Why is it necessary to connect terminal resistors to the Profibus bus? (In-depth analysis)
The diagram below shows the SUB-D9 pin connector of the Profibus bus, with the red section on its left representing the switch for the terminal resistor. The lower part of the connector is used to connect the Profibus cable, with one end and one exit for each cable, each containing A and B data lines. On the right side of the connector are the 9-pin plugs used to connect to the CPU or distributed modules.
When the terminal resistor switch is turned on (set to the ON position), the terminal resistor is connected to the Profibus bus, and the Profibus output is disconnected. The terminal resistor is not a single resistor; it internally comprises three different resistors.
The standard Profibus cable is a shielded twisted pair, consisting of two data lines, A (green) and B (pink). They are connected to the pins (PIN) 8 and 3 of the bus connector, respectively. Pin 6 is the positive pole (+5V), and pin 5 is the reference ground (0V).
In addition to the 220-ohm (Ω) terminal resistor on the Profibus connector, there is also a 390-ohm (Ω) pull-up resistor and a 390-ohm (Ω) pull-down resistor. The pull-up resistor (390Ω) is connected to pin 6 (VP, +5V), and the pull-down resistor (390Ω) is connected to pin 5 (GND, 0V). The terminal resistor (220Ω) is connected to the other two resistors through the switch on the bus connector, as shown in the diagram below:
When the terminal resistor switch on the connector is turned on (set to the ON position), the A and B data lines of Profibus will be connected to the 220-ohm (Ω) terminal resistor, as shown in the diagram below:
Up to this point, we have clarified that when the terminal resistor switch on the Profibus bus connector is turned on (set to the ON position), the bus loop will include terminal resistors as well as pull-up and pull-down resistors. Now, the question is: Why connect terminal resistors? What happens if they are not connected?
The answer is: If terminal resistors are not connected, signal reflections may occur in the communication cable, and terminal resistors help eliminate these signal reflections.
There are two reasons for signal reflections: impedance mismatch and impedance mismatch.
Impedance mismatch refers to situations where the signal encounters a sudden change in impedance at the end of the data line, such as a very low impedance (e.g., a short circuit) or a very high impedance (e.g., an open circuit). Impedance mismatch at these points can cause reflections in the transmitted signal. The solution to eliminate these reflections is to connect a terminal resistor at the end of the cable with the same characteristic impedance as the cable to ensure continuous impedance. Since signal transmission is bidirectional, terminal resistors are needed at both ends.
Another reason for signal reflections is impedance mismatch. During signal transmission, it is essential for the impedance of the data transceiver (load) to match the impedance of the transmission line; otherwise, signal reflections may occur. These reflections are particularly noticeable when the communication line is idle, leading to meaningless data on the line. In Profibus networks, pull-up and pull-down resistors, acting as complementary terminal resistors, ensure that when no data is being transmitted, the voltage difference between the two data lines tends toward a fixed value, thus eliminating interference with data.
The diagram below illustrates the Profibus bus topology with terminal resistors:
In a physical network segment, it is crucial to ensure that terminal resistors are connected at both ends of the network to avoid potential communication failures. This also implies that if an issue arises at a terminal station, it can impact the communication across the entire network. To prevent such problems, Siemens offers active terminal resistors. Active terminal resistors require separate power, and they are installed at both ends of the network segment, ensuring that any device malfunction within the segment does not affect the impedance matching of the network. The following image is a photograph of the active terminal resistors provided by Siemens:
Alright, let's conclude our discussion on the issue of Profibus terminal resistance for now.