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1. Signal Reflection Issues
During communication, when there is an impedance mismatch or discontinuity, signal reflection occurs, similar to light transitioning from one medium to another.
The impact of signal reflection on data transmission: Reflected signals trigger the comparator at the receiver’s input, causing the receiver to receive incorrect signals, leading to errors in data reception.
2. Eliminating Signal Reflection
1. Ensure that the impedance at the end of the cable matches the characteristic impedance of the cable. Since the signal transmission on the cable is bidirectional, a termination resistor of the same value should be bridged at the other end of the communication cable. The typical impedance of cables ranges from 100 to 120 ohms, so a 120-ohm termination resistor is usually recommended. In practice, since the characteristic impedance of the cable may not perfectly match the termination resistor, some signal reflection may still occur;
2. Add bias resistors: connect a pull-up resistor to point A and a pull-down resistor to point B, ensuring that the receiver output is fixed at 1 when the bus is idle. Otherwise, reflected signals when the bus is idle may cause the receiver’s output to drop to 0, leading the controller to mistakenly believe new data is being sent.
3. Signal Grounding
Poor grounding can often lead to unstable operation of electronic systems and even jeopardize the safety of the entire system. In many cases, when connecting an RS485 communication link, a simple twisted pair is used to connect the “A” and “B” terminals, neglecting the connection of the signal ground. This method may work normally in many scenarios but poses significant risks.
4. Common Mode Interference Issues
The RS485 interface uses differential signaling for data transmission, which does not require a reference point for signal detection; the system only needs to detect the potential difference between the two lines. However, people often overlook that the transceiver has a certain common mode voltage range. For example, the common mode voltage range for RS485 transceivers is -7V to +12V. The entire network can only function correctly if this condition is met. When the common mode voltage in the network exceeds this range, it can affect communication stability and even damage the interface. For instance, when driver A sends data to B, the common mode voltage at the sending driver is Va. Due to the independent ground potentials of the two systems, there is a ground potential difference Vg, making the common mode voltage at the receiver’s input Vb equal to Vb = Va + Vg. Although the RS485 standard stipulates Va ≤ 3V, Vg can be significant, causing the receiver’s common mode input Vb to exceed the normal range, preventing the system from functioning correctly.
To solve common mode interference: A low-resistance signal ground can be used to connect the two working grounds, effectively shorting the common mode interference Vg. However, this method may not be effective when the internal resistance of the interference source is low, as it can create a large loop current on the grounding line. In such cases, floating ground techniques can be adopted, isolating the system’s circuit ground from the chassis or earth.
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