LJSocket. How can I know IP-address U3?

The IP address is the same as the IP address of the computer your U3 is connected to. The command-response ports start at 6001 and the Modbus ports starts at 5021. For example, if you have two U3's in LJSocket, port 6001/5021 is for one U3 and 6002/5022 is for the second U3. Both have the same IP address of the computer they are connected to.

Your Modbus command looks correct, so likely you are getting that error since FIO0 is currently set as a digital line, but you are trying to read it as an analog input. There are two ways to resolve this:

1. Use LJControlPanel and configure the power up default of FIO0 to an analog input. Do this for any other lines you intend to read as analog inputs. Note to stop the LJSocket service so LJControlPanel can access your U3, and that settings don't take effect until after you power cycle/reset your U3.

2. Configure FIO0 and other needed lines to an analog input. Using Modbus, write 1 register (u16 value) to address 50590 (FIOAnalog) to configure this. Each bit represents an FIO line where bit 0 is FIO0, bit 1 is FIO1, bit 2 is FIO2, etc.. 1 = analog input and 0 = digital I/O.

For example, to configure only FIO0 to an analog input and the rest as digital I/O lines send a value of 1 (b00000001). For all FIO lines as analog inputs pass 255 (b11111111).

Keep in mind that these are current settings and will reset to the power up default settings if the U3 is reset or power cycled.