issue of Modbus data reading

I am trying to connect and read data from an industrial machine through matlab script, I was able to connect with the machine but I am unable to read proper data tags from the plc server.

code snippet -

ipAddress = ''; % you can enter the ip adress accordingly

tcpPort = 502;

addresses = [ 8, 12];

modbusConnection = modbus('tcpip', ipAddress, tcpPort);

%connect(modbusConnection);

while true

for i = 1:length(addresses)

address = addresses(i);

data = read(modbusConnection, 'holdingregs', address, 1);

fprintf('%s: %f\n', tags(address), data);

end

pause(0.5);

end

Answer 1

[Hassaan]

1. Addressing: Ensure that the addresses you're trying to read (addresses = [8, 12];) are correct and correspond to the data you expect to receive. In some systems, addressing might start at 0, while in others, it might start at 1. This offset can lead to reading the wrong registers.
2. Data Type and Length: Verify the data type and length of the registers you're reading. If the data type or length doesn't match what's actually stored at the address, it can result in incorrect values.
3. PLC Configuration: Ensure that the PLC is configured to allow external reading of these registers. Some PLCs require specific configuration to expose certain data points.
4. Error Handling: Implement error handling in your script to catch any read errors. This can provide more information if the read operation is failing.
5. Tag Mapping: Ensure that tags is a properly defined array or map that corresponds to the addresses. If tags(address) is not defined or doesn't correspond to the correct tag, this could be an issue.
6. Timing and Polling: The polling rate (indicated by pause(0.5)) should be appropriate for your PLC's capabilities and the network latency.

ipAddress = '';  % Enter the IP address
tcpPort = 502; 
addresses = [8, 12];
tags = {'Tag1', 'Tag2'};  % Replace with your actual tag names

modbusConnection = modbus('tcpip', ipAddress, tcpPort);

try
    while true
        for i = 1:length(addresses)
            address = addresses(i);
            try
                data = read(modbusConnection, 'holdingregs', address, 1);
                fprintf('%s: %f\n', tags{i}, data);
            catch e
                fprintf('Error reading address %d: %s\n', address, e.message);
            end
        end
        pause(0.5);
    end
catch e
    fprintf('Error: %s\n', e.message);
end

• tags is an array of strings representing the tag names for each address.
• Error handling is added to catch and display errors during the read operation.

Make sure to replace tags with the actual tag names corresponding to your addresses and ensure that the PLC's IP address is correctly entered. Also, confirm the addressing, data types, and PLC configuration as mentioned earlier. If the issue persists, consult the documentation for your PLC and the Modbus library you're using, as there might be device-specific considerations to account for.

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