RS485 Voltage Levels on COMMU Module
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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.
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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
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@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.
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?
Now I will compare the schematic from RS485 unit with COMMU module.
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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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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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
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@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 ...