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This mode is especially useful when both a Modbus Master and the module’s processor logic need to be able to read and write values to the same internal database addresses. The following diagram shows the data flow for a slave port with pass-through enabled: Step Description 1 Same as normal mode. 2 Same as normal mode. 3 a. In Pass-Through mode, if the Slave driver receives a read request, it looks for the data in module’s internal database, just as it would in Normal mode. b. The data needed to respond to the read command is retrieved directly from the internal database and returned to the Slave driver so it can build a response message. c. In Pass-Through mode, if the Slave Driver receives a write request, it does not send the data directly to the module’s internal database. It puts the data to be written into a special Input Image with a special Block ID code to identify it as a Pass-Through Write Block and substitutes this special block in place of the next regular Read Data Block. The special block is processed by the ladder logic and the data to be written is placed into the WriteData controller tag array at an address that corresponds to the Modbus Address received in the write command. d. During normal backplane communications, the data from the WriteData array, including the data updated by the Pass-Through Write Block, is sent to the module’s internal database. This gives the ladder logic the opportunity to also change the values stored in these addresses, if need be, before they are written to the database.
Note: The ReadData array is not used in Pass-Through mode. 4 Same as normal mode. 5 Same as normal mode. MVI69E-MBS ♦ CompactLogix™ Platform MVI69E-MBS Backplane Data Exchange Modbus Serial Enhanced Communication Module User Manual ProSoft Technology, Inc. Page 74 of 159 5.4.2 Master Mode In Master mode, the MVI69E-MBS module issues read or write commands to slave devices on the Modbus network. These commands are user-configured in PCB; refer to Modbus Port x Commands (page 44). This list is transferred to the module when the module receives its configuration from the processor. The commands can also be issued directly from the CompactLogix or MicroLogix 1500- LRP processor (Special Command Blocks). Command status is returned to the processor for each individual command in the command list. The location of this command status list in the module’s internal database is user-defined. The following flow chart and associated table describe the flow of command data into and out of the module.