@Ghofran-Daoues this is not really a question for this forum, it's really for a libmodbus forum. However, your error indicates it cannot connect to port 502 of the device. There can be a few reasons for this, the first things I would look at are 1) perhaps the device is not accessible on your network; 2) the device is not listening on port 502. To check 1, see if you can ping the device (ping <ip address>) To check 2, (assuming check 1 passes) check your modbus device is listening on port 502 using nmap (nmap -p 502 <ip address>) nmap should report the port is "open" but if it shows as closed, there's your problem, if it reports as "filtered" there is probably a firewall blocking you.

@cskocsis I would first determine which IP it is attempting to listen on as it could be one of 3 addresses. I don't use this app but I suspect you can configure which address it listens on. The issue may be that it is trying to bind on the 0.0.0.0 address (listen on all IP) and is failing on one of the interfaces which may not be the one you want to use.

No way. Suggest Raspberry Pi 3 (large frame buffer, GPU, more CPU/memory power....)

Just a quick update - with the Omega2 this will all get much easier, as the Omega2 has a second serial port on the pins that were labelled GPIO 13 and 14 on the Omega1 (see new pinout). And these pins are available on the dock connector! Also the software side will be probably simpler, because the second serial port will not be occupied by the system console.