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    RS485 Voltage Levels on COMMU Module

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    • R
      RoToR
      last edited by

      Hi,

      I want to use the RS485 from the COMMU module.

      But I am wondering why the differential voltage between A and B from RS485 signal is so small (800mV).
      I tried to connect it to a smart meter, but communication with M5Stack Core and COMMU via modbus protocoll isn´t possible.

      Configuration, software and signal see pictures. What´s wrong? I have no more ideas. So I assume, that voltage leves are to low.

      0_1656018747863_134fqe.JPG
      3_1656018851170_K1024_IMG_20220623_200921.JPG 2_1656018851170_K1024_IMG_20220623_200857.JPG 1_1656018851169_K1024_IMG_20220623_200837.JPG 0_1656018851169_K1024_IMG_20220623_200759.JPG
      0_1656018871010_asdf34t34.jpg

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      • felmueF
        felmue
        last edited by

        Hello @RoToR

        when I send something from the M5Stack and measure between A and B (w/o anything else connected) I get an amplitude of about 1.5 V.

        Have you tried with a 120 Ohm resistor at each end?

        Have you tried w/o the ground wire?

        Thanks
        Felix

        GPIO translation table M5Stack / M5Core2
        Information about various M5Stack products.
        Code examples

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        • R
          RoToR
          last edited by RoToR

          @felmue, thanks for your measurement.

          I don´t understand ...
          3,3V and 5V measured on M5Stack are good.

          With 120 Ohm differential voltage it`s 200mV, so with load even worse.
          Ground wire disconnected, same issue.

          I downloaded the test software to M5Stack and took a separate USB power supply, but the same ...

          It seems, that there is not enough power on SP3485 chip.
          Do I have to enable something?

          I already changed the M5Stack and the COMMU, without success.
          I will cross change again.

          What do you measure without sending a message? I measure 0.57V. Should be much more, when I take a look on the schematic with the assumption, that A/B output is not low-ohmic.

          0_1656064140524_76tu7tif7.JPG

          I just tested a new M5Stack and another COMMU module. Same issue.

          For RS485 communication I took RS485 unit library. Could this be the problem?
          0_1656065322710_234123r4r.JPG

          Now I will compare the schematic from RS485 unit with COMMU module.

          0_1656066061784_32t542tt54.JPG

          It is comparable, should not be the issue.

          Is there a design issue with the COMMU boards? Mine are from October 2011.
          Or are both COMMU modules broken?

          What else could I do?

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          • R
            RoToR
            last edited by RoToR

            I ordered now at Amazon an external UART/RS485 adapter, which will arrive tomorrow. Will test it on Sunday.

            My conclusion up to now is, that the COMMU is unfortunately not suitable for UIFlow, because today I also wanted to test the CAN Bus, but the CAN library is not usable, because it is only for CA-IS3050G Chip, which has only RX and TX.
            If I would have the time, I would programm it in Arduino, but that´s not the case, so for me the COMMU modul is rubbish and a waste of time.
            It would be really a great module, if it would be supported by UIFlow ...

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            • felmueF
              felmue
              last edited by

              Hello @RoToR

              one difference between the RS485/TTL unit and the COMMU is that the RS485 circuit in the former is powered by 5 V whereas the latter is powered by 3.3 V. (Not sure why M5Stack engineers decided to use 3.3 V in the COMMU module.)

              I've done my measurements with the RS485 board that was included with the LAN module. When I power it with 3.3 V, with 120 Ohm resistor and not sending anything I get about 1.5 V between A and B. When I power it with 5 V, same conditions, I get about 2.5 V between A and B.

              Thanks
              Felix

              GPIO translation table M5Stack / M5Core2
              Information about various M5Stack products.
              Code examples

              R 1 Reply Last reply Reply Quote 0
              • R
                RoToR @felmue
                last edited by

                @felmue thanks, I see. They are connected to 3,3V, could be the reason. But I will no more waste my time with this modules. I am testing the other RS485 adapter this weekend and report.
                Hopefully they have the same behavior as yours. These values I expected from the COMMU ...

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                • R
                  RoToR
                  last edited by RoToR

                  Just a short feedback.
                  I tested now the new RS485 to TTL converter.
                  It is working well together with the smart meter. Now communication is possible. All voltage levels are much higher.

                  0_1656706285334_64164.JPG

                  The COMMU module is rubbish! Should not be used for RS485. I lost a couple of hours to fix this issue.

                  Now I have the issue to convert the float values from the smart meter into decimal / integer.
                  Any idea, how to do this with UIflow? is there a function available?

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