Thread Starter


Hi i just started plc, i want to connect plc with modbus tcp, i write my plc code and it is working, but i have to monitor the values from a pc. Because of this i downloaded some monitoring programs (ModbusPoll, ModSim etc. etc) But it is impossible to connect with plc by modbus through port 502, i just started the project and i am stucked. Can you please help me about the communication.

PS: I am using Gefran PLC R-CPU 300

Thanks for your interest,

Ganesh Okade


The first step would be to check if you are able to establish a socket connection to the PLC on port 502. Its fairly simple to do this using the telnet utility on Windows:

telnet <IP address of PLC> 502

When you run this on command prompt from any PC, if there is no MODBUS server running on the PLC (or if you are not able to reach the PLC from your network) you will see an error message something like this "Could not open connection to the host, on port 502: Connect failed". If this occurs, recheck to see if your network cabling is fine or if there is a firewall on your computer which is blocking calls on port 502.

Hope this helps as a starting point for your diagnosis.

Ganesh Okade
Sunlux Technologies Ltd.