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SN65HVD1782: [Vod] checking

Allen Lu
Intellectual 2730 points

Part Number: SN65HVD1782
Other Parts Discussed in Thread: THVD2410, THVD1400

Tool/software:

Hi team,

 i am checking the Vod setting of RS485 on different device.

 i see the TIA/EIA standard have demand on Vod on 1.5V min at least. but i am seeing the SN65HVD1782 device have very low Vod than standard.

my question is:

1. why RL = 54 Ω, 3.15 V ≤ VCC ≤ 3.45 V condition will under spec of standard, does it still work as RS485 standard?

2. why another device doesn't mention the condition of 3.15 V ≤ VCC ≤ 3.45 V like thvd2410

3. what`s the limit and concern when design the Vod range on IC definition ? 

  • 0
    Guru 319830 points

    The standard assumes that you are using 5 V transceivers. With 3.3 V, Vod is lowered to allow some headroom to the supply.

    This is not a problem in practice because all receivers have a wide common-mode range.

  • 0
    TI__Guru* 78111 points

    Hi Allen,

    I think HVD1782 was designed around 5V Vcc and they spec'd the VoD at 3.3V without regard to the RS485 standard. Basically saying the device could be operational at 3.3V but not compliant to the RS485 standard at 3.3V. 

    Allen Lu said:
    1. why RL = 54 Ω, 3.15 V ≤ VCC ≤ 3.45 V condition will under spec of standard, does it still work as RS485 standard?

    RS485 receivers require +/-200mV so as long as this is met, the receivers on the bus should still operate with HVD1782 when its used at 3.3V but the distance and data rate supported at 3.3V likely won't be as good as 5V. 

    Allen Lu said:
    2. why another device doesn't mention the condition of 3.15 V ≤ VCC ≤ 3.45 V like thvd2410

    THVD14XX was designed with 3.3V operation in mind with the goal of meeting RS485 standard.

    Allen Lu said:
    3. what`s the limit and concern when design the Vod range on IC definition ? 

    If VoD is lower, the distance supported is lower. You can think of the cable as a resistor. If you have termination at the end, then there is a voltage divider.

    A---cable resistance------

                                            |

                                         120 ohm resistor

                                            |

    B---cable resistance-------

    The voltage between the 120 ohm resistor will be smaller because VoD is smaller. So at longer distances, lower VoD will affect the voltage drop across the receiving node.

    -Bobby

  • 0 in reply to BOBBY
    TI__Intellectual 2730 points

    HI Boddy,

     tks here. it is clear of the HVD1782  VS standard behavior.

    but we can see the THVD1400 do not mention the Vod on VCC3.3V here.  

    So i purpose if device work with standard , they need to work at 5V to get higher Vod, But we can not guarantee the Vod behavior on 3v3VCC?

  • 0 in reply to Allen Lu
    TI__Guru* 78111 points

    Allen,

    The top VOD spec uses 3V to 3.6V for the Vcc. So the minimum voltage with 60 ohms is 1.5V which we guarantee. I verified this by looking at the char data for this device. 

    For THVD14xx, it was designed to have 1.5V or high VoD at 3V or higher. The older HVD1782 was not designed around 3V Vcc.

    -Bobby