• State Resolved
  • Locked Locked
  • Replies 6 replies
  • Answers 1 answer
  • Subscribers 33 subscribers
  • Views 165 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

THVD1419: Query related to making DE high and RE grounded all the time irrespective of transmission and reception.

akhil v
Prodigy 10 points
Part Number: THVD1419
Other Parts Discussed in Thread: THVD1406

We are using this RS485 half duplex ic and we had a use case where we need to make RE_bar grounded and DE as High all the time irrespective of Data Transmission or Reception, when we are trying to do that we are facing a issue that Data Reception is not happening. But if we enable DE only during transmission and disabling once the transmission is done, then everything is working as Expected. We need to know whether enabling DE all the time is possible, if yes then how, if not can you please explain why.

  • 0
    TI__Mastermind 46607 points

    Hi Akhil,

    Are there any other devices communicating on the bus when you have DE high all the time? 

    When DE is high the transceiver that is active will control the bus - if any other device has DE high at the same time communication won't be possible and bus contention will occur. 

    In general it isn't advisable for DE to be high all the time for a couple reasons:

    1) Unless only one device on the bus acts as a driver (1 driver to multiple receivers) - if only 1 device is the driver than it may be okay

    2) If only 1 device acts as a driver on the bus then the other consideration is power usage - unless there is pretty much no down time the driver will be actively driving the bus the entire time DE is high - this could use a lot of power, especially if you do not need to communicate the entire time. 

    Essentially to the device itself there is no difference between:

    A) /RE = GND and DE = Logic High (constant)

    B) /RE = GND and DE = Logic High (toggled) 

    That is why I am asking the state of the rest of the bus during your test because this type of issue typically would indicate a system level issue and not a direct device level issue. 

    So please let me know: Are there any other devices communicating on the bus when you have DE high all the time?

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 10 points

    Hi Parker,

                Thank you so much for your reply. coming to the query, our system has Microcontoroller communicating with RS485 half duplex sensor, in which both has to communicate with each other in request and response mode. In our controller side we thought of making the DE all the time high and RE_bar as low. Even when sensor is transmitting the data, the DE from controller side is high.

  • 0 in reply to akhil v
    Guru 275590 points

    That would not be possible. Having DE always high works only for a unidirectional connection with exactly one transmitter. With two transmitters (microcontroller and sensor), you need to control DE. Alternatively, use a transceiver with auto-direction control like the THVD1406.

  • 0 in reply to Clemens Ladisch
    Prodigy 10 points

    ok Clemens, got it. Thank you so much for the reply. Is it possible for me to know that how exactly controlling the DE is affecting the transmission of other devices on the bus?

    Thanks.

  • +1 in reply to akhil v
    Guru 275590 points

    When one trasmitter drives the bus high and another transmitter drives it low, then the bus voltage is in the middle and cannot be read by any receiver.

  • +1 in reply to akhil v
    TI__Mastermind 46607 points

    Hi Akhil,


    Clemens is correct - when multiple drivers are on the bus they will experience bus contention - which could result in the driver's shorting each other out. This shouldn't damage the device (unless it overheats during short) - but it will make communication impossible. 

    Best,

    Parker Dodson