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DSAO120A 3BSE018293R1 CPU control card

The MVI94-MCM communication module’s primary services are data concentration and communication gateway. The Modbus master port polls Modbus slave devices based on user defined commands and places the data in the virtual Modbus database. The Flex I/O bus interfaces with this database to a Flex processor. The user is responsible for defining the structure and content of the virtual Modbus database. In order to interface the virtual Modbus database with Modbus slave devices, you must construct a command list. The commands in the list specify the Modbus slave device to be utilized, the function to be performed (read or write), the data area in the device to interface with and the position in the virtual Modbus database to be associated with the device data. Up to 100 commands can be entered for this purpose. The list is processed from top (command #0) to bottom. A poll interval parameter is associated with each command to specify a minimum delay time in seconds between the issuance of a command. If the user specifies a value of 10 for the parameter, the command will be executed no more frequently than every 10 seconds.

Modbus Protocol Specification 

The following pages give additional reference information regarding the Modbus protocol commands supported by the MVI94-MCM. Additionally, a user specified time delay could be inserted between the issuance of each command. This is useful for slow responding slave devices. Write commands have a special feature, as they can be set to execute only if the data in the write command changes. If the data in the command has not changed since the command was last issued, the command will not be executed. If the data in the command has changed since the command was last issued, the command will be executed. Use of this feature can lighten the load on the Modbus network. In order to implement this feature; set the enable code for the command to a value of 2.Each command list record has the same general format. The first part of the record contains the information relating to the communication module and the second part contains information required to interface to the MODBUS slave device.

Read Coil Status (Function Code 01) 

Query This function allows the user to obtain the ON/OFF status of logic coils used to control discrete outputs from the addressed Slave only. Broadcast mode is not supported with this function code. In addition to the Slave address and function fields, the message requires that the information field contain the initial coil address to be read (Starting Address) and the number of locations that will be interrogated to obtain status data. The addressing allows up to 2000 coils to be obtained at each request; however, the specific Slave device may have restrictions that lower the maximum quantity. The coils are numbered from zero; (coil number 1 = zero, coil number 2 = one, coil number 3 = two, and so on). The following table is a sample read output status request to read coils 0020 to 0056 from Slave device number 11.