Datasheet

APPLICATION INFORMATION
CAN Basics
SN65HVD1040
SLLS631D – MARCH 2007 – REVISED DECEMBER 2008 ............................................................................................................................................... www.ti.com
The basics of arbitration require that the receiver at the sending node designate the first bit as dominant or
recessive after the initial wave of the first bit of a message travels to the most remote node on a network and
back again. Typically, this “ sample ” is made at 75% of the bit width, and within this limitation, the maximum
allowable signal distortion in a CAN network is determined by network electrical parameters.
Factors to be considered in network design include the approximately 5 ns/m propagation delay of typical
twisted-pair bus cable; signal amplitude loss due to the loss mechanisms of the cable; and the number, length,
and spacing of drop-lines (stubs) on a network. Under strict analysis, variations among the different oscillators in
a system also need to be accounted for with adjustments in signaling rate and stub and bus length. Table 5 lists
the maximum signaling rates achieved with the SN65HVD1040 with several bus lengths of category 5, shielded
twisted pair (CAT 5 STP) cable.
Table 5. Maximum Signaling Rates for Various Cable
Lengths
Bus Length (m) Signaling Rate (kbps)
30 1000
100 500
250 250
500 125
1000 62.5
The ISO 11898 Standard specifies a maximum bus length of 40 m and maximum stub length of 0.3 m with a
maximum of 30 nodes. However, with careful design, users can have longer cables, longer stub lengths, and
many more nodes to a bus. (Note: Non-standard application may come with a trade-off in signaling rate.) A large
number of nodes requires a transceiver with high input impedance such as the HVD1040.
The Standard specifies the interconnect to be a single twisted-pair cable (shielded or unshielded) with 120 Ω
characteristic impedance (Z
O
). Resistors equal to the characteristic impedance of the line terminate both ends of
the cable to prevent signal reflections. Unterminated drop-lines connect nodes to the bus and should be kept as
short as possible to minimize signal reflections.
Connectors, while not specified by the standard should have as little effect as possible on standard operating
parameters such as capacitive loading. Although unshielded cable is used in many applications, data
transmission circuits employing CAN transceivers are usually used in applications requiring a rugged
interconnection with a wide common-mode voltage range. Therefore, shielded cable is recommended in these
electronically harsh environments, and when coupled with the Standard ’ s – 2-V to 7-V common-mode range of
tolerable ground noise, helps to ensure data integrity. The HVD1040 enhances the Standard ’ s insurance of data
integrity with an extended – 12 V to 12 V range of common-mode operation.
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