AH500 Module Manual Impedance discontinuity: The signal at the end of the transmission cable suddenly encounters too low impedance or even 0 impedance (E.g.: short circuit) or too high impedance (E.g.: broken circuit) and then it will cause the signal reflection. To eliminate such kind of reflection, the terminal resistor with the same characteristic impedances as that of the cable must be connected at the end of the cable and make the impedance of the cable in continuity. Since it is a two-way transmission of the signal on the cable, the other end of the communication cable must also connect a same- size terminal resistor. The following Rt is the terminal resistor. Impedance mismatch:
Another reason causing the signal reflection is the impedance mismatch between data transceiver and transmission cable. The reflection causes the entire network in a mess while the communication circuit is in idle. To prevent the kind of case happening, the upper resistor Rd (connecting the DGND) and the lower resistor Ru (connecting the positive voltage) need be complemented as below. When no data transmission from the station (I.e. in idle state), the complement forces the voltage in different state (I.e. the voltage between conductors) to approach a fixed value. Connect one station to the PROFIBUS network via the PROFIBUS connector and cable as shown in the following figure.
The last stati on 1st station DGND DGND 5 Rd Rd 390Ω 390Ω 5 4 9 A Data w ireA A 4 9 3 8 220Ω B Rt Rt 220Ω Data w ire B B 3 8 2 7 Ru Ru 390Ω 390Ω 2 7 1 6 5V 5V 1 6 B A B A 6 7 8 9 6 7 8 9 5 4 3 2 1 5 4 3 2 1 S tati on 2 Station 3 12.4.2 Network Topology 12.4.2.1 Connecting RS-485 repeater to the PROFIBUS DP network The master module could connect 125 slaves at most and its node address could be set to 0~125 through SYCON.net software. The node address of the slave could be set to 1~125. If the actually connected devices exceeds 32 on the network or the corresponding baud rate is out of the allowed distance, the RS-485 repeaters need to be added to expand the physical network.
The PROFIBUS DP network is divided into several networks using the RS-485 repeaters. Please note that the two ends of every network should be connected with the terminal resistors respectively. For use of the RS-485 repeater, please refer to the user manual of the used repeater. 12-10