Danfoss VLT HVAC Drive FC 100 Series Design Manual

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Danfoss VLT HVAC Drive FC 100 Series Design Manual Manual pdf 129 page image

7.2. FC Protocol OverviewThe FC protocol, also referred to as FC bus or Standard bus, is the Danfoss standard fieldbus. It defines an access technique according to the master-slave principle for communications via a serial bus.One master and a maximum of 126 slaves can be connected to the bus. The individual slaves are selected by the master via an address character in thetelegram. A slave itself can never transmit without first being requested to do so, and direct message transfer between the individual slaves is not possible.Communications occur in the half-duplex mode.The master function cannot be transferred to another node (single-master system).The physical layer is RS-485, thus utilizing the RS-485 port built into the frequency converter. The FC protocol supports different telegram formats; ashort format of 8 bytes for process data, and a long format of 16 bytes that also includes a parameter channel. A third telegram format is used for texts.7.2.1. FC with Modbus RTUThe FC protocol provides access to the Control Word and Bus Reference of the frequency converter.The Control Word allows the Modbus master to control several 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 a variety of preset speeds• Run in reverse• Change of the active set-up• Control of the two relays built into the frequency converterThe Bus Reference is commonly used for speed control. It is also possible to access the parameters, read their values, and where possible, write valuesto them. This permits a range of control options, including controlling the setpoint of the frequency converter when its internal PID controller is used.VLT® HVAC Drive Design Guide 7. RS-485 Installation and Set-upMG.11.B7.02 - VLT® is a registered Danfoss trademark 1297