Using Modbus Plus Global Data

Note: Global data should be used in all applications; it is not only for advanced users.

You should first familiarize yourself with the standard method of reading and writing to a slave from a master, as described in Reading and Writing Modbus Plus Registers. Global data greatly speeds up the time required for a master to retrieve data from a slave such as the RMC.

Each node may send up to 32 registers of global data as it passes the token to the next node on the network. All nodes on the network receive this data and store it in an internal database. Therefore, whenever a node wants to know the global data of a particular node, all it has to do is look in its internal database, which gets updated automatically.

The RMC always broadcasts 32 registers of global data. The user can select which data will be included in the 32 registers of global data. For details on assigning the global data registers, see Using the Status Map Editor.

For a list of the default global data register mapping, see Default Status Map Data.

There are two methods of reading Global Data into the Modicon PLC:

Use the MSTR Modicon Ladder Logic Block to explicitly read the global data. Using this method, you are limited to reading the global data into holding registers only.
Use Modicon’s Peer Cop feature. This method works better in most applications, but takes more time to set up. See Using Modicon’s Peer Cop to Read Global Data topic.