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THVD1505: Can RE_ and DE be terminated with resistors in slave mode?

Lauren Mayfield
Expert 7465 points
Part Number: THVD1505

Hi team,

If my customer is using this part as a slave only, not as a master, how does he need to connect the RE_ and DE pins? Do they need to be controlled by the DSP or can they be terminated with a resistor? 

Is there a sample circuit that can be drawn up for this?

Thanks,

Lauren

  • 0
    TI__Mastermind 46857 points

    Hi Lauren,

    So RS-485 transceivers only implements the Physical layer (as RS-485 is a PHY only standard - data-link/network level control is not defined so it is up to system designer to determine what protocol is needed) - so communication is controlled via whatever controller is also sending/receiving data from the transceiver. I say this because it will vary how to actually set up the system - essentially the peripheral device can either be in RX only mode (use case 1) - i.e. it never needs to communicate back to host or if it needs to be able to respond to the host device (use case 2) then the controller generally will need to control the data flow with at least 1 GPIO - or if the data frame length is constant you may be able to implement auto-direction control. Ultimately enable control on half-duplex RS-485 comes down to: when do I need to transmit and when do I need to receive and then pick a higher level protocol that will give clear conditions on when a state change is necessary - most of the time this is done via software/firmware. Please see below for most common implementations.  

    Use Case 1: Peripheral Device only needs to receive data and never transmit:

    Solution: tie /RE and DE to GND for peripheral device - no external signal needed; peripheral device will always be ready to receive data from host device. Host device can have /RE and DE tied to VCC since it would be TX only in this specific use case.

    Use Case 2: Peripheral Device needs to receive and transmit data - peripheral device default state is receive mode. 

    Solution: short /RE and DE together with a pull-down resistor to GND. This keeps the device defaulting to RX mode but the controller for the peripheral transceiver can force the line high when it needs to send data back to the host device. When to switch between TX and RX is going to be firmware and protocol dependent. 

    Additional Solution with Auto-direction control - please see reference design here to have auto-direct implemented with RS-485 devices (https://www.ti.com/tool/TIDA-01090) - essentially if you have a constant data-frame length the design can be simplified with respect to device control. 

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    TI__Expert 7465 points

    Thanks Parker. Can you please point me to example circuits for both use cases? They're pretty sure they will only use it for Use Case 1 but would like to see examples for both.

  • +1 in reply to Lauren Mayfield
    TI__Mastermind 46857 points

    Hi Lauren,

    Use case 1 can be derived from figure 18 on the THVD1505 datasheet 

    Note 1:   the termination on A/B is only necessary for the last end node on the bus - other nodes should remain unterminated (for systems with greater than 2 nodes)

    Note 2: The red x indicates no connection - since the device is only concerned with receiving data no GPIO needs to control the /RE and DE pins.

    Note 3: The pull-down resistor can be replaced by a direct short to GND since the device will never need to transmit under use case 1.

    Use case 2

    This can also be derived from figure 18 in the datasheet.

    The difference being that the controller now needs a GPIO pin to control both /RE and DE to switch data direction (the controller doesn't need an explicit pin labeled direction - any GPIO should be able to work as long a a VIH(min) of 2V can be reached and no more than 0.8V for the input low voltage. Also the pull-down resistor is required. 

    This is the simplest implementations of the described use case. 

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    TI__Expert 7465 points

    Thank you Parker :)

  • 0 in reply to Lauren Mayfield
    TI__Mastermind 46857 points

    No Problem Lauren!

    If you have any other questions please don't hesitate to reach out!

    Best,

    Parker Dodson