation and field device intelligence. PROFIBUS-DP, as a subset of PROFIBUS specifications, is mainly oriented to the field-level application of the factory. It can complete fast and reliable cyclic communication tasks including programmable controllers, automatic control devices, sensors, and actuators.
HART is a transmission standard developed by Rosemount, and is a communication protocol used between field smart instruments and control room equipment. Its characteristic is to realize digital signal communication on the existing analog signal transmission line. It belongs to the transitional product during the transition from analog system to digital system.
Through cooperation with the HART Foundation, PI specifically defined a HART specification for integrating HART into the PROFIBUS system, and can also provide a basis for the controller to control the field HART device through the PROFIBUS-DP bus. This article describes a method based on this specification and using DPVl to implement a controller to control a field HART device.
1 System design and implementation
Figure 1 shows the overall implementation of the system. The communication in the system is initiated by the Hart management software (Hart Client), and the DP master station (DP Master) is requested by the communication. After receiving the request, the DP master station converts the request data sent by the Hart Client to DPVl The message format, and send the message to the DP slave (DP Slave) through the DP bus. Here, the DP slave also acts as a Hart master (Hart Master Device HMD). Its main function is to send the DP master DP The format message is converted into a Hart format message and sent to the Hart device (Hart Device). For some requests from Hart Client, HMD does not need to send the request to the Hart device, but directly replies, such as reading HMD related parameter data, the implementation process of this type of request is shown in Figure 2. 
2 DPVl implementation
DPVl adds new services on the basis of DPV0. For Class 1 master stations, the newly added services of DPVl are:
â—‡ Aperiodic reading service: MSACl_Read;
â—‡ Acyclic write service: MSACl_Write;
â—‡ Aperiodic alarm: MSACl_Alarm, MSACl_Alarm_Ack, MSACl_Status.
For the 2 types of master stations, the new services include:
â—‡ Aperiodic reading service: MSAC2_Read;
â—‡ Acyclic write service: MSAC2_Write;
â—‡ Establish a communication service with the slave station: MSAC2_IniTIate;
â—‡ Communication between middle finger and slave station: MSAC2_Abort;
â—‡ Data transmission: MSAC2_Data_Transport.
According to the specification, only two acyclic read and write services are used in providing communication for Hart, and Hollysys has used software to implement the functions of DP masters of various versions. Acyclic read and write services to achieve part of the DPVl function. Figure 3 shows the position of the acyclic state transition in the state transition diagram of the master station. 
2.1 Extension of slave station parameter set
According to the DP protocol, after Sl_Flag is expanded, it can provide more control information to the DPVl master station. This byte is related to acyclic read and write is the fourth bit (DPVl_Supported). If this bit is set to 1, the slave supports the DPV1 function, otherwise it works under the DPV0 function. For detailed explanations of other bits, please refer to the relevant literature. The other bytes expanded in the slave parameter set (such as Max_Alarm_Len, Alarm_Mode, etc.) are all related to the alarm, and they are not concerned here, please refer to the literature section.
2.2 Implementation of acyclic read service
DPVl devices are addressed by slots and indexes. The so-called slot is not necessarily an actual physical object, the slot can be a physical or logical data block. The interior of the slot is usually addressed by an index, which can identify variables or represent complete data blocks. Different from the DPV0 message format, the service node number used for acyclic reading is 51. In the data field DU, in addition to the exchanged data, the first four bytes define the function number (FuncTIon_Num) and slot number (Slot ), Index number (Index) and user data length (Data_Length). The message format is shown in Figure 4. 
The function number of the acyclic read service is usually 0x5E. When the read data returns correctly, Read. Bit7 of the function number of res is 0; and when the read operation has no result, Bit7 of the function number is 1 (FuncTIon_Num = 0xDE). At this time, the following 3 bytes are the specific encoding of the error. Such as:
Error_Decode: 
For specific error codes, please refer to the relevant literature.
The acyclic read service is different from the cyclic data exchange read data service. The acyclic read service generally waits for multiple system cycles before it can get the response data.
2.3 Implementation of acyclic write service
The implementation of the acyclic write service is similar to the acyclic read, except that the function numbers are different. The function numbers of the acyclic write service are:
FuncTIon Num = 0x5F.
3 Realization of Hart Communication
When implementing Hart communication through DPVl, the problem of protocol conversion must first be resolved, that is, how to transfer Hart protocol messages to the DP slave station (ie HMD) through the DP master station, and then convert the messages into Hart messages and transmit them to the site Hart equipment. In response, the same problem also exists. The specification defines two transmission formats, transparent and compressed.
As the name implies, the transparent format simply embeds the Hart message into the PROFIBUS message. Of course, this structure will have redundant information. This article uses this format to achieve transmission. Its format is shown in Figure 5. 
Compression format will generally simplify the Hart message embedded in the PROFIBUS message as much as possible, so as to be able to transmit a larger data length, and those omitted message information can be analyzed and filled into the Hart message by HMD, such as the Hart message The ADDRS in can be obtained from the combination of Slot_Num and Index in the PROFIBUS message.
Another problem that needs to be solved is how to implement HMD for the first time. Due to the different structure, HMD can be divided into simple HMD and modular HMD. There is only one HartMaster in a simple HMD. In this Hart Master, there will be a lot of data objects identified by indexes. Among them are data objects of a general catalog (Direetory). The indexes of other data objects can be checked in this data object. The index will be parsed in the GSD file. The format of its keywords is as follows: Subsysy_Dir_Index (1) = Index_Number; therefore, before exchanging Hart data, Hart Client should first read and parse the Directory information. The flow chart is shown in Figure 6. 
4 Conclusion
PROFIBUS-DP occupies a considerable share in industrial automation, therefore, it is very necessary to choose DP as the communication method of Hart Client and Hart equipment. For this reason, in the international market, most manufacturers of DCS and PLC provide DP interfaces. The realization method of this article is helpful for the promotion of DCS and PLC in Hart intelligent equipment and Hart instrument field.
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