In CAN-bus circuit design, theoretically, a transceiver can support up to 110 nodes, but in practical applications, this number is often not achieved. Here, we discuss how to ensure the reliability of communication and the number of nodes in a CAN network through reasonable CAN-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 will discuss the factors that meet the differential voltage amplitude of the receiving node. Only when this prerequisite is met can we consider the influence of other factors of the bus, such as parasitic capacitance and parasitic inductance, on the signal.
1) CAN interface load of the transmitting node
Why consider CAN interface load?
The CAN interface load is the effective resistance value between CANH and CANL. This resistance affects the amplitude of the differential voltage output by the transmitting 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
As the load resistance increases from 45Ω to 66Ω, the node's output differential voltage also increases from 1.84V to 2.16V , showing an approximately linear relationship. To prevent the output differential voltage of the transmitting node from becoming too low, the load resistance should fluctuate within the range tested in Figure 1 during actual network deployment. We analyze that the RL (Transmission Resistor) consists of three components: the terminating resistor, the differential input resistance of the bus node, and the effective resistance of the bus itself.
Termination resistors: Termination resistors need 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 value of the termination resistor can be appropriately increased to reduce the loss of the effective resistance of the bus, such as 150Ω~300Ω.
Differential input resistance: ISO 11898 specifies that the differential input resistance of transceivers is between 10kΩ and 100kΩ. The differential input resistance of the CTM1051M series transceivers is 19kΩ to 52kΩ, with a typical value of 30kΩ. If we network with the most nodes, considering the typical value, the differential input resistance of the entire bus will reach 30kΩ/110 = 273Ω. When connected in parallel with the terminating resistor, it will significantly increase the load on the nodes.
Effective bus resistance: When using twisted-pair cables with a small cross-sectional area, the effective resistance can reach tens of ohms. In long-distance communication, the bus can significantly affect the differential signal. For example, the resistance of commonly used RVS unshielded twisted-pair cables ranges from 8.0 Ω/km to 39.0 Ω/km. In severe cases, this can cause the receiver level to fall below the recognition range.
In addition to the influence of the load resistance, the differential voltage is also affected by the supply voltage. As shown in Figure 2, we tested the differential voltage amplitude of the CTM1051M module under different voltages and loads. It can be seen that when the supply voltage increases by 0.5V, the differential voltage amplitude will increase by about 0.3V .
Figure 2 Differential voltage under different supply voltages
2) Identification level of the receiving node
The receiving node has a certain voltage level recognition range. Typical parameters of the CTM1051M's CAN interface are shown in Table 1. The dominant voltage level of the node input should be greater than 0.9V . In ISO11898, the minimum voltage level at any point on the bus should be greater than 1.2V. When networking, we should ensure that the differential voltage is greater than this value.
Table 1 Typical parameters of CAN interface
2. Actual Network Topology Analysis
The maximum number of network nodes for the transceiver is currently 110. When setting up the network, we consider the above resistance parameters to ensure that the differential voltage on the bus is within a reasonable range.
Figure 3 shows the recommended network topology for the CTM1051M. We need to consider the bus resistance, terminating resistance, and voltage parameters at the transmitting and receiving points. The equivalent circuit is shown in Figure 4.
Figure 3 Recommended Network Setup for CTM1051M
Figure 4. Equivalent circuit of CTM1051M networking
Based on the equivalent circuit, the parameters we can adjust are the terminating resistor RT, the transmitting node voltage VOUT, and the effective bus resistance RW.
In Figure 4, the RW and RIN of each node are difficult to determine accurately, and calculating them using formulas during network setup is quite cumbersome. A simpler method is to measure the node voltages at both ends of the bus. If the bus resistance of the network is too high, the signal loss on the bus from node 1 to node n will be significant. When the differential voltage received by node n is lower than 1.2V , the terminating resistor needs to be increased.
In applications where surge suppressors are used, such as adding an SP00S12 signal surge suppressor between nodes 1 and 2 in Figure 4, its DC equivalent resistance is 9.5Ω , which can be considered as 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 terminating resistance at node 1.
3. Summary
Regardless of the length of the bus network, terminating resistors are required at both ends of the network.
When the communication distance is long, the terminal resistance value should be increased appropriately to reduce the signal attenuation caused by the bus resistance, such as 150Ω~300Ω;
In situations with strong interference, use shielded twisted-pair cables, with the shielding layer connected to ground at a single point.
The differential level output of the transceiver's CAN interface will vary with the supply voltage; therefore, the supply voltage should be ensured to be within the range specified in the manual.
4. New generation high-reliability CAN-bus isolated transceiver module
In outdoor or other complex industrial 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 overvoltages on the signal transmission system. For these harsh application environments, Zhiyuan Electronics has launched the new generation high-protection-level isolated CAN transceiver CTM1051HP. Based on a standard isolation transceiver, it integrates the functions of a bus protector, effectively preventing the impact of lightning strikes, surges, and overvoltages on the CAN bus, greatly enhancing bus reliability. Furthermore, the CTM1051HP maintains the same size as conventional CAN isolation transceivers, making it widely applicable in various compact or space-constrained products, and also serving as an optimization solution for existing CAN isolation transceiver circuits.
