7.1.4 EMC Precautions
The following EMC precautions are recommended in order to achieve interference-free operation of the RS-485 network.
Relevant national and local regulations, for example regarding protective ground connection, must be observed. The RS-485 communication cable must
be kept 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 generally 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 degrees.
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 individual slaves are selected by the master 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 adjustable frequency protocol supports different
message 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 message
format is used for texts.
7 RS-485 Installation and Set-up
VLT AQUA Low Harmonic Drive Instruction
Manual
7-4
MG.20.T1.22 - VLT
®
is a registered Danfoss trademark
7
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