7.1.5 EMC Precautions
To achieve interference-free operation of the RS485
network, Danfoss recommends the following EMC
precautions.
NOTICE
Observe relevant national and local regulations, for
example regarding protective earth connection. To avoid
coupling of high-frequency noise between the cables,
the RS485 communication cable must be kept away from
motor and brake resistor cables. Normally, a distance of
200 mm (8 inches) is sufficient. Maintain the greatest
possible distance between the cables, especially where
cables run in parallel over long distances. When crossing
is unavoidable, the RS485 cable must cross motor and
brake resistor cables at an angle of 90
°
.
1
2
Minimum
200 mm (8 in)
90°
130BD999.11
1
Fieldbus cable
2
Minimum 200 mm (8 in) distance
Illustration 7.3 Minimum Distance between Communication
and Power Cables
7.2 FC Protocol Overview
7.2.1 Overview
The FC protocol, also referred to as FC fieldbus, is the
Danfoss standard fieldbus. It defines an access technique
according to the master/slave principle for communications
via a fieldbus.
1 master and a maximum of 126 slaves can be connected
to the bus. The master selects the individual slaves 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 RS485, thus utilizing the RS485 port
built into the frequency converter. The FC protocol
supports different telegram formats:
•
A short format of 8 bytes for process data.
•
A long format of 16 bytes that also includes a
parameter channel.
•
A format used for texts.
7.2.2 FC with Modbus RTU
The 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 stop.
-
Quick stop.
-
DC Brake stop.
-
Normal (ramp) stop.
•
Reset after a fault trip.
•
Run at various preset speeds.
•
Run in reverse.
•
Change of the active set-up.
•
Control of the 2 relays built into the frequency
converter.
The bus reference is commonly used for speed control. It is
also possible to access the parameters, read their values,
and where possible, write values to them. Accessing the
parameters offers a range of control options, including
controlling the setpoint of the frequency converter when
its internal PI controller is used.
RS485 Installation and Set-...
Design Guide
MG18N202
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