How can I send modbus message to the PLC

I am trying to send and receive modbus messages between Wago PLC and MATLAB. Wago PLC is the master and matlab will be the slave.

(I checked my PLC programm with a modbus slave simulator and it works)

I can read the modbus message in matlab but I can not send a modbus message to the PLC.This is what I write in Matlab.

obj=udp('192.168.1.2',502); % Ip address and port number of my plc and i will use UDP connection

set(obj, 'LocalPort', 502); % local port is 502

fopen(obj);

Message_Taken=fread(obj);% i read the modbus message

%The message from the PLC is like this and this data is in double format in matlab;

84 %Transaction ID

63 %Transaction ID

0 %Protocol ID

0 %Protocol ID

0 %Length

13 %Length

0 %Unit Identifier

23 %Function Code

0 %Reference number for Read

0 %Reference number for Read

0 %number of words for read

2 %number of words for read

0 %reference number for write

1 %reference number for write

0 %word count for write

1 %word count for write

2 %byte count (2X word count for write)

0 %register value1

254 %register value1

Transaction_Id_1=uint16(Message_Taken(1,1));

Transaction_Id_2=uint16(Message_Taken(2,1));

Transaction_Id=uint16(Transaction_Id_1+Transaction_Id_2);%16b Transaction Identifier

Protocol_Id=uint16(0);% 0 for modbus

Lenghf =uint16(6);% number of following bytes

UnitID=uint16(0);

Function_Code = uint16(3);% I use Function code 3 in slave

UnitIDFunCod = uint16(UnitID + Function_Code); % Concatenation of UnitID & FunctionCode in one int16 word

ByteCount =uint16(2);% number of data bytes

DATA=int16(29); % data that needs to be sent

Message_Sent= [Transaction_Id;Protocol_Id;Lenghf;UnitIDFunCod;ByteCount;DATA] fwrite(obj,Message_Sent,'uint16'); fclose(obj); delete(obj);

It gives no errors but I dont see the sent data in my PLC. I am quite new to modbus and if you figure out my mistake please contact me [e-mail]

[Raimundas Steponavicius]

Hi Ferit,

Did you figure it out ? I am looking for the answers to the same question, but unsuccessfully so far.