Modbus TCP master send messages

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Recently used C++ to write a modbus tcp master application.

but there is a problem to ask: If I have a layer of application A, B, C, D four (or more) used to request modbus tcp slaver data.
when I call send() , How to control the sending interval of the message, I now use the sleep(), but the feeling is stupid! If I set sleep (100), then I received the D message request data will be 300MS, if the message more, this time will be longer, simply can not hide the real-time data read.

That in this question and answer tcp communication, how to control the message to send continuously?

What I have tried:

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Posted 26-Jun-17 1:47am

robin333

Updated 27-Jun-17 7:32am

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KarstenK · Answer 1 · 2017-06-26T01:53:00

Solution 1

Dont use sleep. Send the data in a function and set up a timer callback function, which checks the result.

Posted 26-Jun-17 1:53am

KarstenK

Rick York · Answer 2 · 2017-06-27T07:32:00

There are lots of ways to handle communications with PLCs and other intelligent devices. When ever possible, I like to set them up in continuous poll/response loops. The exact mechanism will depend on whether you need synchronous communication or you can get away with it being asynchronous. In my experience with Modbus TCP/IP, it is a very fast protocol so I could have a poll/response period of about 20mS and this could be throttled with a short sleep. I always made that an adjustable parameter and it was usually set to 10mS. The way I implemented this is I made a separate thread that would only read and write data blocks. The data blocks were data structures that contained a buffer, a starting address, and a size. The comm thread would write its send block, sleep a few milliseconds, read its receive block, sleep a little more, and then repeat this. Sometimes, the communications was done by a separate process that had its data blocks in shared memory so multiple client processes could access the data. Either way, it's fairly simple matter to implement this.