Cannot read modbus Micromotion - MCM


Thread Starter


I have problem about reading modbus from Micromotion Transmitter 1700 AN to MVI46-MCM prosoft (with SLC500). I have configured the MCM Port 2 to be used for reading port via RS485, but in the N31:0 MCM register is showing all zero or nothing data transfered. does any give me solving for this problem?



Carl Burgess

The MVI-46 Modules need quite a bit of code to get data from the internal database to the PLC data files. Stuff isn't just going to appear in the N31 file without some ladder. The examples that come with the module are only a starting point. (I note N31 is the file used in the supplied examples.)

Typically you'll need to work out what Modbus polls are required to your slave device, and define these in a data file. Then you'll need some code to transfer this poll information to the MVI module. There will also be some ladder code to transfer data from the MVI internal database to another data file using COP command and the M1 /M0 address space. Then there's a bit of logic to handle warm / cold boot.

We've used these units on several projects. It took a good day or so to get to grips with the documentation, but once you can get your head around it, they just work...

The big plus for these units is the ability to plug a terminal (e.g. hyperterminal) into the diagnostic port and see what is going on. You can view the data going across the modbus link, view the internal database, and see what polls are being executed. If you think there's nothing wrong with your code, then this is the place to start.


ProSoft Technology Technical Services


The first thing I would do is verify whether you're reading all zeros, reading data but mapping to the wrong location, or reading nothing at all due to an error.

While online, look at the N30:16 register. This register will show the number of command requests for port 2, and N30:17 will show the number of valid responses. If these numbers increment together, then your commands are getting a valid response.

If you should have further questions, please don't hesitate to contact us using the link below.

ProSoft Technology - Technical Services Team