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ISO15: RS485 communication not working

BHUSHAN SAHAY
Expert 1085 points
Part Number: ISO15

Greetings,

I am using ISO15DW 3.3V RS-485 Transceiver. Here is the schematic of my hardware:-

EXTTRANAL CONTROLLER.pdf

I have used dsPIC microcontroller for this purpose. When I directly connect RS485-RX and RS485-TX pins of dsPIC microcontroller(removing ISO15DW and R3,R6,C15 and C16) to USB-TO-TTL module then I am able to see dsPIC transmitted signals on UART terminal of my PC. However, when I place the ISO15 ic and other related components and connect to USB-TO-RS485 module then I am seeing only 0x00 on my UART terminal. Kindly tell me how to troubleshoot this issue.

Thanks

Best Regards

Bhushan

  • 0
    TI__Mastermind 49415 points

    Hi Bhushan,

    Sorry to hear about the issue.
    Could you please confirm how are you connecting this PCB to your PC? Is it through the USB?

    1. Assuming the answer to above question as yes, I believe there is ISO15DW and an RS-485 to USB dongle between dsPIC and PC, not quite sure if the issue is either or both. To rule that out, I would recommend you to monitor ISO15DW Side1 (D, R) and Side2 (A/B) pins on oscilloscope. This will confirm if the issue is with ISO15.

    2. I also noticed that dsPIC is powered by 3.3V while ISO15 VCC1 is powered with 5V. This may or may not result into communication error but it is worthwhile to test both dsPIC and ISO15 powered with the same voltage (either 3.3V or 5V). Since ISO15 VCC1 is 5V, the output voltage at R can be upto 5V. If the dsPIC inputs are not 5V tolerant then it is possible that the dsPIC is not behaving the way it is expected to.

    Please note that using different supply voltages on MCU and ISO15 is strongly not recommended. Even though it might work fine for normal operating conditions, it could be an issue when tested across temperatures and across samples.

    Please try out the above 2 experiments that I have recommended to identify the cause of the issue, thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Expert 1085 points

    Greetings Koteshwar Rao,

    Thanks for your guidance.

    dsPIC(the microcontroller) UART RX-TX lines are connected to ISO15 D and R pins. DE pin is shorted with RE pin and connected to one of the pin of microcontroller( as I am only transmitting from microcontroller so I have given logic HIGH on this pin). Then from output side of ISODW, A and B pins are connected to USB-TO-RS485 module D+ and D- pins. Then I have connected the module to PC.

    TX and RX pins of dsPIC are 5V tolerant pins. I also tried using 3.3V for powering ISO15 ic.

    My analysis:-

    (1)I replaced the ISO15 ic with a new ISO15 ic. I started getting transmitted signals on the terminal. I also probed A and B lines of ISODW and found correct signals. But after sometimes the terminal started showing garbage values and then I started getting oscillating signals on A and B lines(using DSO probe between A and B).

    (2)On input sides of ISO15, correct UART signals are coming.

    Kindly suggest.

    Thanks

    Best Regards

    Bhushan 

  • 0 in reply to BHUSHAN SAHAY
    TI__Mastermind 49415 points

    Hi Bhushan,

    Thanks for sharing details related to the connection and the tests you have carried out.

    I am assuming the USB-TO-RS485 module has RS485 side (usually referred as A/B or Y/Z pins) and the USB side (usually referred as D+/D-). I am assuming the RS485 side of the module is connected to ISO15. It would be very helpful to review if you could share the schematic showing the USB-TO-RS485 module as well, along with the part number.

    One of the experiments I had recommended earlier is to narrow down if the issue is with ISO15 or with USB-TO-RS485. The experiment I recommended is to monitor D and A pins of ISO15 on DSO and the waveform should match in pattern though they will be different in voltage levels. Based on this the issue can be narrowed down to either ISO15 or to the USB-TO-RS485. Narrowing it down will make it easier to debug the issue.
    Please monitor D & A pins of ISO15 on DSO when you see garbage values being displayed in the terminal. Thanks.

    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Expert 1085 points

    Greetings Koteshwar Rao,

    Thanks for your guidance. Once I will try again your recommended steps and will update.

    Thanks again

    Best Regards

    Bhushan