The Industrial Raspberry Pi Forum

Dear all,
I use a RevPiCore 3+ with a USB-2-RS485 adapter to read modbus signals from a sensor. According to the sensors manufacturer, the registers I want to read are 8 and 9. If I set these up in the virtual Modbus RTU Master in piCtory and read them with piTest I saw mainly zeros. Only when I startet to look at the process image more broadly with, e.g. "piTest -r 326,10", with 326 the address of register 8 which I received from "piTest -v REGISTER_8", I saw the correct value for register 8. I played around with rebooting the revpi after changing the pictory configuration, checking with my old application, unplugging the usb connector, etc. etc. until I found out that if I read register 9, I get the value for register 8. If I read register 10, I get the value for register 9.
I double checked with the manufacturer and my own program that these registers are correct.
What am I doing wrong? Is there a bug in piCtory?
What kind of data do you need to be able to follow my explanation better?

Hr RMeissnerCC, the question is how the Extended Data configuration of the Modbus Master looks like?
Have a look at the tutorial How to Use RevPi Core as Modbus TCP Master.
Instead of using a real Modbus Slave you could use i.e. ModSim32 (please don't be alarmed by the 90s layout of the website).
Could you be so kind to share your PiCtory configuration file "/etc/revpi/config.rsc" with us?

Hi Dirk,
I used your youtube tutorial to setup the Modbus RTU Master.
I also attached the config.rsc as zip. Is the file correct?
Best, Robert

Hi Dirk,
I guess the solution was that the manufacturer starts the address at 0. You have written that in the How to Use ... tutorial. Unless you find something in the config I deem this post as answered.
Best, Robert

Hi Robert, this is a common problem as Modbus registers by definition start counting from 1 instead of 0. But to address them i.e. with a 16Bit variable you usually switch to the offset 0 for being able to to address them from (2^0) to (2^16 - 1)...

Good morning Dirk,
thank you
Does this offset in register address apply to writing to the registers, too?
Best, Robert