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RS485 internal function

zanzibar
Intellectual 605 points

Hi,


Regarding HVD08, I see the following logic diagram :

Can you explain me the function of the part highlighted in yellow ?

Thanks !

  • 0
    Intellectual 605 points


    This is the logical NAND function with DE and ~RE as inputs.

  • 0 in reply to zanzibar
    TI__Mastermind 21811 points

    Hi Zanzibar,

    This gate enables the user to have all combinations of the driver and receiver enabled and disabled. I do think that the picture in the datasheet can be confusing, but it operates as any normal transceiver. I will check back with the members of the team that were around when the device was released (12+years ago), and see if there is anything specific that they were looking to convey.

    DE = 0, /RE = 0 → Driver off, receiver on

    DE = 0, /RE = 1 → Driver off, receiver off (both off, lowest power)

    DE = 1, /RE = 0 → Driver on, receiver on (both on, highest power)

    DE = 1, /RE = 1 → Driver on, receiver off

    Please let me know if you have any other questions, and I will let you know if I hear more.

    Thanks,

    John

  • 0 in reply to John Griffith
    TI__Mastermind 21811 points

    Hi Zanzibar,

    I just spoke with an engineer on my team, and they said that this allows for a faster turn on state. Basically when both DE and /RE are held high, the receiver is biased, the driver is on, but the output buffer is off. When you switch the /RE pin low, the output will be ready with a very low delay, as opposed if the receiver would need to be biased and allowed time to resolve.

    Therefore, in conclusion, the device operates as you would expect, but the receiver turn on time when the driver is active is minimized.

    Thanks,

    John