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Capstone Turbine Corporation • 21211 Nordhoff Street • Chatsworth • CA 91311 • USA Technical Reference: ModBus Translator Module ModBus Protocol Introduction The ModBus protocol is an open standard that defines a command-response method of communicating digital information between a master and slave device. The electrical connection between devices is known as a bus. In ModBus, two types of devices attach to the bus, master and slave devices. A master device issues commands to slaves. A slave device, such as the ModBus Translator device, issues responses to master commands that are addressed to them. Each bus must contain exactly one master and may contain as many slaves as the electrical standards permit. When RS-485 ModBus queries are sent to the translator board, the board responds with data it has buffered from the MicroTurbine system. ModBus commands to write registers in the translator board are immediately queued up and transmitted to the MicroTurbine system over the RS-232 network. Both origination and user responses are buffered and can be read back. The ModBus protocol specifications define two types of transmission modes: ASCII and RTU. This manual describes only the more common RTU mode. 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. 410040-001 Rev A (February 2003) Page 7 of 35 This information is proprietary to Capstone Turbine Corporation. Neither this document nor the information contained herein shall be copied, disclosed to others, or used for any purposes other than the specific purpose for which this document was delivered. Capstone reserves the right to change or modify without notice, the design, the product specifications, and/or the 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
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