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THVD1520: Application Inquiry

Renan Santos Adriano
Mastermind 22360 points
Part Number: THVD1520

Hello Team,

I'm posting on behalf of   customer:

I have two microcontrollers communicating via UART and I am trying to textend over a 485 bus with the THVD1520 part. However, it doesn't seem to be working.

 In the application notes (within datasheet), I've seen the application image. But I'm unsure of what to use to drive the DE pin. I have the standard handshaking pins available. Any help?

 I also am unsure if DE and nRE should be driven the same way on both ends?

Regards,

Renan

  • 0
    Guru 317000 points

    An RS-485 driver has three output states: high, low, and inactive. This allows multiple transceivers on the same bus.

    In general, DE is high only when sending, and /RE is connected to the same signal. But if you want to listen to what you're sending, keep /RE always low.

    If your microcontroller does not have a signal specifically designed for the RS-485 DE signal, then you need to use a GPIO and set/clear it manually.

    If both devices need to be able to send at the same time, then you must use full-duplex transceivers (and four signal lines) instead.

  • 0
    TI__Guru 52282 points

    Hi Renan,

    Clemens is correct - if the microcontroller you are using doesn't have specific RS-485 enable control pins a GPIO can be programmed to act as the enable and will turn on the device (high on DE) when it needs to send data and low other wise (its wise to use a pull-down to ensure a known logic state at all times and then drive high when you wish to send data) - programming a GPIO pin is one of the most common ways to control this pin. 

    The handshaking pins that your customer is referring to are for RS-232 which defines handshaking protocols - RS-485 only defines the electrical specifications so all higher level control/firmware is more or less designers choice - generally for this type of application it is 1 GPIO connected to DE and /RE so the device will switch modes is the simplest implementation  - but if you have /RE always low then it will be able to listen to what it sends - if you need to send and receive data at the same time a full duplex device is needed.

    Please let me know if you have any other questions!

    Best,

    Parker Dodson