12. Communication Design Examples The relation between the transmission speed (bps) and the transmission distance (foot) for RS-485 standard communication interface: Transmission distance (feet) z The Limit for Cables : Transmission speed (bps) Users should choose shielded twisted pair cables for wiring because the quality of cables will greatly influence the transmission signal. If users use low quality cables (such as PVC twisted pair cables), the signal attenuation will be higher and the transmission distance will be significantly shortened. In addition, the communication could be interfered easily due to the poor noise immunity of low quality cables.
Therefore, in situations of high transmission speed, long distance or high noise, the high quality twisted pair cable (such as Polyethylene twisted pair cable) should be used. However, in situations of low transmission speed and low noise, PVC twisted pair cable will be a compatible and cost saving choice though the signal loss of PVC cable could be 1,000 times bigger than high quality cable. If the transmission distance is too long to increase the signal attenuation, users can use RS-485 repeater (IFD-8510) to magnify the signal. z Wiring Topology: For RS-485 wiring, the nodes should be near the master cable as much as possible.
Generally, daisy chain topology structure is recommended for RS-485 wiring. Topology is the link structure of the connection. The topology of RS-485 must be station-by-station structure, that is, stations should be connected from 1 to 2, 2 to 3, etc. Star and ring topological structures are not permitted. z Signal Grounding (SG): Though the RS-485 network can be connected by twisted cables only, it is easily to be interfered by noise and should be connected under the condition that the CMV (Common Mode Voltage) between stations should not exceed the max. allowable CMV of RS-485 transmission IC. If the CMV exceeds the working voltage range of IC, RS-485 will stop working.
DVP-PLC Application Examples 12-2