Datasheet

500 mV Threshold
900 mV Threshold
ALLOWABLE JITTER
NOISE MARGIN
NOISE MARGIN
RECEIVER DETECTION WINDOW
75% SAMPLE POINT
SN55HVD251
SN65HVD251
SLLS545E –NOVEMBER 2002–REVISED MARCH 2010 www.ti.com
APPLICATION INFORMATION
oscillators in a system must also be accounted for
The basics of bus arbitration require that the receiver
with adjustments in signaling rate and stub & bus
at the sending node designate the first bit as
length. Table 4 lists the maximum signaling rates
dominant or recessive after the initial wave of the first
achieved with the SN65HVD251 in high-speed mode
bit of a message travels to the most remote node on
with several bus lengths of category-5, shielded
a network and back again. Typically, this sample is
twisted-pair (CAT 5 STP) cable.
made at 75% of the bit width, and within this
limitation, the maximum allowable signal distortion in
Table 4. Maximum Signaling Rates for Various
a CAN network is determined by network electrical
Cable Lengths
parameters.
BUS LENGTH (m) SIGNALING RATE (kbps)
Factors to be considered in network design include
30 1000
the 5 ns/m propagation delay of typical twisted-pair
100 500
bus cable; signal amplitude loss due to the loss
250 250
mechanisms of the cable; and the number, length,
and spacing of drop-lines (stubs) on a network. Under
500 125
strict analysis, variations among the different
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 on a bus. (Note: Non-standard application may come with a trade-off in signaling rate.) A bus
with a large number of nodes requires a transceiver with high input impedance such as the HVD251.
The Standard specifies the interconnect to be a single twisted-pair cable (shielded or unshielded) with 120-Ω
characteristic impedance (Zo). 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 ISO 11898 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 –2-V to 7-V common-mode range of tolerable
ground noise specified in the standard, helps to ensure data integrity. The HVD251 extends data integrity beyond
that of the standard with an extended –7-V to 12-V range of common-mode operation.
Figure 24. Typical CAN Differential Signal Eye-Pattern
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