Danfoss VLT Refrigeration Drive FC 103 Design Manual

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Danfoss VLT Refrigeration Drive FC 103 Design Manual Manual pdf 81 page image

3.9.9.4 Network ConfigurationTo enable Modbus RTU on the frequency converter, set thefollowing parameters:Parameter SettingParameter 8-30 Protocol Modbus RTUParameter 8-31 Address 1–247Parameter 8-32 Baud Rate 2400–115200Parameter 8-33 Parity / Stop Bits Even parity, 1 stop bit (default)Table 3.31 Modbus RTU Parameters3.9.10 Modbus RTU Message FramingStructure3.9.10.1 Frequency Converter with ModbusRTUThe controllers are set up to communicate on the Modbusnetwork using RTU mode, with each byte in a messagecontaining 2 4-bit hexadecimal characters. The format foreach byte is shown in Table 3.32.StartbitData byte Stop/parityStopTable 3.32 Format for Each ByteCoding system 8–bit binary, hexadecimal 0–9, A–F.2 hexadecimal characters contained in each8-bit field of the message.Bits per byte 1 start bit.8 data bits, least significant bit sent first;1 bit for even/odd parity; no bit for noparity.1 stop bit if parity is used; 2 bits if noparity.Error check field Cyclic redundancy check (CRC).3.9.10.2 Modbus RTU Message StructureThe transmitting device places a Modbus RTU messageinto a frame with a known beginning and ending point.This allows receiving devices 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 3.33.Start Address Function Data CRCcheckEndT1-T2-T3-T48 bits 8 bits N x 8 bits 16 bits T1-T2-T3-T4Table 3.33 Typical Modbus RTU Message Structure3.9.10.3 Start/Stop FieldMessages start with a silent period of at least 3.5 characterintervals. This is implemented as a multiple of characterintervals at the selected network baud rate (shown as StartT1-T2-T3-T4). The first field to be transmitted is the deviceaddress. Following the last transmitted character, a similarperiod of at least 3.5 character intervals marks the end ofthe message. A new message can begin after this period.The entire message frame must be transmitted as acontinuous stream. If a silent period of more than 1.5character intervals occurs before completion of the frame,the receiving 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.This causes a timeout (no response from the slave), sincethe value in the final CRC field is not valid for thecombined messages.3.9.10.4 Address FieldThe address field of a message frame contains 8 bits. Validslave device addresses are in the range of 0–247 decimal.The individual slave devices are assigned addresses in therange of 1–247. (0 is reserved for broadcast mode, whichall slaves recognise.) A master addresses a slave by placingthe slave address in the address field of the message.When the slave sends its response, it places its ownaddress in this address field to let the master know whichslave is responding.3.9.10.5 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 slave. When amessage is sent from a master to a slave device, thefunction code field tells the slave what action to perform.When the slave responds to the master, it uses thefunction code field to indicate either a normal (error-free)response, or that an error occurred (called an exceptionresponse). For a normal response, the slave simply echoesthe original function code. For an exception response, theslave returns a code that is equivalent to the originalfunction code with its most significant bit set to logic 1. Inaddition, the slave places a unique code into the data fieldof the response message. This tells the master what erroroccurred, or the reason for the exception. Also refer toSystem Integration Design GuideMG16G202 Danfoss A/S © 08/2015 All rights reserved. 793 3