Hello Siby;
Please take a few minutes to explain your problem a little further:
-You have for the moment 3 Siemens CPUs, connected over Profibus-DP, in a working configuration;
- For some reason, you wish to change the connection from Profibus-DP to Modbus RTU;
-On the master PLC (Modbus master) you have setup a CP341 with a X27 (RS422/RS485) interface module (since you are connecting to more than one slave) and the dongle that comes with the Modbus Master loadable driver software package. You have used the parametrizing software to setup the Modbus driver parameters of the CP341 (Modbus address, data speed, duplex/half-duplex connection, etc...), and dowloaded it to the CPU. You have then programmed the Modbus Master function blocks included in the Modbus Master driver to establish links and Modbus functions with the slaves.
-On each Modbus slave PLC you have similarly setup a CP341 with its X27 interface module, installed the dongle that comes with the Modbus slave driver package, parametrized the CP341 as a Modbus interface similar to the master (and downloaded to its CPU)and inserted the Modbus slave function blocks from the driver package to respond with appropriate data to the Modbus master's requests;
-Finally you have installed appropriate wiring between master and slaves, as per the instruction manuals of the drivers (note: this can be tricky, especially if you have other types of slaves on the Modbus network);
Is this correct?
If you have bought and installed all packages as per the instruction manuals and programming examples from Siemens, you should have at least functionning LEDs on the master and slave modules, or diagnostic LEDs available for troubleshooting. Sometimes there can be NDR (New Data recieved) signal synchronization to be verified, or Modbus function triggering to be checked, but at least you should have some kind of validation of the network.
Please make sure you have in your possession all required elements for the setup of the network: CP3431 with X27 submodules; Modbus master and Modbus slave loadable drivers, with appropriate dongle; parammetrizing software for the Modbus interfaces; and correct cabling between the Modbus partners. Then tell us what diagnostics you find on the CPUs and CPs (LEDs, diagnostic buffer, Hardware diagnostics).
Hope this helps,
Daniel Chartier
