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THVD1426: MCU direction control

Eduardo Garcia
Prodigy 65 points
Part Number: THVD1426


Tool/software:

Hello:

I am using THVD1426DR in one application set up in auto-direction control at 3MBaud. I wanted to test this circuit with the direction being controlled instead by an IC (as an option on our PCB).

I followed the diagram on Figure 9-5, which alludes to connecting the MCU DIR pin to both RE' and SHDN' (similar to other RS485 ICs that have RE' and DE). However reception does not work on this configuration as the SHDN' pin following DIR would be low. Transmission works. My question is, if I wanted to have the option of MCU control while using this IC, would I need to instead connect RE' to the DIR pin of the MCU and have SHDN' always high?

Thank you.

  • +1
    TI__Guru* 76201 points
    Eduardo Garcia said:
    . My question is, if I wanted to have the option of MCU control while using this IC, would I need to instead connect RE' to the DIR pin of the MCU and have SHDN' always high?

    This device has a unique feature in the sense that the RE# pin can only be disabled for a small amount of time. 

    "When RE is logic high or left open and D input switches from logic low to logic high, the receiver output is high-impedance for the duration of tdevice-auto-dir. After the duration of tdevice-auto-dir, the receiver turns ON and outputs a logic high or low depending upon the differential bus input voltage. "

    So you won't really be able to disable the receiver. You would probably need to place a FET or something infront of the R pin to disable its output when you don't want it.

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 65 points

    Hi Bobby:

    Thank you for your message. Does that mean that the device cannot behave correctly when trying to use it as a device that the direction can be controlled by the MCU?

    Also I have some results from my side - I implemented the circuit as shown on figure 9-5 of the datasheet (which is the same circuit as when other non-automatic RS485 chips are used). In this configuration I could only send data. If I attach the DIR pin only to RE' and leave SHDN' tied high, I am able to get data on both directions. We will change to another type of chip but just wanted to share my results in case figure 9-5 was not what was expected for this IC.