是什么决定了CAN网络的最大节点数量和最大总线长度？

本文选自NXP的《AH1014 Application Hints - Standalone high speed CAN transceiver》

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Maximum number of nodes

The number of nodes, which can be connected to a bus, depends on the minimum load resistance a transceiver can drive. 

NXPs 3rd generation high speed CAN transceivers provide an output drive capability down to a minimum load of RL,min = 45Ohm for VCC >4,5 V (4,75V for the TJA1048). 

The overall busload is defined by the termination resistance RT, the bus line resistance RW and the transceiver's differential input resistance Ri(dif). The DC circuit model of a bus system is shown in Fig 48. 

For worst case consideration the bus line resistance RW is considered to be zero. This leads to thefollowing relations for calculating the maximum number of nodes:

Table 15 gives the maximum number of nodes for two different termination resistances. Notice that connecting a large number of nodes requires relatively large termination resistances.

Maximum bus line length

The maximum achievable bus line length in a CAN network is determined essentially bythe following physical effects:

1. Loop delays of the connected bus nodes (CAN controller, transceiver etc.) and the delay of the bus line.

2. Relative oscillator tolerance between nodes.

3. Signal amplitude drop due to the series resistance of the bus cable and the input resistance of bus nodes .

Effects 1 and 2 result in a value for the maximum bus line length with respect to the CAN bit timing. 

Effect 3, on the other hand, results in a value with respect to the output signal drop along the bus line. 

The minimum of the two values has to be taken as the actual maximum allowable bus line length. As the signal drop is only significant for very long lengths, effect 3 can often be neglected for high data rates.

Table 16 gives the maximum bus line length for the bit rates 125 kbit/s, 250 kbit/s and 500 kbit/s, along with values specified in the SAE J2284 standards associated to CAN.

The calculation is based on effects 1 and 2 assuming a minimum propagation delay between any two nodes of 200 ns and a maximum bus signal delay of 8 ns/m. Notice that the stated values apply only for a well-terminated linear topology. Bad signal qualitybecause of inadequate termination can lower the maximum allowable bus line length。