MODBUS UDP exists ?

[arbj 0]

Hi, I have read about MODBUS TCP, but is there a MODBUS UDP ?? The MODBUS site library only mentions MODBUS TCP, but there are implementations in PHP that uses UDP. Please also mention what other fieldbuses use UDP. I am very keen on using UDP in my slave device project to communicate with a PLC. I have been told to implement Industrial Ethernet UDP on my device, is this the same as MODBUS UDP ? I know PROFINET uses TCP but being a closed system I cannot access the specs for the protocol. thanks a

[JRoss 84]

I'm no Ethernet expert, but I believe the main difference between UDP and TCP is that UDP doesn't require an acknowledgement from the receiver and TCP does. When using a fieldbus, you generally want an acknowledgement so that you know your messages are getting through, especially if you're using it for something like I/O, where you have safety considerations.

[arbj 0]

Thanks for the reply. Yes UDP is a connectionless protocol. But how does a PLC know which addresses to access on the slave device. In PROFIBUS a GSD file is used to setup the PLC, this informs the PLC which registers may be used to perform certain functions. I think MODBUS is also similar. thanks a

[JRoss 84]

That depends on the PLC, the fieldbus, and the connection type being used. There are a lot of combinations.

[PdL 71]

Usually a Modbus slave comes with a specific adress mapping, specified in a datasheet or manual. This shows what function codes and adressing is available in the slave device. The master programming is done using this information. Reading or writing a false address will generate an exception error in the slave.

[arbj 0]

thanks PdL It is getting clear for me now. Instead of developing an interface I am considering using a off the shelf gateway like Anybus Communicator for interfacing with various fieldbuses, what setup files do I have to provide for my clients (I am assuming I have to provide the address mapping for my system also ??) to use my system, will anybus communicator have setup files of its own ? I read in the MODBUS and PROFIBUS site that to test for system compliance one needs to be member, is this required even after I buy a gateway device like the Anybus Communicator?? Are there any low cost gateway devices say for PROFIBUS, MODBUS for ControlNet ??? I think Anybus Communicator would be very expensive ?? thanks a