Danfoss VLT AQUA Drive FC 202 Design Manual

Also see for VLT AQUA Drive FC 202: Service guide Installation guide Installation guide Operating guide Installation guide

Contents

7.6.4 Frequency Converter with ModbusRTUThe frequency converter communicates in Modbus RTUformat over the built-in RS-485 interface. Modbus RTUprovides access to the control word and bus reference ofthe frequency converter.The control word allows the Modbus master to controlseveral important functions of the frequency converter:• Start• Stop of the frequency converter in various ways:Coast stopQuick stopDC Brake stopNormal (ramp) stop• Reset after a fault trip• Run at various preset speeds• Run in reverse• Change the active set-up• Control the built-in relay of the frequencyconverterThe bus reference is commonly used for speed control. It isalso possible to access the parameters, read their values,and, where possible, write values to them, permitting arange of control options, including controlling the setpointof the frequency converter when its internal PI controller isused.7.7 Network Configuration7.7.1 Frequency Converter with ModbusRTUTo enable Modbus RTU on the frequency converter, set thefollowing parameters:Parameter Setting8-30 Protocol Modbus RTU8-31 Address 1–2478-32 Baud Rate 2400–1152008-33 Parity / Stop Bits Even parity, 1 stop bit (default)7.8 Modbus RTU Message FramingStructure7.8.1 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 two 4 bit hexadecimalcharacters. The format for each byte is shown in Table 7.10.StartbitData byte Stop/parityStopTable 7.9 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 7.10 Bit Detail7.8.2 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 7.12.Start Address Function Data CRCcheckEndT1-T2-T3-T48 bits 8 bits N x 8bits16 bits T1-T2-T3-T4Table 7.11 Typical Modbus RTU Message StructureRS-485 Installation and Set... VLT® AQUA Drive FC 202 Design Guide172 MG20Z102 - VLT® is a registered Danfoss trademark77