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I understand the reason for choosing a termination resistor of 120 Ohms for the CAN bus to avoid reflections but why was the nominal impedance of the CAN bus cables required to be 120 Ohms by the ISO 11898-2?

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    \$\begingroup\$ It was conveniently close to what some wire pair gave for impedance , based on ratios of conductor to insulator. Ribbon wire , twisted pair etc. \$\endgroup\$ – Tony Stewart EE75 Jul 31 '20 at 20:41
  • \$\begingroup\$ I think 93 ohms was common at one time for low capacitance coax on arcnet or something like that. Ribbon cable was 120 ohms and many others. due to voltage rating. so not 220 ,100 or 75 or 50 \$\endgroup\$ – Tony Stewart EE75 Jul 31 '20 at 20:46
  • \$\begingroup\$ @TonyStewartSunnyskyguyEE75 that's an answer, may as well make it one. \$\endgroup\$ – TimWescott Jul 31 '20 at 20:52
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You mean, why not 220 ,100 or 75 or 50 Ohms?

It was conveniently close to what some wire pair gave for impedance , based on ratios of conductor to insulator diameter for certain Ribbon wire, twisted pair etc.

I think 93 ohms was common at one time for low capacitance coax on arcnet or something like that. Ribbon cable was 120 ohms and many others, due to voltage rating and thus insulation to conductor ratio. Thinner insulation with a lower gap between conductors is lower impedance so 120 Ohms is I recall the typical 60~70 pF/m (est.) and 500 to 1000 nH/m (ballpark) but \$Z_o=\sqrt{\frac{L}{C}}\$ so I'll let you work it out.

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The impedance (odd mode impedance to be exact) of most twisted pair cable is around this value - its not an exact figure - it doesn't have to be - see my calculation below.

100 ohm is close enough! Calculate VSWR for given load and source

enter image description here

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