Adjustable Frequency Protocol Overview - Danfoss VLT FC 102 Series Design Manual

Installation and Set-up
8.1.4 EMC Precautions
The following EMC precautions are recommended to
achieve interference-free operation of the RS-485 network.
Observe relevant national and local regulations, for
example regarding protective ground connection. Keep the
RS-485 communication cable away from motor and brake
resistor cables to avoid coupling of high frequency noise
from one cable to another. Normally a distance of 200 mm
(8 inches) is sufficient, but keeping the greatest possible
distance between the cables is recommended, especially
where cables run in parallel over long distances. When
crossing is unavoidable, the RS-485 cable must cross motor
and brake resistor cables at an angle of 90°.
Min. 200 mm
90° crossing
Figure 8.4 Cable Routing
MG11BC22
Design Guide
Brake resistor
Danfoss A/S © Rev. 06/2014 All rights reserved.
8.2 Adjustable Frequency Protocol
Overview
The FC protocol, also referred to as FC bus or Standard
bus, is the Danfoss standard serial communication bus.
It defines an access technique according to the master-
follower principle for communications via a serial bus.
One master and a maximum of 126 followers can be
connected to the bus. The master selects the individual
followers via an address character in the message.
A follower itself can never transmit without first being
requested to do so, and direct message transfer between
the individual followers 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 message 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
8.2.1 FC 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 adjustable frequency
drive:
•
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
•
Change of the active set-up
•
Control of the two relays built into the adjustable
frequency drive
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