Danfoss VLT Series Design Manual

Also see for VLT series: Service manual Installation guide Operating instructions Operating instructions Service manual

Contents

Danfoss VLT Series Design Manual Manual pdf 135 page image

7.7. Network Configuration7.7.1. VLT HVAC with Modbus RTUTo enable Modbus RTU on the VLT HVAC, set the following parameters:Parameter Num-berParameter name 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 Framing Structure7.8.1. VLT HVAC with Modbus RTUThe controllers are set up to communicate on the Modbus network using RTU (Remote TerminalUnit) mode, with each 8-bit byte in a message containing two 4-bit hexadecimal characters. Theformat for each byte is shown below.Start bit Data bit Stop/parityStopCoding System 8-bit binary, hexadecimal 0-9, A-F. Two hexadecimal characterscontained 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 no parity1 stop bit if parity is used; 2 bits if no parityError Check Field Cyclical Redundancy Check (CRC)7.8.2. Modbus RTU Message StructureThe transmitting device places a Modbus RTU message into a frame with a known beginning andending point. This allows receiving devices to begin at the start of the message, read the addressportion, determine which device is addressed (or all devices, if the message is broadcast), and torecognise when the message is completed. Partial messages are detected and errors set as aresult. Characters for transmission must be in hexadecimal 00 to FF format in each field. Thefrequency converter continuously monitors the network bus, also during ‘silent’ intervals. Whenthe first field (the address field) is received, each frequency converter or device decodes it todetermine which device is being addressed. Modbus RTU messages addressed to zero are broad-cast messages. No response is permitted for broadcast messages. A typical message frame isshown below.Start Address Function Data CRC check EndT1-T2-T3-T4 8 bits 8 bits N x 8 bits 16 bits T1-T2-T3-T4Typical Modbus RTU Message StructureVLT® HVAC Drive Design Guide 7. RS-485 Installation and Set-upMG.11.B2.02 - VLT® is a registered Danfoss trademark 1357