RS-485 is a differential serial communication standard widely used in industrial automation, building management, and remote sensing. It supports long-distance communication, noise immunity, and multi-drop configurations. However, its performance relies heavily on proper termination and biasing.
When integrating RS-485 to Ethernet Converters, many engineers face issues related to signal reflection, voltage instability, and communication errors. These problems often stem from incorrect or missing termination and biasing configurations.
Understanding the RS-485 Bus
RS-485 operates over a differential pair (A and B lines). It supports multiple devices on a single twisted pair cable and can communicate over distances exceeding 1,200 meters at low speeds.
Key RS-485 features:
- Up to 32 unit loads (devices) per bus
- Differential signaling for noise rejection
- Tri-state driver logic (only one device drives at a time)
- Requires external resistors for signal conditioning
The design of a reliable RS-485 network depends on three things:
- Proper termination
- Correct biasing
- Consistent topology (usually a daisy-chain)
Why Termination is Important
1. Reflection Problem
When a signal reaches the end of an unterminated line, it reflects back. These reflections interfere with ongoing data transmission. This causes signal distortion, data errors, or failed communication.
2. Termination Solution
To prevent reflection, a resistor matching the cable’s characteristic impedance is placed at each end of the main bus. This absorbs the signal energy and eliminates reflections.
Typical impedance of RS-485 cable: 100 to 120 ohms
Recommended practice:
- Install a 120-ohm resistor at each end of the RS-485 trunk
- Do not terminate drop lines or stubs
- Avoid star topologies that create multiple reflection points
Without termination, communication may still work on short lines or low speeds. However, for cables longer than 100 meters or speeds above 115.2 kbps, termination becomes essential.
Why Biasing is Required
1. Floating Bus Condition
RS-485 drivers are tri-stated when idle. This leaves the bus floating without a known voltage level. If noise enters the line during idle time, the receiver may interpret it as a logic state, causing errors or false triggering.
2. Biasing Solution
Biasing forces the RS-485 lines to a known voltage when no device is driving the bus. This is typically done by:
- Connecting a pull-up resistor on line B to +V (e.g., 5V)
- Connecting a pull-down resistor on line A to GND
This setup produces a positive voltage difference between B and A. It ensures the receiver sees a valid idle logic level, avoiding misinterpretation.
Choosing Termination Resistors
1. Standard Values
- Use 120-ohm resistors at each end of the bus
- Ensure resistors are installed across A and B lines
- Use resistors rated at least 0.25 watts for robustness
2. Impact on Load
Two 120-ohm terminations in parallel appear as a 60-ohm load to the driver. The RS-485 driver must support this load. Most modern drivers are designed to handle 54 ohms or more.
Do not install more than two termination resistors. Extra terminations reduce signal strength and increase power demand.
Choosing Biasing Resistors
1. Target Differential Voltage
The RS-485 receiver requires a minimum of 200 mV differential voltage for a valid logic level. Bias resistors must provide at least this voltage when the bus is idle.
2. Resistor Calculation
Assume a termination resistance of 120 ohms at each end:
- Total load = 60 ohms (parallel)
- Desired voltage = 200 mV
- Required current = 200 mV / 60 ohms ≈ 3.3 mA
- Bias resistor total resistance = 5V / 3.3 mA ≈ 1,500 ohms
- Use 750 ohm pull-up and pull-down resistors
This configuration provides a differential idle voltage of ~200 mV.
3. Placement Guidelines
- Only one pair of biasing resistors should exist on a bus
- Typically placed near the master or central controller
- Avoid multiple biasing sources to prevent voltage conflicts
Special Considerations for RS-485 to Ethernet Converters
1. Converter Function
A RS-485 to Ethernet Converter connects a serial RS-485 network to an Ethernet network. It allows remote access to RS-485 devices using protocols like Modbus TCP or HTTP over IP.
Converters often include:
- RS-485 line driver and receiver
- Built-in termination jumpers
- Optional biasing resistors
- Serial configuration software
Key Design Rules
1. Termination:
- Enable the converter’s termination only if it is at one end of the RS-485 bus
- If the converter sits mid-bus, keep termination disabled
2. Biasing:
- Check if the converter has internal bias resistors
- Only one device (usually the converter near the master) should enable biasing
3. Cable Length and Speed:
- For distances over 100 meters or speeds above 250 kbps, always use termination
- If multiple converters are used in different locations, maintain a clear topology
4. Power Supply:
- Ensure the converter can handle the power drawn by the biasing resistors
- External power is preferred over port-powered converters for larger loads
5. Network Topology:
- Use a straight daisy chain with converters at the ends or intermediate points
- Minimize stub lengths to reduce reflection
- Keep stub length under 0.5 meters if possible
Common Mistakes and How to Avoid Them
1. Installing Too Many Terminations
More than two terminations lower the impedance too much. This increases driver load and causes signal attenuation.
Fix: Install 120-ohm resistors only at the two far ends of the RS-485 trunk.
2. Enabling Multiple Biasing Points
Multiple bias sources create voltage conflicts. The idle voltage becomes unpredictable, resulting in communication errors.
Fix: Enable biasing in only one location on the bus.
3. Leaving the Bus Floating
Without biasing, the idle state is undefined. Noise on floating lines may cause false data or timeouts.
Fix: Always apply proper biasing resistors.
4. Long Stub Lines
Stubs act like antennas or transmission lines. Long stubs cause signal reflections and degraded waveforms.
Fix: Keep stub length as short as possible, ideally under 0.5 meters.
5. Ground Loop Issues
Improper shield grounding creates ground loops and EMI problems.
Fix: Ground the shield at one point only—usually at the master device.
Performance and Reliability
Proper termination and biasing are essential to maintain the performance and reliability of RS-485 networks, especially in systems that include RS-485 to Ethernet Converters. When these techniques are applied correctly, the network benefits in several key ways.
First, signal integrity improves significantly, leading to fewer data errors during transmission. The RS-485 lines maintain a stable communication link over long distances, often up to 1,200 meters, without loss of signal quality. Proper termination also eliminates reflections that would otherwise interfere with the signal, while biasing ensures the network always has a defined idle voltage, preventing receivers from misinterpreting noise as data.
In contrast, neglecting proper termination and biasing can cause serious communication issues. Common problems include:
- CRC errors, caused by corrupted or distorted signals
- Random disconnects of devices from the network
- Device timeouts, where nodes fail to respond within the expected time
- Corrupted data packets, resulting in unreliable or incomplete data
These issues can lead to downtime, troubleshooting challenges, and poor system performance. However, when a network is well-terminated and correctly biased, it can run reliably for years with minimal maintenance, making it a dependable backbone for industrial and automation systems.
Conclusion
Termination and biasing are critical design elements in RS-485 networks. Ignoring them leads to communication instability, especially when RS-485 to Ethernet Converters are part of the system.
By following resistor selection guidelines and configuring converters correctly, you can ensure stable, error-free communication. Whether you are designing a new system or troubleshooting an existing one, always verify termination and biasing before diving into complex diagnostics.