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MODBUS Protocol is a protocol used for PROFIBUS, it identifies that a device controller can recognize and use information frame, and it describes the process that how main station controller visits slave devices, also, it stipulates how slave devices makes response, inspects and reports transmission errors.
�MODBUS encoder, using bundled software, can conduct auto-detect and set of Baud rate, allocate Site URL, select the accounting direction of encoder. �MODBUS protocol Adopting MODBUS RTU protocol. 8BIT data bits, no odd-even check, 1BIT stop bit.


Information Frame:





Address Code


Function Code


Data Area


Error-Checking Code




8BITS


8BITS


N*8BITS


16BITS





Address Code: Primary device puts the device's address which is to contact with into the information frame's address field to select slave device. When slave device sends a reply information, it puts its address into the reply information frame's address in order to let primary device know which slave device made that response. The range of address code is from 1 to 254.Function Code: When information is delivered from primary device to slave device, the function code would inform slave code what action should be carried out. When slave device replies, it uses function code to judge whether it's a normal reply or an objectionable reply. (Function code's top position of objectionable reply is 1). ADK series encoder only support function code03 till now.





Code


Implication


Operation




03


reading data


return to encoder's current value





Data Area: The information that primary devices send to slave devices includes some additional information that needs to be carried out by encoder. Without error, the data area sent back from slave devices include the data required by primary devices. If there's an error, objectionable code is included in this area.
�Error-Checking Code: Error-Checking adopts CRC method, mainframe and encoder can use checking code to judge whether there's any wrong when accept information. When the data is transmitted, low-order of CRC value ahead, and high-order behind. The generator polynomial that CRC adopts isg?x?=X16 X12 X 5 1?