How should the terminal resistance of the CAN/RS-485 bus be set?

The function of terminal resistors is to eliminate signal reflections, but terminal resistors are not required in all situations. Sometimes, terminal resistors may affect signal quality. This article provides an in-depth analysis of the terminal resistor settings for the CAN/RS- 485 bus .

If we check Baidu Encyclopedia, the file of terminal resistors seems to be very clean:

It seems that adding such a magic weapon at both ends of the communication link can avoid signal reflection and make the signal propagation smoother. Although this is not wrong, there are still some details that cannot be sorted out. Let's take a look at the terminal resistance in the CAN bus and 485 bus.

1. CAN bus

In the CAN bus, the terminal resistor is indispensable. Its existence mainly includes two points:

It can ensure that the level quickly enters the recessive state;

Improve signal quality.

1. Ensure that the level quickly enters the recessive state

During the dominant state, the parasitic capacitance of the bus will be charged, and when returning to the recessive state, these capacitances need to be discharged. If there is no resistive load between CANH and CANL, the capacitance can only be discharged through the differential resistance inside the transceiver. We added a 220PF capacitor between CANH and CANL of the transceiver for a simulation test, with a bit rate of 500kbit/s, and the waveform is shown in the figure below.

CAN bus 220pf without terminal resistor

CAN bus 220pf increase terminal resistance

2. Improve signal quality

When the signal is at a higher conversion rate, signal reflection will occur when the signal edge energy encounters impedance mismatch; if the geometric structure of the transmission cable cross-section changes, the characteristic impedance of the cable will change accordingly, which will also cause reflection.

At the end of the bus cable, the impedance changes sharply, causing the signal edge energy to be reflected, and ringing will occur on the bus signal. If the ringing amplitude is too large, it will affect the communication quality. Adding a terminal resistor that is consistent with the characteristic impedance of the cable at the end of the cable can absorb this part of the energy and avoid the generation of ringing, as shown in the following figure.

CAN bus without terminal resistor

Add terminal resistance to CAN bus

Two 485 buses

1. Benefits of adding terminal resistance to RS-485 bus

The terminal resistance of the 485 bus is mainly used to suppress signal reflection.

Improve signal quality When setting up an RS-485 bus network, a shielded twisted pair with a characteristic impedance of 120Ω is usually used. Since the input impedance of the RS-485 transceiver is generally high (for example, the input impedance of RSM485ECHT is 96kΩ, and a maximum of 256 nodes can be connected), when the signal is transmitted to the end of the bus, the instantaneous impedance will change suddenly (taking RSM485ECHT as an example, the impedance changes from 120Ω to 96kΩ), causing the signal to reflect and affect the signal quality. The waveforms of RSM485ECHT without and with terminal resistors at a communication rate of 1200m and 500kbps are shown in the figure below. The terminal resistor significantly improves the signal quality.

RSM485ECHT 1200m 500kbps without terminal resistor

RSM485ECHT 1200m 500kbps plus terminal resistor

2. The problem of increasing terminal resistance in RS-485

Although terminal resistance can improve signal quality, it also has the following problems:

Reduce the amplitude of the driving signal

The greater the load on the RS-485 bus, the lower the amplitude of the differential voltage output by the RS-485 transceiver. The waveforms of the RSM485ECHT without and with terminal resistance at 5m and 500kbps are shown in the figure below.

RSM485ECHT 5m 500kbps without termination

RSM485ECHT 5m 500kbps plus terminal

Increased voltage drop on communication lines

Increasing the terminal resistance increases the current on the communication cable, resulting in a larger voltage difference, which reduces the signal amplitude at the receiving end. The signal waveforms of the RSM485ECHT at the head and tail ends of 1200m, 115.2kbps are shown in the figure below (0.75mm2 communication line). The signal at the end drops by about 0.7V compared to the signal at the head end.

RSM485ECHT 1200m 115.2kbps plus terminal resistor first end waveform

RSM485ECHT 1200m 115.2kbps Add terminal resistor end waveform

Increased power consumption of the transceiver

Adding terminal resistance has little effect on the working current in the receiving state, but it will greatly increase the working current in the driving state. Taking RSM485ECHT as an example, the working current of RSM485ECHT is about 20mA when it is in the receiving state, about 27mA when it is in the driving state without adding terminal resistance, and about 83mA when it is in the driving state with adding terminal resistance. It can be seen that the terminal resistance greatly increases the power consumption of RS-485 transceiver. For applications with power consumption requirements, terminal resistance should be used with caution.

3. RS-485 terminal resistance setting

It is recommended not to add terminal resistance when the communication speed is low or the communication distance is short.

When the communication speed is low or the communication distance is short, signal reflection has little effect on the communication signal, and not adding terminal resistance can greatly reduce power consumption. By adding larger pull-up and pull-down resistance values, the RS-485 bus can have a higher differential voltage amplitude when idle, thereby improving the reliability of communication.

The communication distance is long and the communication speed is fast, and the signal quality requirement is high.

At this time, the terminal resistance can be increased to prevent signal reflection problems caused by impedance mutation and improve signal quality, but it should be ensured that the differential voltage of the bus is not within the threshold level range when the bus is idle.

When power consumption is high and the communication distance is long

As reflected signals bounce back and forth on the bus, they are gradually consumed.

RSM485ECHT 1200m 9600bps without terminal headend waveform

RSM485ECHT 1200m 9600bps without terminal end waveform

Conclusion

For the CAN bus, in order to ensure the stability of communication, terminal resistance must be added; RS-485 bus is different. The terminal resistance needs to be set according to the situation. It is recommended to reserve the terminal resistance position in the design or use ZLG Zhiyuan Electronics isolation transceiver series products, which can effectively solve bus interference, communication anomalies and other problems. Compared with the traditional design, it has a built-in complete isolation DC-DC circuit, signal isolation circuit, bus transceiver circuit and protection circuit, with higher integration and reliability, suitable for occasions requiring high-stability communication, and can effectively help you improve the bus communication protection level.