Danfoss VLT Series Design Manual page 138

7. RS-485 Installation and Set-up
7.2. FC Protocol Overview
The FC protocol, also referred to as FC bus or Standard bus, is the Danfoss Drives standard serial
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communication bus. 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 the telegram. 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 adjustable frequency
drive. The FC protocol supports different telegram formats; a short 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. VLT HVAC with Modbus RTU
The FC protocol provides access to the control word and bus reference of the adjustable frequency
drive.
The control word allows the modbus master to control several important functions of the adjust-
able frequency drive:
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Start
Stop of the adjustable frequency drive in various ways:
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.
MG.11.B4.22 - VLT ® is a registered Danfoss trademark.
VLT ® HVAC Drive Design Guide