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SN65HVD08: RS-485 transceivers with 3.3V and 5V supply

Danilo A.
Mastermind 26350 points
Part Number: SN65HVD08
Other Parts Discussed in Thread: THVD1420

Hi Team,

I understand from the E2E blog below that RS-485 transceivers with 3.3V and 5V supply comply with the RS-485 standard. Our customer's application needs long distance RS485 communication. He would like to know if the RS-485 transceivers with 3.3V and 5V supply have the same distance coverage. If not, which one has longer distance coverage? Do we have some documents comparing the performance of the two?

https://e2e.ti.com/support/interface-group/interface/f/interface-forum/93260/rs-485-compatibility-between-3-3v-supply-and-5v-supply-devices

Regards,

Danilo

  • +1
    Guru 273200 points

    The RS-485 specification requires a certain minimum drive strength. All RS-485 transceivers, including 3.3 V ones, comply with the specification.

    Section 9.2.1.1 of the SN65HVD08 datasheet shows the relationship between cable length and data rate. It's the same as for 5 V transceivers.

    Please note that the more modern THVD1420 is cheaper.

  • +1
    TI__Mastermind 46477 points

    Hi Danilo,

    So there is a difference in distance - but its not very straight-forward.

    What Clemens said is correct in that - both 3.3V and 5V RS-485 transceivers both meet the minimum requirements and the data-rate vs. bus length chart in section 9.2.1.1 of the datasheet is assuming minimum conditions (1.5V across 54 Ohm load). However reality is a bit different - as the effective max bus length will increase with an increase in output voltage - the output voltage is derived from VCC - so higher VCC on modern parts will output higher VOD's which will increase effective bus length. The caveat is the vast majority of our transceivers will have their minimum VOD be 1.5V regardless of 3.3V or 5V supply - so it will really will be a case by case basis which supply will actually net you the largest max effective bus length - but they will comply with minimum. Also something important to note is that some older 5V devices don't have a very large VOD - and could be comparable to more modern 3.3V devices VODs - so it really is about output voltage - which generally is higher with higher VCC - but some older devices do not follow that rule completely.

    Please let me know if you have any other questions!

    Best,

    Parker Dodson