12. Communication Design Examples Introduction: The wiring principles of RS-232 / RS-485 communication are keeping the connection as short as possible and keeping away from high noise source. The RS-232 communication interface is structured by one to one connections and usually with a shorter connection, so the standard cable or the cable provided by Delta is compatible for common RS- 232 applications.
However, for the high-speed RS-485 with long distance connection, high communication speed, large number of stations, high signal attenuation and the possible problems of improper ground potential, mismatched terminal impedance, noise interference, and wiring methods, the inferior communication quality may occur if the above factors are not considered properly. Therefore, users should pay attention to the following notes about the wiring of RS-485 communication: z The Limit for the Number of Stations: The limit for the number of stations connected to DVP-PLC is 254. For RS-485 communication, its hardware interface is compatible with max.16 stations.
If more than 16 stations are required, a RS-485 repeater (IFD-8510) should be applied. Each repeater supports 16 more stations. Users can add stations by adding repeaters till the limit of 254. z The Limit for Distance: In RS-485 communication, it is a function from the data signaling rate to the maximum cable length for transmission. The value of maximum cable length is generally influenced by the factors such as signal distortion and noise. The below graph of the function from signaling rate to cable length is measured by using 24AWG copper twisted pair telephone cable (diameter: 0.51mm) with the 52.5PF/M bypass capacitor and the 100Ω terminal load (please refer to GB11014-89 Appendix A).
From the figure, suppose the max. acceptable signal loss is 6dBV, when data signaling rate is lower than 90Kbit/S, the limit for cable length will be 1200m (4Kft.). However, the graph is conservative and a longer cable length is accessible in practical application. Users can get different cable length by different cable diameters. For example, if the data signaling rate is 600Kbit/S and the cable is 24AWG, the maximum cable length will be 200m. If the cable is 19AWG (diameter: 0.91mm), the maximum cable length could be longer than 200m. If the cable is 28AWG (diameter: 0.32mm), the maximum cable length can only be shorter than 200m. DVP-PLC Application Examples 12-1