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SN65HVD3088E: Questions about termination

Motoyasu Hattori
Genius 11225 points
Part Number: SN65HVD3088E


Hi,
My customer has the following questions. Please answer it.

Termination resistance RL that determines the differential output voltage

(1) In the circuit shown in your data sheet Figure 8, the differential output voltage is 2.3V (typ) when the terminating resistance is RL=54Ω, but how much is output when RL=33.6Ω?
Also, if you select a terminal resistance value other than those listed in the data sheet, may the operation be out of warranty?

(2) It seems that the differential voltage difference will be reduced by making the resistance value of RL smaller than 55Ω. Is there a possibility that wrong operation(H, L error) will occur between SN65HVD3088 transmitter and receiver?

(3) Is half-duplex communication with 110Ω on one side and 55Ω on one side with the circuit shown in the data sheet fig.20, is it operable and within the guaranteed operation?
He is considering Vcc=5V, transmission rate=10Mbps.

Thanks and best regards,
M.HATTORI.

  • 0
    TI__Guru 62920 points

    Hello Hattori-san,

    Due to the non-zero output resistance of the driver circuit, the differential output voltage will decrease as the output current increases.  Lower termination resistance results in higher output currents and thus lower output voltage.  The relationship can be seen in Figure 5 of the datasheet.  The SN65HVD3088E is specified to meet the requirements of RS-485 and RS-422, which require minimum output voltages of 1.5 V across a 54-Ohm bus load and 2 V across a 100-Ohm bus load (respectively).  However, the device is able to operate with higher loads if the reduced output amplitude is acceptable to the application.

    There are some things to consider when operating with lower termination resistances:

     - The receiver input sensitivity is fixed, so with a lower differential signal output level the amount of signal loss that could be allowed in a cable would be lessened.  This could be an issue for very long-reach applications.

     - When sourcing higher output currents, the power dissipation in the driver is increased.  This could be an issue for high-temperature applications.

    Please let me know if the information is not clear or if you have further questions.

    Regards,
    Max