In-depth exploration of LabVIEW--Using DDE to realize data exchange between processes

LabVIEW is multi-threaded. There are many ways to exchange data between two threads. Process (PROCESS) and thread (THREAD) are two different concepts. When we start an executable file, we actually start a process. The process manager of WINDOWS can observe the currently active processes. The exchange of data between processes can be simply understood as the exchange of data between multiple executable files.

There are several ways to exchange data between processes: clipboard (CLIPBOARD), dynamic data exchange (DDE), memory mapped file (MAP FILE) and general files. Of course, TCP/IP, SHARE VARIABLE, DATASOCKET can also be used, but these are all network data exchange and are not suitable for local process communication.

In a previous article, we have introduced how to use the clipboard for communication. Today, we will introduce dynamic data exchange (DDE).

DDE (Dynamic Data Exchange), that is, dynamic data exchange, is a complete communication protocol on the Windows platform, which enables applications to exchange data and send instructions to each other. The DDE process is a dialogue process between two programs, one party asks a question to the other party and then waits for an answer. The party that asks the question, that is, the application that requests to be informed of the information, is called the client, and the party that answers, that is, the application that provides the information, is called the server. An application can be both a client and a server at the same time: when it requests data from other programs, it acts as a client; when other programs need it to provide data, it becomes a server. But at a certain moment, an application can only act as a client or a server.

The content of the DDE conversation is agreed upon through three identifiers: ① Server Name (Service Name): Each application of the DDE source has a unique server name, usually an executable file without a suffix; ② Topic (Topic): Some data units that are meaningful to the source program, that is, the topic of the conversation. Many applications use the document name as the topic of the DDE conversation; ③ Item (Item): The data actually transferred between two applications in the DDE conversation. Before establishing DDE, the client program must fill in the three identifiers of the service program.

There are three types of DDE links: ① Hot link: The server sends data in the project specifically set for DDE dialogue. When the data changes, the link will act in real time and automatically update the data; ② Cold link: When the data changes, the client must explicitly request an update before the data will be updated; ③ Warm link: When the data changes, the server notifies the client, and the client decides whether to update the data based on its own requirements.

DDE is essentially implemented by sending messages. In VC and CVI, event callback functions can be registered to achieve automatic data exchange, but unfortunately LABVIEW does not provide an event-driven method for DDE. Just like its serial port operation, it is all done through polling (POLLING), so it involves the problem of DDE speed coordination between two processes.

DDE is an important way of inter-process communication in the early days of WINDOWS. It is not used much now, but many applications, such as OFFICE, MATLAB, etc., including various popular configuration software, still provide support for DDE, so it is necessary to understand it.

After 7.1, LABVIEW cannot find the DDE library in the template and needs to be manually added to USER LIB.

C:Program FilesNational InstrumentsLabVIEW 8.5vi.libPlatformdde.llb

is mainly divided into two parts: client and server, client and server VI.


DDE data exchange must first start the server, otherwise the client cannot connect.

The server operation process is:
register server---》register ITEM----》set ITEM value-----》cancel ITEM registration---》cancel server registration




Corresponding client reading data block diagram




Note that the server cycle is updated every 500MS, and the client is updated every 500MS. The server and client are basically synchronized. If the client speed is higher than the server, it will cause the same data on the server to be read multiple times. Similarly, if the server runs fast and the client runs slowly, data will be lost. This is the disadvantage of no event response. It is difficult to ensure the synchronization of sending and receiving. Therefore, the above program is only suitable for situations where data exchange requirements are not high, such as monitoring.

DDE is a hierarchical structure, SERVER--》TOPIC---》ITEM.
A SEVER can include multiple TOPICs (similar to a group), and each TOPIC can include multiple ITEMs (projects). We can register multiple TOPICs and multiple ITEMs in a loop to achieve batch data exchange.




In the above figure, 10 ITEMs are registered for TOPIC1 at the same time through a loop, namely ITEM0----》ITEM9, and data is written to ITEM0--》ITEM9 in the main loop.





Similarly, we can classify the data we have to communicate into multiple SERVERs and multiple TOPICs in detail to exchange a large amount of data.

The advanced synchronization technology of DDE will be further introduced in subsequent articles.