Danfoss VLT AutomationDrive FC 300 Design Manual

Also see for VLT AutomationDrive FC 300: Instruction manual Operating instructions Design guide Service manual Operating instructions

Contents

Danfoss VLT AutomationDrive FC 300 Design Manual Manual pdf 250 page image

10.11 Modbus RTU Message FramingStructure10.11.1 Frequency Converter with ModbusRTUThe controllers are set up to communicate on the Modbusnetwork using RTU (Remote Terminal Unit) mode, witheach byte in a message containing 2 4-bit hexadecimalcharacters. The format for each byte is shown inTable 10.10.StartbitData byte Stop/ParityStopTable 10.9 Example FormatCoding System 8-bit binary, hexadecimal 0–9, A-F. twohexadecimal characters contained in each 8-bit field of the messageBits Per Byte 1 start bit8 data bits, least significant bit sent first1 bit for even/odd parity; no bit for noparity1 stop bit if parity is used; 2 bits if no parityError Check Field Cyclical Redundancy Check (CRC)Table 10.10 Bit Detail10.11.2 Modbus RTU Message StructureThe transmitting device places a Modbus RTU messageinto a frame with a known beginning and ending point.Receiving devices are able to begin at the start of themessage, read the address portion, determine whichdevice is addressed (or all devices, if the message isbroadcast), and to recognise when the message iscompleted. Partial messages are detected and errors set asa result. Characters for transmission must be inhexadecimal 00 to FF format in each field. The frequencyconverter continuously monitors the network bus, alsoduring ‘silent’ intervals. When the first field (the addressfield) is received, each frequency converter or devicedecodes it to determine which device is being addressed.Modbus RTU messages addressed to zero are broadcastmessages. No response is permitted for broadcastmessages. A typical message frame is shown in Table 10.12.Start Address Function Data CRCcheckEndT1-T2-T3-T48 bits 8 bits N x 8bits16 bits T1-T2-T3-T4Table 10.11 Typical Modbus RTU Message Structure10.11.3 Start/Stop FieldMessages start with a silent period of at least 3.5 characterintervals, implemented as a multiple of character intervalsat the selected network baud rate (shown as Start T1-T2-T3-T4). The first transmitted field is the device address.Following the last transmitted character, a similar period ofat least 3.5 character intervals marks the end of themessage. A new message can begin after this period. Theentire message frame must be transmitted as a continuousstream. If a silent period of more than 1.5 characterintervals occurs before completion of the frame, thereceiving device flushes the incomplete message andassumes that the next byte is the address field of a newmessage. Similarly, if a new message begins before 3.5character intervals after a previous message, the receivingdevice considers it a continuation of the previous message,causing a time-out (no response from the follower), sincethe value in the final CRC field is not valid for thecombined messages.10.11.4 Address FieldThe address field of a message frame contains 8 bits. Validfollower device addresses are in the range of 0–247decimal. The individual follower devices are assignedaddresses in the range of 1–247. (0 is reserved forbroadcast mode, which all slaves recognise.) A masteraddresses a follower by placing the follower address in theaddress field of the message. When the follower sends itsresponse, it places its own address in this address field tolet the master know which follower is responding.10.11.5 Function FieldThe function field of a message frame contains 8 bits. Validcodes are in the range of 1-FF. Function fields are used tosend messages between master and follower. When amessage is sent from a master to a follower device, thefunction code field tells the follower what action toperform. When the follower responds to the master, it usesthe function code field to indicate either a normal (error-free) response, or that an error has occurred (called anexception response). For a normal response, the followersimply echoes the original function code. For an exceptionresponse, the follower returns a code that is equivalent tothe original function code with its most significant bit setto logic 1. In addition, the follower places a unique codeinto the data field of the response message. This code tellsthe master what error occurred, or the reason for theexception. See 10.11.10 Function Codes Supported byModbus RTU.RS-485 Installation and Set... VLT® AutomationDrive FC 300 Design Guide 90-1200 kW248 MG34S202 - Rev. 2013-08-191010