Hello,

I am working on data logging inverters that communicate using ModBus.
I can retrieve most values with a simple python script sending requests via RS485, but 30% of the requests returns an "Illegal data adress error". Not much info on this error except: "the address might not exist"... which it does though. Some forums indicated that the value is stored in a "block" of register and cannot allow a user to access only one of the registers individually.
So i looked up my registers and it looks like the ones returning an error are values stored as 32bit integers or floats. This means i cannot access them without requesting 2 registers at the same time? For example:
- Register 40200: value1 (32Uint)
- Register 40202: value2 (16int)
I can access value at 40202 but neither value in 40200 and 40201. How do i even ask the slave that i need both register values at the same time? Can my request hold an information containing how many registers i want?

Thanks in advance for your help!

 

I believe you're correct, you need to read the 2 registers in a single Modbus request to read the 32-bit values from your slave device. If register 40200 has a 32-bit unsigned integer, you must read both 40200 and 40201 in a single request.

This restriction is vendor-specific, but some devices do prevent a Modbus Master from reading only 1 register of a 32-bit value stored in 2 consecutive registers, so that the values cannot be out of sync.

Are you using a Modbus library to make Modbus requests, or are you manually creating your own requests in your python script?

A Modbus Read Holding Registers (function code 03) request allows you to read up to 125 consecutive registers with a single request. Refer to section 6.3 of the spec here:
https://modbus.org/docs/Modbus_Application_Protocol_V1_1b3.pdf

 

Hey!
thanks for your reply, i am indeed using a modbus library (minimalmodbus), problem is, i didn't see anywhere in the code a way to read multiple registers at once? I will look more into it tomorrow, and i will check that link too!
Thanks for your answer

 

Thank you for letting us know which library you're using.

With the MinimalModbus library, it looks like you need to use the "read_long" function for 32-bit integers and "read_float " function for 32-bit floating point instead of "read_register".

https://minimalmodbus.readthedocs.io/en/stable/apiminimalmodbus.html

 

Thank you for letting us know which library you're using.

With the MinimalModbus library, it looks like you need to use the "read_long" function for 32-bit integers and "read_float " function for 32-bit floating point instead of "read_register".

https://minimalmodbus.readthedocs.io/en/stable/apiminimalmodbus.html

Hey thanks for looking into it! Can't believed I missed it, i'll try it out on the inverter next week! thank you very much