IC Realtime ICIP-P2012T Product Manual - Page 25

7 APPENDIX Ⅱ ABOUT RS485 BUS 7.1 RS485 Bus Main Feature RS485 is semi duplex communication cable of impedance 120Ω. Its max load amount is 32 effective loads (including main control device and devices to be charged). 7.2 RS485 Bus Transmission Distance When we take 0.56mm (24AWG) twisted-pair as communication cable, the max transmission distance (theoretically) are listed below (according to different baud rates). Baud Rate 2400 BPS 4800 BPS 9600 BPS Max Distance 1800M 1200M 800M In the following situations, the max transmission distance shall become shorter accordingly: z The communication cable is a little bit thin; z The surrounding environment has strong electromagnetic interference; z There are too much devices connected to the RS485 bus; And vice versa, the max transmission distance shall become longer. 7.3 The Problem in Practical Use In practical usage, we usually adopt star type connection. The terminal resistance shall connect to the furthest two devices (Such as device 1# and device 15# in Figure 7-1 ). But this connection way does not conform to RS485 Bus standard. When the distances between devices are too long, the signal reflection occurs and anti-jamming decreases, thus the signal reliability becomes very low. You can see speed dome is not under control or speed dome is running automatically and can not stop. Figure 7-1 In this situation, we recommend RS485 distributor. This device can turn star type connection into the connection that conforms to RS485 bus industry standard, which can avoid the above mentioned problems and enhance communication reliability. See Figure 7-2. 17