The Role of Termination Resistors on Siemens PROFIBUS DP Connectors (Commonly Known as ‘DP Connectors’).
Core Function Summary:The core function of termination resistors is to match the impedance at both ends of the communication bus to eliminate reflected wave interference caused by impedance discontinuities at the ends of the transmission line.
In simple terms, it is to prevent communication signals from “bouncing” back at the end of the cable, colliding with subsequent signals, causing data distortion and communication errors.
Detailed Principle Explanation
1. Signal Reflection:
· The PROFIBUS DP network uses the RS-485 standard for data transmission, which is a high-speed serial communication.
· When electrical signals are transmitted in the cable, if they reach the end of the cable or a disconnected node, they encounter an impedance change (from the characteristic impedance of the cable to infinity).
· According to transmission line theory, this portion of energy cannot be completely absorbed and will reflect back along the line.
· This reflected signal will superimpose with the normal signals sent subsequently, leading to waveform distortion, making it impossible for the receiver to correctly identify logical “1” and “0”, ultimately causing communication interruptions, increased error rates, or unstable nodes.
2. Impedance Matching:
· The characteristic impedance of PROFIBUS cables is typically 220 ohms.
· The termination resistor on the Siemens DP connector is designed to match the cable impedance. It consists of a 220-ohm resistor and a 330-ohm resistor connected in series between the data lines (A and B lines).
· When the termination resistor switch is set to the “ON” position, this resistor network is connected to the bus.
· The benefit of this is that when the signal reaches the end of the bus, its energy is consumed by this resistor (converted to heat) rather than being reflected back, ensuring the signal is “clean” and complete.
Correct Termination Resistor Setting Rules
This is the most critical part of practical operation, and the rules are very simple and strict:
Termination resistors should only be set to “ON” (enabled) at the connectors at both ends of the entire PROFIBUS bus. All DP connectors in the middle of the bus must have their termination resistors set to “OFF” (disabled).
Correct Configuration Example: Suppose you have a PROFIBUS bus with the following connection order: PLC (Master) —-> Site 1 —-> Site 2 —-> Site 3 (End)
· DP connector on PLC (Master): Termination resistor set to ON
· DP connector on Site 1: Termination resistor set to OFF
· DP connector on Site 2: Termination resistor set to OFF
· DP connector on Site 3 (End node): Termination resistor set to ON
Common DP Connector Switch Descriptions:
· ON: Termination resistor is active. At this time, the incoming (In) terminal of this DP connector is terminated by the resistor network, and the outgoing (Out) terminal may be disconnected (depending on the design), thus cannot connect to the next node.
· OFF: Termination resistor is inactive. Signals can pass freely through the incoming (In) and outgoing (Out) terminals.
Important Reminder:
· Never set the termination resistors of multiple nodes to ON, as this will cause the bus impedance to be too low, resulting in the communication voltage being pulled down, and the entire network will not function.
· If the bus only has two nodes (for example, one PLC and one ET200S station), then both nodes are at the end of the bus, and the termination resistors of both must be set to ON.
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Termination Resistors and Power Supply
It is important to note that for termination resistors to function properly, power must be supplied to the DP connector. The 5V DC power terminal on the DP connector (usually bridged) must be connected.
· This is because one end of the termination resistor network is connected to a 5V voltage, while the other end is connected to data line B through a 330-ohm resistor and to data line A through a 220-ohm resistor. This bias voltage ensures that the bus is in a defined logical state (“1”) when idle, enhancing its anti-interference capability.
· If the DP connector at the end of the bus is not powered, even if the resistor is set to ON, it will not achieve true termination matching, and reflections may still occur at the end of the network.
Summary
Setting Rules: Only the physical first and last nodes of the bus need to have their termination resistors set to ON, while all intermediate nodes must be set to OFF.
Physical Composition: Typically a series combination of a 220Ω resistor and a 330Ω resistor, connected between the A/B lines.
Necessary Condition: For the termination resistors to be effective, the DP connector must be connected to a 24V DC power supply (to generate the bias voltage).
Consequences: Incorrect settings (having the wrong ones ON or the wrong ones OFF) can lead to communication instability, node loss, or even total network failure.
Correctly setting the termination resistors is the primary and necessary step in debugging and maintaining a PROFIBUS network.