In the CAN-bus circuit design, in theory, the number of nodes supported by the transceiver can be up to 110, but this number is often not achieved in practical applications. Today we will talk about how to ensure the reliability and number of nodes in the CAN network through a reasonable CAN-bus bus design.
Factors affecting the number of CAN bus nodes
There are many factors affecting the number of bus nodes. In this paper, we discuss the differential voltage amplitude of the receiving node. Only when this precondition is met, we can consider other factors such as parasitic capacitance and parasitic inductance on the signal.
1, the CAN interface load of the sending node
Why consider the CAN interface load?
The CAN interface load is the effective resistance value between CANH and CANL. This resistor affects the amplitude of the differential voltage output by the transmitting node. After the networking, the load resistance RL of each node in the network is close.
2. Identification level of the receiving node
The receiving node has a certain level identification range. The typical parameters of the CTM1051M CAN interface 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.
Actual networking analysis
At present, the maximum number of networking nodes of the transceiver is 110. When considering networking, we consider the above resistance parameters to ensure that the differential voltage on the bus is within a reasonable range.
According to the equivalent circuit, the parameters that we can adjust are terminal resistance RT, transmitting node voltage VOUT, and bus effective resistance RW.
In Figure 4, the RW and RIN of each node are difficult to determine accurately. It is cumbersome to calculate by formula when networking. The simple method is to measure the node voltage at both ends of the bus. If the bus resistance of the network is too large, the loss of the signal from the node 1 to the node n bus will be large. When the differential voltage received by the node n is lower than 1.2V, the termination resistance needs to be increased.
In the case of using a surge suppressor, for example, adding a SP00S12 signal surge suppressor between node 1 and node 2 of FIG. 4, the DC equivalent resistance is 9.5 Ω, which can be equivalent to the effective resistance of the bus. When the voltage received by node 1 is too low, the loss due to the surge suppressor can be compensated by reducing the effective resistance of the bus and increasing the termination resistance at node 1.
To sum up
Regardless of the length of the bus network, terminal resistors need to be added at both ends of the network;
When the communication distance is long, the terminal resistance value is appropriately increased to reduce the attenuation of the signal by the bus resistance, such as 150 Ω to 300 Ω;
Shielded twisted pair is used in places with strong interference, and the shield is grounded at a single point;
The differential level of the CAN interface output of the transceiver will vary with the supply voltage. Make sure that the supply voltage is within the range specified in the manual.
