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I want to use the SN65HVD31DR to connect two microcontrollers together. The connecting cable has an impedance of Z0 = 80Ω. According to the data sheet it is recommended to terminate the connection cable on both sides. So there would be two 80Ω resistors connected in parallel which would effectively make a parallel resistance of 40Ω. According to the datasheet the differential load resistance RL must not be less than 54Ω.

So is it right that I am not allowed to terminate the connection line with two 80Ω resistors and I have to use a cable with a higher impedance so I can use higher termination resistors? Or do I misunderstand something?

Schematic Datasheet

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    \$\begingroup\$ You don't have to calculate the equivalent resistance of the termination resistors, You just have to use the correct termination resistor for the cable at each end. \$\endgroup\$ – Lior Bilia Feb 24 '19 at 16:20
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You don’t misunderstand it as far as I can tell. The load of 40 ohms is a little too heavy for the device in question so, take a look at figure 37 (on the use of protection devices) and note that 10 ohm resistors are inserted in the driver output lines. If you can implement this (with or without the TVS diodes) then it might be a way forward for you.

You should also note that using a terminating resistor that is a little higher in value won’t degrade the signal quality too much. Maybe also consider 100 ohm terminators.

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