Thanks for answer,

there is actually many questions for me, and I will glad to learn something, if you have time to explain me. Obviously, I'm going to use Modbus (RTU)... but this table confused me: (See attachment)

1. In the first table Address is "1"...why?

Answer: The number one is the slave node's ID or address number.   Modbus RTU over RS-485 is multidrop, meaning there can be multiple slaves on the network and the master (PLC) can poll each one individually one at a time.  So the Modbus message must include the slave node ID number/address that is being polled by the master.   The number is just an example, it can be any number from 1 through 247.  The slave must be configured for that node ID number.

2. In Function 3: MODBUS Register Read: for example Request says DevAddr 0x03 - this is Hex value, but on the first table is Address "1" maybe Decimal !?

Answer: I don't see in the screen shot where you see "DevAddr 0x03", but I would interpret that as the slave node ID number 3, which is different than the number 1 used in the example.

Your PLC uses whatever slave node ID number (Device address) the slave is configured for.

3.  StAdd H..!?  StAdd L..!?

Answer:   StAdd H = Start Address (High order) byte
StAdd L = Start Address (Low Order) byte

The slave's memory map will provide the register address for the data.  That memory address is two bytes, for example, P1 sensor pressure is a floating point value in registers 0x0002 and 0x0003.  SO the start address = 0x0002.

The high order byte = 00
The low order byte = 02

The screen shot seems to use decimal numbers, which is unusual, especially when representing 32 bit floating point data values which is just a bunch of binary bits best represented in hexadecimal until the lot of them are converted to a decimal number that humans understand.