Why must active terminal resistors be installed at both ends of some RS485 buses?
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Why must active terminal resistors be installed at both ends of some RS485 buses?

Posted Date: 2024-02-07

Why must active terminal resistors be installed at both ends of some RS485 buses?

RS485 is a commonly used serial communication standard used for long-distance data transmission. In a typical RS485 bus, active termination resistors need to be installed at both ends. This article will discuss in detail why active terminal resistors must be installed at both ends of some RS485 buses, and analyze them from the perspectives of electrical characteristics, signal matching, current balance and noise suppression to help readers better understand this issue.

1. Analysis of electrical characteristics

The RS485 bus usually consists of a twisted pair, which contains two signal lines A and B. At first glance, it seems that these two signal lines only need to be connected at both ends of the bus to achieve communication. However, in fact, the RS485 bus requires active termination resistors at both ends due to its electrical characteristics.

1. Differential signal transmission

The RS485 bus uses differential signal transmission, that is, by comparing the voltage difference on line A and line B to transmit information. At the same time, each node on the bus receives signals through differential amplifiers. Without the terminating resistor, the voltages at both ends of the bus will not be exactly the same, causing the received differential signal to be inaccurate. Installing active terminating resistors helps enhance the electrical characteristics of the bus and makes the transmission of differential signals more reliable.

2. Impedance matching

Active termination resistors at both ends of the bus can provide input impedance that matches the line characteristics, making signal transmission more stable. When a signal is transmitted from the sending end to the receiving end, the active termination resistor ensures that the characteristic impedance of the bus matches the characteristic impedance of the line, minimizing signal reflection and interference.

2. The importance of signal matching

Installing active termination resistors helps signal matching and ensures the electrical characteristics and signal quality of the bus.

1. Signal echo suppression

On the RS485 bus, due to the fast signal transmission speed, when the signal reaches the bus terminal, part of the signal may return due to reflection. These reflected signals can cause bus interference and data errors. Active termination resistors can effectively suppress signal reflections and improve data reliability by providing appropriate impedance matching at the bus terminals.

2. Signal quality improvement

The installation of active terminating resistors at both ends of the bus can help improve signal quality. By limiting reflections and interference during signal transmission, signal distortion and distortion can be reduced, making data transmission on the bus more stable and accurate.

3. Advantages of current balancing

The RS485 bus needs to balance the currents sending data in both directions. When an active termination resistor is installed on only one end, this resistor can help balance the current on the bus and prevent data loss or corruption.

1. Current direction balance

In the RS485 bus, the current between line A and line B can be balanced by adjusting the transmitter level on the bus. The installation of active terminating resistors helps maintain a balanced distribution of current on the bus and prevents data loss or damage caused by reduced current accuracy.

2. Enhance data integrity

Configuring active terminal resistors at both ends of the bus can improve current balance and enhance data integrity of the bus. By ensuring that the signals are current balanced, the input common-mode voltage at the receiver input stage is reduced and data bit errors due to current imbalance are reduced.

4. The important role of noise suppression

Active termination resistors also play an important role in noise suppression in the RS485 bus.

1. Electromagnetic interference suppression

In industrial environments, the RS485 bus may be affected by electromagnetic interference. Installing active termination resistors can help reduce electromagnetic interference at the input of the bus receiver and reduce the impact of noise on signal quality.

2. Suppress common mode noise

The installation of active terminal resistors can suppress the impact of common mode noise on the bus and ensure the reliability of data transmission. Common mode noise refers to the interference signal that interferes with line A and line B at the same time. By installing active terminal resistors, data errors caused by common mode noise can be eliminated or reduced.

in conclusion:

By analyzing the reasons for electrical characteristics, signal matching, current balance and noise suppression, we can conclude that installing active terminal resistors at both ends of the RS485 bus is to enhance the electrical characteristics of the bus, ensure signal matching, and balance current distribution. and improve noise suppression capabilities. This can effectively improve the stability, accuracy and reliability of data transmission. Therefore, reasonable installation and adjustment of active terminal resistors are of great significance for the normal operation of the RS485 bus.


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