Quote:
The High-Speed ISO 11898 Standard specifications are given for a maximum signaling rate of 1 Mbps with a bus length of 40 m and a maximum of 30 nodes. It also recommends a maximum un-terminated stub length of 0.3 m. The cable is specified to be a shielded or unshielded twisted-pair with a 120 ohm characteristic impedance*. The Standard defines a single line of twisted-pair cable with the network topology as shown in Figure 6. It is terminated at both ends with 120ohm resistors, which match the characteristic impedance of the line to prevent signal reflections.
source: http://www.ti.com/lit/an/slla270/slla270.pdf, page 5
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*The
characteristic impedance of the cable cannot be measured with an ohmmeter.