For example, when the Modbus command 3 (3 hex) is sent to a standard Modbus device, it reads and returns the value of one or more registers. Once a Modbus TCP Slave Device has been configured, Modbus commands sent to its Unit ID (Modbus address) are handled differently than the same command would be when addressed to any other Modbus device on the network. O %QWs are mapped from register n to n+m -1 and are read only (m = Input registers quantity, each %QW register is 2 bytes). O %IWs are mapped from register 0 to n-1 and are R/W (n = Holding register quantity, each %IW register is 2 bytes). The I/Os are mapped to Modbus registers from the master perspective as follows: Number of %QW registers to be used in the exchange (2.40) (each register is 2 bytes) Number of %IW registers to be used in the exchange (2.40) (each register is 2 bytes) Sends the requests to the Modbus TCP slave device (1.247), instead of the embedded Modbus server (255). NOTE: The timeout applies to the IP Master Address unless the address is 0.0.0.0. The connections are not closed on this address.
For more information, refer to the Ethernet Port Read-Only System Variables.įor further information about Modbus TCP, refer to the website. If no Modbus request is received within the timeout duration, the diagnostic information i_byMasterIpLost is set to 1 (TRUE). The timeout duration associated to the privileged connection allows you to verify whether the controller is being polled by the privileged master. The Modbus TCP Slave Device can define a privileged Modbus client application, whose connection is not forcefully closed (embedded Modbus connections may be closed when more than 8 connections are needed). Inputs/outputs are seen from the slave controller: inputs are written by the master, and outputs are read by the master. The embedded Modbus server of the slave controller needs no configuration, and is addressed by specifying a Unit ID equal to 255. This server is addressed by the Modbus client application by specifying a configured Unit ID (Modbus address) in the range 1.247. The Modbus TCP Slave Device adds another Modbus server function to the controller. This Modbus TCP Slave Device functionality allows you to furnish to this area the controller I/O objects which can then be accessed with a single Modbus read/write registers request. This I/O area is used whenever an external master needs to access the %IW and %QW objects of the controller. This functionality creates a specific I/O area in the controller that is accessible with the Modbus TCP protocol. To configure your M241 Logic Controller as a Modbus TCP Slave Device, you must add Modbus TCP Slave Device functionality to your controller (see Adding a Modbus TCP Slave Device ). This section describes the configuration of the M241 Logic Controller as a Modbus TCP Slave Device. M241 Logic Controller as a Slave Device on Modbus TCP
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