Danfoss VLT HVAC Drive FC 102 Design Manual

Also see for VLT HVAC Drive FC 102: Operating instructions Service guide Programming guide Installation guide Installation guide

Contents

7.6.3 Modbus RTU OverviewRegardless of the type of physical communicationnetworks, the Modbus RTU Overview describes the processa controller uses to request access to another device. Thisprocess includes how the Modbus RTU responds torequests from another device, and how errors are detectedand reported. It also establishes a common format for thelayout and contents of message fields.During communications over a Modbus RTU network, theprotocol determines how each controller:• Learns its device address• Recognizes a message addressed to it• Determines which actions to take• Extracts any data or other information containedin the messageIf a reply is required, the controller constructs the replymessage and sends it.Controllers communicate using a master-slave technique inwhich only one device (the master) can initiatetransactions (called queries). The other devices (slaves)respond by supplying the requested data to the master ortaking the action requested in the query.The master can address individual slaves or initiate abroadcast message to all slaves. Slaves return a message(called a response) to queries that are addressed to themindividually. No responses are returned to broadcastqueries from the master. The Modbus RTU protocolestablishes the format for the master’s query by placinginto it the device (or broadcast) address, a function codedefining the requested action, any data to be sent and anerror-checking field. The slave’s response message is alsoconstructed using Modbus protocol. It contains fieldsconfirming the action taken, any data to be returned andan error-checking field. If an error occurs in receipt of themessage, or if the slave is unable to perform the requestedaction, the slave will construct an error message, and sendit in response, or a timeout occurs.7.6.4 Adjustable Frequency Drive withModbus RTUThe adjustable frequency drive communicates in ModbusRTU format over the built-in RS-485 interface. Modbus RTUprovides access to the control word and bus reference ofthe adjustable frequency drive.The control word allows the Modbus master to controlseveral important functions of the adjustable frequencydrive:• Start• Stop of the adjustable frequency drive in variousways:- Coast stop- Quick stop- DC brake stop- Normal (ramp) stop• Reset after a fault trip• Run at a variety of preset speeds• Run in reverse• Change the active set-up• Control the built-in relayThe 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. This permits arange of control options, including controlling the setpointof the adjustable frequency drive when its internal PIcontroller is used.Installation and Set-up Design Guide190 Danfoss A/S © Rev. 2014-01-13 All rights reserved. MG16C12277