unavoidable, the RS-485 cable must cross motor and brakeresistor cables at an angle of 90°.Fieldbus cableMin.200mm90° crossing130BA080.11Illustration 7.2 EMC Precautions7.2 FC Protocol OverviewThe FC protocol, also referred to as FC bus or Standardbus, is the Danfoss standard fieldbus. It defines an accesstechnique according to the master-slave principle forcommunications via a serial bus.One master and a maximum of 126 slaves can beconnected to the bus. The master selects the individualslaves via an address character in the telegram. A slaveitself can never transmit without first being requested todo so, and direct message transfer between the individualslaves 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 utilising the RS-485 portbuilt into the frequency converter. The FC protocolsupports different telegram formats:• A short format of 8 bytes for process data.• A long format of 16 bytes that also includes aparameter channel.• A format used for texts.7.2.1 Modbus RTUThe FC protocol provides access to the control word andbus reference of the 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 of the active set-up• Control of the two relays built into 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 PID controlleris used.RS-485 Installation and Set... Design GuideMG20Z202 Danfoss A/S © Rev. 06/2014 All rights reserved. 1497 7
