Fc Protocol Overview; Network Configuration - Danfoss VLT Series Design Manual

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 field-
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 frequency converter. 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
7
The FC protocol provides access to the Control Word and Bus Reference of the frequency con-
verter.
The Control Word allows the Modbus master to control several important functions of the fre-
quency converter:
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The Bus Reference is commonly used for speed control. It is also possible to access the parame-
ters, read their values, and where possible, write values to them. This permits a range of control
options, including controlling the setpoint of the frequency converter when its internal PID con-
troller is used.

7.3. Network Configuration

7.3.1. VLT HVAC Frequency Converter Set-up
Set the following parameters to enable the FC
protocol for the VLT HVAC.
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Start
Stop of the frequency converter 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
Change of the active set-up
Control of the two relays built into the frequency converter
MG.11.B2.02 - VLT
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VLT HVAC Drive Design Guide