Fuji Electric FRENIC-VG Series, User Manual

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5.1 Standard RS-485 Communications Ports
5-15
Chap. 5
USING ST
ANDARD RS-
485
5.1.9 RAS
5.1.9.1 Communication errors
Depending on the usage environment, noise generated by the inverter may prevent normal
communications or cause equipment such as instrumentation on the master and converters to
malfunction. The following measures may be effective in such cases. Please also refer to the
appendix on electrical noise.
(1) Measures on side receiving noise
Isolated
converter: Deletes common mode noise exceeding the specified operating voltage range of
the receiver, such as with long wiring layouts. However, the isolated converter
itself may malfunction as a result of noise so it is necessary to use a converter with
strong noise immunity.
Twisted pair
shield wire: The shield protects against static induction noise. Make sure to earth one side.
The twisting protects against electromagnetic induction noise. Where possible, use wires with a short
twist pitch. When using long wiring layouts where crosstalk could be a problem, consider using
separate shields for sending and receiving.
Terminating
resistor: To suppress ringing due to signal reverberation, position terminating resistors
with resistance equivalent to the cable impedance (100
Ω ) at both ends of the
wiring.
Separate wiring: Do not bundle power wires (input: R, S, T; output: U, V, W) with RS-485
communication wires. Separating the wiring can help to suppress inductance
noise.
Effect of shield
The shield acts as an antenna, picking up noise.
If the circiut is looped between the shield and
the ground and the earth is not close, the
electrical potential may differ, resulting in noise
from the flow of current through the loop.
Additionally, noise may be caused by variations
in the magnetic flux in the loop.
In section X in the diagram, the effect of static
induction can be completely eradicated.
Effect of twist
If there is a uniform magnetic flux downwards as you look at the page and it changes (augmentation), electromotive
forces are generated in the direction of the arrows in the diagram.The strength of electromotive forces (A) to (D) is the
same, and the directions are as shown. On line Tx+, (B) and (C) move in opposite directions and therefore cancel each
other out. This is the same for (A) and (D) and, therefore, normal mode noise due to electromagnetic inductance does
not occur. However, it may not be possible to completely suppress noise due to conditions such as a non-uniform twist
pitch. If the lines are parallel, normal mode noise will occur.
Shield earthed on one side
Good example
Twisted wire
Changes (augmentation) to magnetic lines
Shield earthed on both sides
Bad example
Shield not fixed
Magnetic
flux
Signal wire