12. Communication Design Examples However, no matter what degree the CMV is, we suggest users connect each SG of stations (please refer to “Wiring Topology”) by using shielded twisted pair cables so as to reduce the CMV. This wiring method provides the shortest circuit for communication and improves the noise immunity as well. z Terminal Resistor: All cables have their own characteristic impedance (120Ω for Twisted Pair). When the signal is transferred to the terminal and the terminal impedance is different from the characteristic impedance, echo signal will occur to distort the waveform (convex or concave).
This situation is not obvious for short cables but become serious when the cable length increases. In this case, a terminal resistor needs to be applied for maintaining the normal communication. z Methods to Reduce the Noise: When RS-485 network is connected according to the above rules and applied with a 120Ω terminator also, most of the noise interference can be reduced. If the interference continues, that means there is a strong noise source near the network.
In addition to keeping the cable away from the strong noise source (such as electromagnetic valve, AC motor drive, AC servo drive, or other power equipment and their power lines), the best way to reduce the noise is to add a noise suppressor to the noise source. The figure below is the noise suppressing methods for AC motor drive, AC servo drive, and other power equipment. (To apply X capacitors, Y capacitors, or X+Y capacitors) C = 0.22μf~0.47μf/AC630V. Generally, the RS-485 communication cable is made of twisted pair and transmits the signal by the potential difference between the twisted pair, and therefore it is called differential mode transmission.
Differential mode interference is transferred between 2 cables and belongs to symmetric interference, which can be reduced by applying a stabilizing resistor to the circuit together with twisted pair cables. On the other hand, common mode interference is transferred between the communication cable and the earth, which belongs to asymmetric interference. Common mode interference can be eliminated by the following methods: 1. Use shielded twisted-pair cables and ensure it is well-grounded. 2. Use galvanized pipes to shelter the strong electric field. DVP-PLC Application Examples 12-3