RS-485 User's Manual MEH448d - Fuji Electric America

2.2.4 Measures against noise
Depending on the operating environment, normal communications cannot be performed or
instruments and converters on the host side may malfunction due to the noise generated by the
inverter. This section describes measures to be taken against such problems. Consult Appendix
A "Advantageous Use of Inverters (Notes on electrical noise)" in User's Manual of each inverter
type.
[1] Measures for devices subjected to noise
Using an isolated converter
An isolated converter suppresses common mode noise that exceeds the specified operating
voltage range of the receiver in case of long-distance wiring. However, since the isolated
converter itself may malfunction, use a converter insusceptible to noise.
Using a category 5 compliant LAN cable
Category 5 compliant LAN cables are generally used for RS-485 communications wiring. To
obtain an improved preventive effect on electromagnetically induced noise, use Category 5
conformed LAN cables with four twisted-pair-cores and apply one twisted pair, DX+ and DX-. To
ensure a high preventive effect on electrostatically induced noise, use Category 5 conformed
LAN cables with four shielded-and-twisted-pair-cores, and ground the shield at the master-side
end.
Effect of twisted pair cables
A uniform magnetic flux directing from the face to back of the paper exists, and if it increases,
electromotive force in the direction of → is generated. The electromotive forces of A to D are the same
in intensity, and their directions are as shown in the above figure. In the cable DX+, the direction of
electromotive forces B is reverse to that of electromotive force C, then the electromotive forces B and C
offset each other, and so do electromotive forces A and D in the cable DX-. So, normal mode noise
caused by electromagnetic induction does not occur. However, noise cannot be completely suppressed
under such conditions as an uneven twist pitch. In the case of twisted cables, the normal mode noise is
considerably reduced. But in the case of parallel cables, there may be a case where noises are not
sufficiently reduced.
Shield effect
1) When the shield is not grounded,
the shield functions as an antenna and receives noise.
2) When the shield is grounded at both ends,
if the grounding points are separated from each other, the ground potential may be different between
them, and the shield and the ground form a loop circuit in which a current flows and may cause noise.
Additionally, the magnetic flux within the loop may vary and generate noise.
3) When the shield is grounded at either end,
the effect of electrostatic induction can be completely eliminated within the shielded section.
Connecting terminating resistors
Insert a resistor equivalent to the characteristic impedance of the cables (100 to 120Ω) into both
end terminals of the wiring (network) to prevent ringing due to the reflection of signals.
Separating the wiring
Separate the power lines (input L1/R, L2/S, and L3/T and output U, V, and W) from the RS-485
communications line, because induced noise can be prevented.
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