How to ensure the reliability and number of nodes in CAN network communication

In CAN-bus circuit design, the transceiver can theoretically support up to 110 nodes, but this number is often not reached in practical applications. Here we will talk about how to ensure the reliability of communication and the number of nodes in the CAN network through reasonable CAN-bus bus design.

1. Factors affecting the number of CAN bus nodes

There are many factors that affect the number of bus nodes. In this article, we discuss the differential voltage amplitude that satisfies the receiving node. Only when this prerequisite is met can we consider the impact of other bus factors such as parasitic capacitance and parasitic inductance on the signal.

1) CAN interface load of the sending node

Why consider CAN interface load?

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The CAN interface load is the effective resistance value between CANH and CANL. This resistance will affect the amplitude of the differential voltage output by the sending node. After networking, the load resistance RL of each node in the network is close. As shown in Figure 1, we tested the output differential voltage amplitude of the CTM1051M small-volume CAN isolation module under different loads.

Figure 1 Differential voltage under different loads

When the load resistance increases from 45Ω to 66Ω, the output differential voltage of the node also increases from 1.84V to 2.16V, and the two are approximately linear. In order to prevent the output differential voltage of the sending node from being too low, the load resistance should fluctuate within the test range of Figure 1 in actual networking. We analyze that RL consists of three components: terminal resistance, differential input resistance of bus nodes, and effective resistance of the bus itself.

Terminal resistance: Terminal resistance needs to be added at both ends of the bus. When the bus distance is long, the effective resistance of the bus is large and the loss is large. The terminal resistance value can be appropriately increased to reduce the loss of the effective resistance of the bus, such as 150Ω~300Ω.

Differential input resistance: The differential input resistance range of transceivers specified in ISO 11898 is between 10kΩ and 100kΩ. The differential input resistance of the CTM1051M series transceivers is 19kΩ to 52kΩ, and its typical value is 30kΩ. If we network with the maximum number of nodes and consider the typical value, the differential input resistance of the entire bus will reach 30kΩ/110=273Ω, which will significantly increase the node load when connected in parallel with the terminal resistance.

Bus effective resistance: When using a twisted pair with a smaller cross-sectional area, its effective resistance reaches tens of ohms. For long-distance communication, the bus will have a great impact on the differential signal. For example, the resistance of the commonly used RVS unshielded twisted pair ranges from 8.0Ω/km to 39.0Ω/km. In severe cases, the level of the receiving point will not reach the recognition range.

In addition to the influence of load resistance, the differential voltage is also affected by the power supply voltage. As shown in Figure 2, we tested the differential voltage amplitude of the CTM1051M module under different voltages and different loads. It can be seen that when the power supply voltage increases by 0.5V, the differential voltage amplitude will increase by about 0.3V.

Figure 2 Differential voltage at different supply voltages

2) Receiving node recognition level

The receiving node has a certain level recognition range. The typical parameters of the CAN interface of CTM1051M are shown in Table 1. The node input dominant level should be greater than 0.9V. In ISO 11898, the minimum level of any point on the bus should be greater than 1.2V. When networking, we should make the differential voltage greater than this value.

Table 1 Typical parameters of CAN interface

2. Actual Network Analysis

Currently, the maximum number of networking nodes of the transceiver is 110. When networking, we consider the above resistance parameters to ensure that the differential voltage on the bus is within a reasonable range.

Figure 3 shows the network topology recommended by CTM1051M. We need to consider bus resistance, terminal resistance, and voltage parameters of the sending and receiving points. Draw its equivalent circuit as shown in Figure 4.

Figure 3 CTM1051M recommended networking

Figure 4 CTM1051M network equivalent circuit

According to the equivalent circuit, the parameters we can adjust are the terminal resistance RT, the sending node voltage VOUT, and the bus effective resistance RW.

In Figure 4, it is difficult to accurately determine the RW and RIN of each node. It is cumbersome to calculate with formulas when networking. A simple method is to measure the node voltages at both ends of the bus. If the bus resistance of the network is too large, the signal loss from node 1 to node n will be large. When the differential voltage received by node n is lower than 1.2V, the terminal resistance needs to be increased.

When a surge suppressor is used, for example, a SP00S12 signal surge suppressor is added between node 1 and node 2 in FIG4 , and its DC equivalent resistance is 9.5Ω, it can be equivalent to the effective resistance of the bus. When the voltage received by node 1 is too low, the loss caused by the surge suppressor can be compensated by reducing the effective resistance of the bus and increasing the terminal resistance at node 1.

3. Summary

Regardless of the length of the bus network, terminal resistors are required at both ends of the network;

When the communication distance is long, increase the terminal resistance value appropriately to reduce the attenuation of the bus resistance on the signal, such as 150Ω~300Ω;

Use shielded twisted pair cables in situations with strong interference, and connect the shielding layer to the ground at a single point;

The differential level output by the transceiver CAN interface will change with the change of the power supply voltage. Make sure that the power supply voltage is within the range specified in the manual.

4. New generation of high reliability CAN-bus isolated transceiver module

In outdoor or other complex industrial field environments, in addition to using CAN bus isolation transceivers for signal isolation transmission, it is also necessary to consider how to avoid the impact of harmful signals such as lightning strikes, surges, and overvoltage on the signal transmission system. In response to such harsh application environments, Zhiyuan Electronics has launched a new generation of high-protection-level isolation CAN transceiver CTM1051HP, which combines the functions of a bus protector on the basis of ordinary isolation transceivers, can effectively avoid the impact of lightning strikes, surges, and overvoltage on the CAN bus, and greatly enhance the reliability of the bus. In addition, compared with conventional CAN isolation transceivers, CTM1051HP has the same volume, which can be widely used in various high-volume or compact products, and can also be used as an optimization solution for existing CAN isolation transceiver circuits.