Effect of junction capacitance of RS-485 protection circuit on signal quality

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/ 2019-07-04 / Tech

Abstract: RS-485 bus is widely used in industrial sites. There may be high levels of static electricity or surge on site. Engineers usually use gas discharge tubes and TVS tubes to build protection circuits, but the junction capacitance of this circuit is high. Improper application will affect bus communication. This article will introduce you to a peripheral circuit with a low junction capacitance.

 

1, commonly used RS-485 protection circuit

 

Figure 1  Protection Circuit 1

 

Protection as shown in Figure 1 Circuit,The gas discharge tube bleeds most of the surge current at the interface, and the common mode inductor filters out the interference of the common mode signal. The TVS further reduces the residual voltage after the gas discharge tube, thereby protecting the latter circuit. RSM485ECHT module application diagram shown in Figure 1 can achieve contact static ± 8kV, common mode surge ± 4kV, differential mode surge ± 2kV, to meet the requirements of most industrial sites on the RS-485 node static and surge levels.

 

The protection circuit shown in Figure 1 has strong protection, but its junction capacitance is large. A-RGND or B-RGND junction capacitance is about 2.5nF. When there are many nodes on the bus that use the Figure 1 protection circuit for networking, the bus has a large capacitance, the signal reflection and the signal edges tend to be flat, which makes the signal quality worse, and may even cause communication anomalies.

 

2, Signal reflection caused by bus capacitance

 

When the signal is transmitted on the communication line,When the protection circuit on the RS-485 node is reached, the junction capacitance of the protection circuit changes the instantaneous impedance of the signal, some of the signal will be reflected, and the other part will be distorted and continue to propagate.

 

Figure 2 shows the RSM485ECHT single-node transmit waveform, Figure 3 is a schematic diagram of the RS-485 bus connected to six protection circuits, each node The distance between them is about 30cm, and they are connected by hand in a twisted pair. Figure 4 and Figure 5 respectively connect the waveform test point 1 and the waveform test point 6 of the circuit shown on the bus. The waveform of the position marked in Fig. 3, the rise/fall time of the waveform becomes long, and the waveform of the waveform test point 1 becomes a step shape.

 

 

Bus 6 protection circuit connection diagrams

 

 

RSM485ECHT is connected to 6 protection circuit waveform test point 1 waveform

 

 

RSM485ECHT connected to 6 protection circuit waveform test point 6 waveform

 

RSM485ECHT RS The -485 interface has strong driving capability. The following test results are used to test the RS-485 transceiver chip test waveform commonly used in the market. It can be seen that the waveform has been seriously interfered, and the reflected waveform has reached the RS-485 chip threshold. In the vicinity of the plane, communication abnormalities may occur. Therefore, in practical applications, a transceiver with strong driving capability should be selected.

 

3, low junction capacitance Protection circuit

 

When there are a large number of communication nodes, you can use the protection circuit shown in Figure 8 with A-RGND or B-RGND The junction capacitance is only 20pF. Although the TVS junction capacitance is large, the common diode junction capacitance is very small, and the junction capacitance of the TVS and the ordinary diode is in series, so the junction capacitance of the protection circuit can be reduced. Use the figure 8 to map  The network shown in Fig. 3, the waveform of test point 1 is shown in Fig. 9, and the waveform of test point 6 is as shown in Fig.As shown in 10, the waveform does not change substantially.

 

Figure 8  Protection Circuit 2 (Low Junction Capacitance)


 

4. Summary

 

The protection circuit mounted on the bus changes the instantaneous impedance of the signal. Lead to signal reflection, when the number of nodes on the bus is large, the capacity of the bus is large, which will cause interference to the bus waveform and affect the quality of the communication signal. Therefore, in order to reduce the influence of the protection circuit on the bus communication, the driver can be selected in practical applications. If the protection circuit shown in Figure 1 is used, the low junction capacitor TVS should be selected, or the low junction capacitor protection circuit as shown in Fig. 8 can be selected.

 


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