Serial communication through a ADG725

Hi

I am working on a project where we need to calibrate multiple sensors at the same time. The sensors communicate through a RS485 bus (using modbus). The RS485A-signal is connected to the ADG725’s DA-pin and the RS485B-signal is connected to the ADG725’s DB-pin. Each of the SxA and SxB pairs are connected to a sensor.

A microcontroller then controls which of the connected sensors the PC “sees”.

The problem is that the RS485 signal seem to reach all of the SxA and SxB pins, meaning that each connected sensor sees the request and responds. However, only one of the sensors’ response is returned to the PC. The responding sensor corresponds to the address selected on the ADG725.

So, my questions are:

  • Was I wrong to assume that the ADG725 could be used for two-way serial communication?
  • If not, what could cause this issue, and what are some possible solutions?

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  • 0
    •  Analog Employees 
    on Jun 8, 2021 11:13 AM

    Hi Anders,

    See answers to your questions below:

    • The ADG725 has a bandwidth of 34 MHz, therefore it should work correctly with a 10 Mbps two-way serial communication protocol (RS485).
    • From the description of the issue you have given, the correct behavior of the switch is occurring when the sensor response is returned to the PC i.e. (through the microcontroller), but for some reason when communication occurs on the RS485 line all sensors are receiving and transmitting a response.

    In order to try and identify the issue would you be able to give more information on the following:

    • The DA and DB lines when RS485 communication is triggered (i.e. scope shots etc.)
    • The 2-wire RS-485 transmission network between the ADG725 and RS485 input, was this circuit implemented referencing a design guide ? ( There seems to be no termination resistor present in your network design, it's best practice that each end of the network has a termination resistor equal to the characteristic impedance of the cable.)

    Kind regards,

    Kevin

  • Hi

    Thanks for your response Blush.

    I fixed the issue by making 4 changes.

    1. I added a 120Ohm termination resistor at the exit of each multiplexer output.
    2. I added a 120Ohm termination at the input of my PCB.
    3. I connected my USB-to-RS485's ground to my PCB's ground.
    4. One of the two sensors I used to test the PCB was weird so I substituted it for another.

    It now works flawlessly (at least for now).

    I hope my paint skills are adequate.

  • 0
    •  Analog Employees 
    on Jun 14, 2021 3:44 PM in reply to Anders_sorensen

    Hi Anders,

    Great to see that the additions you made to your circuit helped solve the issue.

    Please don't hesitate to contact, if you need any further assistance.

    Regards,

    Kevin

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