1 when I write to ttyS0 a new line data Is random sent to only One client at time, i would this to be to all at same time.
Unfortunately this is a standard feature of serial port communication: a TTY device is a simple FIFO. Once the data is read from the TTY device, it's gone. This is why having multiple processes accessing the same TTY device simultaneously is usually not a good idea.
A RS-232 serial line is fundamentally one-to-one: devices and protocols designed for serial ports are designed to expect just one link partner at the other end of the line.
If you need multiple TCP clients with a single serial port, I'm afraid you might have to make your own program that would do the communication the way you need it to happen.
2 what can I use instead of cat to have a bidirectional communication?
microcom is a busybox command that would seem to fit the purpose here. According to the busybox man page, it uses stdin & stdout, so it should work with tcpsvd... but I have not tested it.
3 How can I have flow done byte by bye sent not line be line?
Assuming I understood your question, stty -F /dev/ttyS0 raw should do it, but if you switch to microcom instead of cat, it should already do that for you.
I assumed you are restricted by whatever is available on the router, so the above answer assumes only busybox, as I cannot know what other commands might be available.
But if you can add more software, then I would recommend using something designed for the purpose, like ser2net.
