Use a proper terminal program (e.g. picocom, minicom). Or use screen: 'screen /dev/ttyS1 115200'. This allows to use one command for RX and TX.
Thank you for your response. "Screen" works much better than "cat".
What's more, I didn't know Linux was waiting for 32 bytes before printing them. Many of my tests were shorter... And when I tried to send a byte as 0x05, of course Linux did not print them because it did not have a character to put on... It works better with "0x66" which is printed as a "f".
I've never seen Linux waiting for 32 bytes on the serial line before it prints anything. If this is what you are seeing the problem might be somewhere else.