Many people know that terminating resistors are connected to both ends of the PROFIBUS bus, but they may not be clear on why. This article will discuss the PROFIBUS bus terminating resistors.
The image below shows the SUB-D 9-pin connector for the Profibus bus. The red part on the left is the terminating resistor switch. The lower end of the connector is used to connect the Profibus cable, with one input and one output. Each cable contains two data lines, A and B. The right side of the connector is a 9-pin plug, used to connect to the CPU or distributed module.
When the terminating resistor switch is turned on (ON position), the terminating resistor is connected to the PROFIBUS bus, and the PROFIBUS output line is disconnected. The terminating resistor is not a single resistor; it contains three different resistors internally. To better illustrate the terminating resistor, let's first look at the pin definitions of the PROFIBUS 9-pin D-type connector, as shown in the following diagram:
The standard Profibus cable is a shielded twisted-pair cable with two data lines, A (green) and B (pink), which connect to pins 8 and 3 of the bus connector, respectively. Pin 6 is positive (+5V), and pin 5 is reference ground (0V). In addition to a 220-ohm terminating resistor, the Profibus connector includes a 390-ohm pull-up resistor and a 390-ohm 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 terminating resistor (220Ω) is connected to the other two resistors via a switch on the bus connector, as shown in the diagram below.
When the connector's terminating resistor switch is turned on (to the ON position), the Profibus A and B data lines will be connected to a 220-ohm (Ω) terminating resistor, as shown in the diagram below:
So far, we've understood that when the terminating resistor switch on the Profibus bus connector is turned on (to the ON position), there will be a terminating resistor, as well as pull-up and pull-down resistors in the bus circuit. The question now is: why connect the terminating resistor? And what would happen if it weren't connected?
The answer is: without a terminating resistor, signal reflections may occur in the communication cable, while a terminating resistor can eliminate signal reflections in the communication cable.
There are two reasons for signal reflection: impedance discontinuity and impedance mismatch.
Impedance discontinuity refers to a situation where a signal encounters a sudden change in cable impedance at the end of a data line, either very low (e.g., a short circuit) or very high (e.g., an open circuit). This impedance discontinuity causes signal reflection. To eliminate this reflection, a terminating resistor with the same characteristic impedance as the cable is connected to the end of the cable, ensuring impedance continuity. Since signal transmission is bidirectional, terminating resistors are required at both ends.
Another cause of signal reflection is impedance mismatch. During signal transmission, the impedance of the data transceiver (load) must be the same as the impedance of the transmission line; otherwise, signal reflection will occur. This reflection mainly manifests as meaningless data appearing on the line when it is idle. In a PROFIBUS network, pull-up and pull-down resistors, as supplementary terminating resistors, ensure that the voltage difference between the two data lines tends to a fixed value when there is no data transmission, thereby eliminating interference.
The following diagram shows the Profibus bus topology with terminating resistors:
Within a physical network segment, terminating resistors must be connected at both ends of the network; otherwise, communication failures may occur. This means that if a terminal station experiences a problem, it will affect the communication of the entire network. To prevent such problems, Siemens also provides active terminating resistors. Active terminating resistors require separate power supplies and are installed at both ends of the network segment, ensuring that a problem with any device within the segment will not affect the network's resistance matching. The image below shows a photograph of an active terminating resistor provided by Siemens:
