NI NI-9853, Руководство пользователя

Страница: 9 / 16

Characteristic Value
120 Ω nominal,
140 Ω max
Length-related resistance 70 mΩ/m nominal
Specific line delay 5 ns/m nominal
Table 3. ISO 11898 Specifications for Characteristics of a CAN_H and CAN_L Pair of Wires
Termination Resistors
The termination resistors (Rt) should match the nominal impedance of the CAN
cable and therefore comply with the values in the following table.
Characteristic Value Condition
Termination resistor, R
t
100 Ω min, Minimum power dissipation: 220 mW
120 Ω nominal,
130 Ω max
Table 4. Termination Resistor Specification
Cable Lengths
The cabling characteristics and desired bit transmission rates affect the allowable
cable length. You can find detailed cable length recommendations in the ISO 11898,
CiA DS 102, and DeviceNet specifications.
ISO 11898 specifies 40 m total cable length with a maximum stub length of 0.3 m for
a bit rate of 1 Mb/s. The ISO 11898 specification says that significantly longer cable
lengths may be allowed at lower bit rates, but you should analyze each node for
signal integrity problems.
Number of CAN Nodes
The maximum number of nodes depends on the electrical characteristics of the
nodes on the network. If all nodes meet the ISO 11898 requirements, you can
connect at least 30 nodes to the bus. You can connect higher numbers of nodes if
the nodes’ electrical characteristics do not degrade signal quality below ISO 11898
signal level specifications.
The NI-9853 electrical characteristics allow at least 110 CAN ports on a network.
© National Instruments
9
NI-9853