Server / Slave OperationΒΆ
A typical application for the operation of a controller as (Open)Modbus on TCP/IP server is the linking of an operating terminal via Ethernet. With this application, the operating terminal operates as Modbus client and sends telegrams for reading or writing variables to the controller which executes them accordingly. Since (Open)Modbus on TCP/IP is a standardized protocol, every device supporting this protocol can be connected this way independent of the device type and manufacturer.
For our design example a closed installation-internal network is used. The network consists of four subscribers, two operating terminals and two controllers. The devices are connected to each other via a switch. The operating terminals shall have the IP addresses 10.49.91.251 and 10.49.91.252. The controllers shall have the IP addresses 10.49.91.253 and 10.49.91.254. Both operating terminals shall be able to access both controllers. Since the terminals are not time-synchronized, simultaneous access of both terminals to the same controller must be possible.
Example for OpenModbus on TCP/IP - controller operated as server
Designing the operating terminals as Modbus clients is not subject of this description. For information about this, please contact the corresponding manufacturer. Please refer to the description of the ETH_MOD_MAST: ETH_MOD_MAST ETH_MOD_MAST function block contained in the AC500 documentation for the cross-reference list for client access to the operands of the controller.
Caution
The specified number of parallel server connections results in a permanent reservation of a corresponding number of sockets for these connections. These sockets cannot be used by other protocols. If many clients are used that shall be able to access the controller simultaneously, this can result in a lack of sockets for other protocols. In this case the maximum possible number of simultaneous server connections has to be reduced.
Now, repeat the configuration for the second controller using a different IP address of e.g. 10.49.91.254. All other parameter settings are identical to the settings for the first controller.