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ISO3082 working properly from RS485 to TTL but not from TTL to RS485

Green Power
Intellectual 835 points
Other Parts Discussed in Thread: ISO3082

Hi everyone,


I am using the ISO3082 as Dual Isolated Half-Duplex RS485/TTL conversor. I am following the Reference Desing TIDU590, but on the TTL side I have a microcontroller which receives R, and D in a UART and D and /RE are controlled with a GPIO as Digital output (8mA) .

I have tested this configuration: MCU with the SN65HV82 (RS845/TTL conversor but not isolated)  without problems however I am having problems with the Isolated one.

I can receive properly from RS485 to the microcontroller (I check in the oscilloscope A and B signals, perfectly inverted). But from MCU to RS485, something is not working:  I can check the answer signal (D) from the MCU to the ISO3082, also the control signal (D/RE). But at the output of the ISO3082,  A and B signals followed D but they are not inverted so it is not recognized as RS485. 

Any idea of what could it be happening?

Thanks in advance

Best regards

  • 0
    Intellectual 835 points
    I forgot to say than I am using 3V3 supply and signals at the logic side (MCU side) and 5V at the RS485 side.
    Also with control signals I refer to DE and /RE, both together controlled by a GPIO from the MCU configured as Push-Pull output (8mA).
  • 0 in reply to Green Power
    TI__Intellectual 2855 points
    Hi,

    To understand the system better can you please share the schematics of ISO3082 section of the system?
    What is the bus ( A & B) voltage when MCU is transmitting data, probe A,B and D/RE at IC pins. Is this issue observed on multiple boards?

    Regards
    Tejas
  • 0 in reply to Tejas Hommaradi
    Intellectual 835 points
    Hi Tejas,

    The schematics in the RS485 side is exactly the same that in the reference design : www.ti.com/.../tidrbn8.pdf
    The 5V supply in the RS485 comes from an external isolated and stable 5V source.

    On the MCU side, the ISO3082 is supplied from 3.3V, R and D (10k pull-up) goes directly to an UART in the microcontroller, /RE and D are together and connected to and GPIO configured as Digital output (10k pull-down).

    voltages in A and B, transmitting RS485 to MCU (it works properly A and B are inverted):

    - A goes from 1.3V to 3.8V and B more less the same levels.

    Voltages in A and B, transmitting MCU to RS485 (not inverting):

    -A goes from 1V to 4.3V approximately and B goes from 0.6V to 3.6V

    I can send you a screen shot of the signals (Is it possible to attached a file?)

    On the other side of the RS485 link I am using this USB-RS485 converter uk.rs-online.com/.../
    and between them 30 meters of 4-wire twisted shielded cable (special for RS485) .
  • 0 in reply to Green Power
    TI__Intellectual 2855 points

    Hi,

    You can use "Insert/Edit Media" or "Insert file" option to share files. Also did you get a chance to check if this issue exists on multiple boards? or tried using fresh IC? 

    In previous reply you mentioned /RE and D are tied together, I hope /RE and DE are tied together and D is independently connected to UART of MCU.

    what is connected on the other side of the RS-485 Bus?   

    can you also share the plots for following conditions too during MCU transmitting data?

    1) Remove R14,R16 resistors and replace R15 with 60ohm and measure the voltage of A & B at IC pins

    2) Remove R11 and R12 resistor and probe voltages at A&B at IC Pin.

    Looking forward for plots.

    Regards

    Tejas