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Capstone Turbine Corporation • 21211 Nordhoff Street • Chatsworth • CA 91311 • USA Technical Reference: Modbus Translator Module Modbus RTU Message Framing Each message from either a master or slave consists of a continuous stream of characters. A silent interval of 3.5 characters times (3.5 * 11 bits / 9600 bps = 3.5ms) or more separates these communication streams. A typical response from this device would be: • Enable the output interface drivers (RS-485) • Wait 5 character times • Send the response as a continuous stream • Wait 3.5 character times • Disable the output interface drivers (RS-485) Modbus RTU Message Content The Modbus RTU message stream consists of an address byte, a function code byte, a number of message bytes, and two check bytes. The address byte, which is in the range 1 to 247, specifies the identity of the slave device. The function code byte in a master command indicates the operation that the slave is to perform. The function code byte in a slave response is the same value as the master command function code if no error occurs, otherwise it has 128 added to it. The message bytes in a command contain additional information needed to perform the command. Message bytes in a response contain the data requested if no error has occurred or a one-byte exception code upon errors. The check bytes are generated using the CRC-16 polynomial generator sequence (x16 + x15 + x2 + 1) with the remainder pre-initialized to all 1’s. The least significant byte of the CRC is transmitted first. Modbus Function and Exception Codes The Modbus only supports the function codes shown below. Note that the values are shown in hexadecimal (base 16). The table below shows the value returned when an error occurs. Table 1 – Error Response Function Codes Master Function Code Slave Error Code Name Meaning 0316 8316 0616 8616 1016 9016 Read Holding Registers Preset Holding Registers PresetMultipleRegisters Read values from system run-time data Set control values to command system Set control values of multiple register values The Modbus will return exception codes back to the master upon certain conditions. All function codes greater than 127 decimal (7F16) indicate a slave error response. The message byte indicates the exception code. The table below shows error code value definitions. 410078 Rev A (July 2009) Page 18 of 53 Capstone reserves the right to change or modify, without notice, the design, specifications, and/or contents of this document without incurring any obligation either with respect to equipment previously sold or in the process of construction.PDF Image | Technical Reference Modbus Translator Module (MOXA®)
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