9. Bus network aspects of high speed CAN-Toy模板网

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AH1014 Systems & Applications, Automotive Innovation Center

9.1 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 50. For worst case consideration the bus line resistance RW is considered to be zero. This leads to the following relations for calculating the maximum number of nodes:
9. Bus network aspects of high speed CAN

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.
9. Bus network aspects of high speed CAN

9.2 Maximum bus line length

The maximum achievable bus line length in a CAN network is determined essentially by the 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 (for a detailed description refer to [21]).

Effects 1 and 2 result in a value for the maximum bus line length with respect to the CAN bit timing [21]. 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.

9. Bus network aspects of high speed CAN

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 [20] standard 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 quality because of inadequate termination can lower the maximum allowable bus line length.

9.3 Topology

The topology describes the wiring harness structure. Typical structures are linear, star- or multistar-like. In automotive, shielded or unshielded twisted pair cable usually functions as a transmission line. Transmission lines are generally characterized by the lengthrelated resistance RLength, the specific line delay tdelay and the characteristic line impedance Z. Table 17 shows the physical media parameters specified in the ISO11898 and SAE J2284 standard. Notice that SAE J2284 specifies the twist rate rtwist in addition.
9. Bus network aspects of high speed CAN

Ringing due to signal reflections

Transmission lines must be terminated with the characteristic line impedance, otherwise signal reflections will occur on the bus causing significant ringing. The topology has to be chosen such that reflections will be minimized. Often the topology is a trade-off between reflections and wiring constraints.

CAN is well prepared to deal with reflection ringing due to some useful protocol features:

• Only recessive to dominant transitions are used for resynchronization.

• Resynchronization is allowed only once between the sample points of two bits and only, if the previous bit was sampled and processed with recessive value.

• The sample point is programmable to be close to the end of the bit time.

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Linear topology

The high speed CAN standard ISO11898 defines a single line structure as network topology. The bus line is terminated at both ends with a single termination resistor. The nodes are connected via not terminated drop cables or stubs to the bus. To keep the ringing duration short compared to the bit time, the stub length should be as short as possible. For example the ISO11898 standard limits the stub length to 0.3 m at 1 Mbit/s. The corresponding SAE standard, J2284-500, recommends keeping the stub length below 1 m. To minimize standing waves, ECUs should not be placed equally spaced on the network and cable tail lengths should not all be the same [20] . Table 18 along with Fig 51 illustrate the topology requirements of the SAE J2284-500 standard. At lower bit rates the maximum distance between any two ECUs as well as the ECU cable stub lengths may become longer.
9. Bus network aspects of high speed CAN

In practice some deviation from that stringent topology proposals might be necessary, because longer stub lengths are needed. Essentially the maximum allowable stub length depends on the bit timing parameters, the trunk cable length and the accumulated drop cable length.

The star topology is neither covered by ISO11898 nor by SAE J2284. However, it is sometimes used in automotive applications to overcome wiring constraints within the car. Generally, the signal integrity suffers from a star topology compared to a linear topology
9. Bus network aspects of high speed CAN