Chapter 12 Network Module AH10PFBM The In hole has 2 pins, A1 and B1 and the Out hole has two pins, A2 and B2. A1 and A2 are connected to the green wires in the cable and B1 and B2 are connected to the red wires in the cable. The terminal resistor switch is used to select whether to connect the terminal resistor to the station or not. If the switch is set to “Off”, A1 and A2 are connected; B1 and B2 are connected and the In hole is not connected to the terminal resistor. If the terminal resistor switch is set to “On”, A1 and A2 are disconnected; B1 and B2 are disconnected and the In hole is connected to the terminal resistor.
When the PROFIBUS connector is inserted to the PROFIBUS interface of the station, A1 and B1 are connected to pin 8 and pin 3 of the PROFIBUS interface of the station respectively. The terminal resistors inside the PROFIBUS connector are illustrated in the right side figure as below. When each station is connected to the network via the connector and cable, every physical network supports 32 physical devices according to the RS-485 serial port communication protocol and the stations at the ends of every physical network should be connected with terminal resistors in order to prevent the current/voltage surf and ensure the communication quality.
Every PROFIBUS connector contains the built-in resistors. User could select whether to connect the station to the terminal resistor via the terminal resistor switch. The switch On indicates that the resistor is connected to the station and the switch Off indicates that the resistor is disconnected to the station. When the terminal resistor switch is set to On, it means one physical network ends and the signal of the next network connected to the Out hole is aborted. Therefore, in the connectors of the two stations at the two ends of every physical network, the cables should be connected to the In holes and meanwhile the terminal resistor switches are set to On.
The stations in the middle of one network, the cables should be connected to the In hole and Out hole successively and meanwhile the terminal resistor switches are set to Off as below. 12.4.1.5 Terminal Resistor The PROFIBUS cable must connect the terminal resistors including the following 3 resistors, Rt, Ru and Rd at its two ends. The standard PROFIBUS connector contains the built-in terminal resistor which user could select whether to be connected to the station or not via the switch. User could purchase the standard PROFIBUS connector to add the terminal resistor. The terminal resistor is used to eliminate the signal reflection in the communication cable.
There are two reasons leading to the signal reflection: Impedance discontinuity and Impedance mismatch. 12-9