To build a stable and reliable Profibus-DP network, termination resistors can be considered a “small component with a big role”! Many engineers often overlook their importance, resulting in spending several days troubleshooting inexplicable communication failures, only to find out that it was the termination resistors causing the trouble. Today, we will analyze this seemingly simple yet crucial component—Profibus-DP termination resistors.
Why Are Termination Resistors Needed? Starting from Signal Reflection
Imagine throwing a stone into a pool of water; the ripples will spread outwards, and when they hit the pool’s edge, they will bounce back, creating new ripples that interfere with the original clear pattern. The electrical signals on the Profibus bus behave similarly.
Without termination resistors, when the signal reaches the end of the bus, it will reflect back, and the reflected signal will overlap with the newly sent signal, causing signal distortion, which can lead to communication errors in severe cases.
In simple terms: Termination resistors act like “wave absorbers” for the bus; they can absorb the signal energy reaching the end, preventing the signal from bouncing back and interfering with new signals.
The Dangers of Signal Reflection:
- Signal waveform distortion, leading to reception errors
- Unstable communication, occasional disconnections
- Difficult system diagnostics, making it hard to locate the cause of errors
I once encountered this issue in a steel mill’s rolling mill control system: devices on the bus would randomly disconnect, sometimes several times a day, sometimes only once every few days. When measuring the signal with an oscilloscope, I found significant reflected waveforms. Upon inspection, I discovered that one end’s termination resistor was loose, causing intermittent contact. Unreliable termination resistor connections are harder to troubleshoot than having no termination resistors at all! After fixing it, the system has been running stably for over three years.
Correct Specifications for Profibus-DP Termination Resistors
Profibus-DP uses the RS-485 transmission physical layer, and the standard termination resistor consists of the following components:
Standard Termination Resistor Circuit
+5V
│
│
↓
线路B ──────┬────── 390Ω ──────┐
│ │
│ │
220Ω │
│ │
│ │
线路A ──────┴────── 390Ω ──────┘
│
│
↓
GND
This circuit includes three resistors:
- A 220Ω resistor connected between lines A and B to match the cable impedance
- Two 390Ω resistors connected to +5V and GND to provide bias voltage
Important Note: The 390Ω bias resistors ensure that the bus remains at a defined level when no data is being transmitted, preventing noise from triggering the receiver. Some introductory tutorials only mention the 220Ω resistor, neglecting the importance of the bias resistors!
Types and Selection of Termination Resistors
In practical applications, Profibus-DP termination resistors mainly come in the following forms:
1. Termination Resistors Integrated in D-Sub Connectors
This is the most common form, where the connector usually has a switch or knob to turn the termination resistor on or off.
Advantages:
- Convenient to use, no additional components required
- The switch is intuitive and can be configured quickly
- Includes all necessary resistors (220Ω and 390Ω)
Disadvantages:
- The switch may be accidentally toggled
- May have poor contact after long-term use
Selection Advice: Prefer products from well-known brands like Siemens, Phoenix Contact, or Wago for guaranteed quality. If using Siemens products, models 6ES7 972-0BA52-0XA0 (with programming interface) or 6ES7 972-0BB52-0XA0 (without programming interface) are good choices.
2. Standalone Termination Resistor Boxes
This form encapsulates the termination resistors in a standalone small box, connected to the end of the bus via short wires.
Advantages:
- Flexible layout, can be installed in the rail slot
- Less likely to be accidentally touched
- High reliability, good contact
Disadvantages:
- Takes up additional space
- Requires separate power supply (for some models)
Selection Advice: Consider this option in harsh environments or where vibration is significant, such as Siemens model 6ES7 972-0DA00-0AA0.
3. Built-in Termination Resistors
Some devices (like certain remote I/O stations) have configurable termination resistors integrated internally.
Advantages:
- Space-saving, reduces external components
- Simple configuration, usually set via DIP switches
Disadvantages:
- Not all devices have this feature
- Configuration is not intuitive and requires opening the device casing
Selection Advice: If the device is built-in, it can be used directly, but the manual should be carefully read to confirm its specifications.
4. Homemade Termination Resistors
In emergencies, termination resistors can also be made by oneself.
Materials Needed:
- 1 precision 220Ω resistor (1% tolerance, 1/4W or higher)
- 2 precision 390Ω resistors (1% tolerance, 1/4W or higher)
- A stable 5V power supply
Notes: Homemade termination resistors should be considered a temporary solution; using standard products is strongly recommended in production environments!
Correct Installation Location for Termination Resistors
In a Profibus-DP network, the installation location of termination resistors is a key factor in ensuring stable communication.
Basic Principle:
Termination resistors should only be installed at the two physical endpoints of the bus; no intermediate devices should have termination resistors enabled.
Incorrect Installation Method:
主站(TR开) ── 从站1(TR开) ── 从站2(TR开) ── 从站3(TR开) 【错误]
Correct Installation Method:
主站(TR开) ── 从站1(TR关) ── 从站2(TR关) ── 从站3(TR开) 【正确]
Note: TR = Termination Resistor
Termination Resistor Installation for Different Network Topologies
1. Standard Linear Topology:
主站(TR开) ── 从站1 ── 从站2 ── ... ── 最后一个从站(TR开)
2. Extended Linear Topology with Repeaters:
主站(TR开) ── 从站1 ── 中继器(TR开|TR开) ── 从站2 ── 从站3(TR开)
Note: Each side of the repeater requires a termination resistor, as it divides the network into two independent electrical segments.
3. Star Topology (Using Active Star Couplers):
┌── 从站1 ── 从站2(TR开)
│
主站(TR开) ── 星形耦合器 ── 从站3 ── 从站4(TR开)
│
└── 从站5(TR开)
Note: Termination resistors need to be installed at the end of each branch.
Technical Points: In star topology, active star couplers already implement signal isolation and amplification, making each branch an independent electrical segment.
Handling Special Cases
1. Systems with Frequent Device Changes
In some testing systems or scenarios where devices need to be frequently connected/disconnected, it is recommended to use active termination resistors (termination resistors with power supply).
Solution: Install a permanent active termination module at the end of the bus, ensuring that even if the last device is disconnected, the bus termination can still function normally.
2. Long-Distance Transmission
When the bus length exceeds 200 meters, the quality of termination resistors becomes even more critical.
Solution: Use high-quality active termination resistors and consider adding repeaters at intermediate positions.
Practical Experience: In an 800-meter-long Profibus network, we installed a repeater every 200 meters and high-quality termination resistors at both ends of each electrical segment, ensuring stable operation even in environments with strong electromagnetic interference, with very few communication failures.
Connecting and Checking Termination Resistors
The correct installation process for termination resistors:
1. Preparation Before Installation
- Confirm the physical endpoints of the bus
- Prepare suitable termination resistors
- Turn off the system power
2. Installation Steps
- For D-Sub connectors, toggle the switch at the end of the connector to the “ON” position
- For standalone termination resistor boxes, connect them to the output port of the bus end device
- For built-in termination resistors, set the corresponding DIP switches according to the device manual
3. Verification After Installation
After installation, the following methods can be used to verify whether the termination resistors are functioning correctly:
Method 1: Resistance Measurement
- Disconnect the bus power
- Use a multimeter to measure the resistance between lines A and B
- Under normal circumstances, the reading should be about 110Ω (two 220Ω termination resistors in parallel)
- If the reading is significantly higher than 110Ω, it indicates that the termination resistors are not properly connected
- If the reading is significantly lower than 110Ω, it indicates that there may be extra termination resistors connected
Method 2: Voltage Measurement
- Turn on the bus power
- Use a multimeter to measure the voltage between lines A and B
- Under normal circumstances, there should be a voltage difference of about 1.0-1.5V in idle state
- If the voltage is close to 0 or unstable, it indicates that there may be an issue with the termination resistors
Method 3: Using Bus Diagnostic Tools
Professional Profibus diagnostic tools (such as Profitrace) can visually display signal quality and reflection conditions, making it the most accurate way to verify termination resistors.
Maintenance Tip: Before the system is put into use, take photos to document the correct termination resistor configuration status and annotate it in the system documentation. This will provide a reliable reference for future maintenance or troubleshooting.
Common Problems and Solutions
Problem 1: Occasional Communication Interruptions in the System, Especially in High-Temperature Environments
Possible Cause: Some low-quality termination resistors may drift in parameters when the temperature rises. Solution: Replace with industrial-grade high-temperature stable termination resistors, consider using active termination resistors.
Problem 2: Communication Becomes Unstable After Adding Devices
Possible Cause: The newly added devices may have changed the physical structure of the bus, and the original termination resistor location is no longer at the physical endpoint. Solution: Recheck the network topology to ensure that termination resistors are only enabled at the actual physical endpoints.
Problem 3: All Devices Over a Certain Distance on the Bus Are Unstable
Possible Cause: There may be a termination resistor mistakenly enabled somewhere in the middle, causing signal reflection. Solution: Systematically check the termination resistor settings of each device to ensure that only endpoint devices are enabled.
Problem 4: Communication Issues After Replacing Devices
Possible Cause: The termination resistor settings of the new device may differ from those of the original device. Solution: Check the termination resistor settings every time a device is replaced to maintain consistency.
Real Case: I once encountered a very strange problem at a water treatment plant—the system was more prone to communication failures on rainy days. After detailed investigation, it was found that water had entered the junction box, causing the termination resistors to become damp and their parameters to change. The solution was to replace it with waterproof termination connectors and add protective covers, which immediately resolved the issue.
Advanced Applications and Optimization of Termination Resistors
In addition to basic installation, there are some optimization techniques that can further enhance system reliability:
1. Active Termination Resistors
Standard termination resistors need to draw power from the bus devices; if the last device loses power, the termination resistors will also fail. Active termination resistors have an independent power supply, ensuring that even if the devices lose power, the termination resistors can still function normally.
Application Scenario: Systems where devices are frequently started and stopped, such as flexible production lines.
2. Programmable Termination Resistors
Some advanced systems use intelligent termination resistors that can be controlled via the bus, allowing parameters to be automatically adjusted based on system status.
Application Scenario: Systems with frequently changing network configurations, such as testing platforms.
3. Termination Resistors with Diagnostic Functions
These termination resistors have built-in diagnostic circuits that can monitor bus status and report anomalies.
Application Scenario: Critical systems, such as safety-related applications.
Optimization Suggestion: For important systems, it is recommended to choose termination resistors with status indicator LEDs, which can intuitively indicate whether the bus is active, facilitating troubleshooting.
Practical Exercise Suggestions
To gain a deeper understanding of the role of Profibus-DP termination resistors, you can try the following experiments:
- Set up a simple Profibus network (1 master + 2-3 slaves)
- Use an oscilloscope to observe the signal waveform when termination resistors are correctly installed
- Try removing one or both ends of the termination resistors and observe the signal changes
- Incorrectly enable a termination resistor on an intermediate device in the bus and observe the phenomenon
- Compare the performance differences of termination resistors from different brands
- Test the impact of different cable lengths on termination resistor effectiveness
Record Table: Termination Resistor Configuration and System Performance
| Termination Resistor Configuration | Baud Rate | Signal Quality | Error Rate | System Stability |
|---|---|---|---|---|
| Correctly installed at both ends | 1.5Mbps | Good | Very low | Stable |
| Only installed at the master end | 1.5Mbps | Significant signal reflection | Medium | Occasional faults |
| Only installed at the slave end | 1.5Mbps | Significant signal reflection | Medium | Occasional faults |
| Not installed at all | 1.5Mbps | Severe distortion | Very high | Basically no communication |
| Intermediate device incorrectly enabled | 1.5Mbps | Multiple reflections | High | Frequent faults |
Termination resistors may seem simple, but they are the key to the stability of the Profibus-DP network. Correctly selecting and installing termination resistors, ensuring they are only enabled at physical endpoints, and regularly checking their status will greatly enhance the reliability of your Profibus network. When troubleshooting on-site, always prioritize checking termination resistors, as this can save you a lot of troubleshooting time!
Remember: In industrial communication, the simpler the component seems, the more likely it is to be overlooked, and the more it can become a bottleneck for system reliability. Paying attention to every detail is essential for building a truly stable automation system.