7.2 Adjustable Frequency Protocol
Overview
The adjustable frequency drive protocol, also referred to as
adjustable frequency drive bus or standard bus, is the
Danfoss standard serial 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 master selects the individual
slaves via an address character in the message. 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 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.
7.2.1 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
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. This permits a
range of control options, including controlling the setpoint
of the Adjustable frequency drive when its internal PID
controller is used.
RS-485 Installation and Set...
VLT
®
HVAC Drive Design Guide
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