All you have to do is open two terminals. In the first terminal you cat everything from the device, e.g.

cat /dev/ttyS0

in the other terminal, you can send arbitrary hex characters and text to the terminal e.g. as follows:

echo -e "\x7E\x03\xD0\xAF und normaler Text" > /dev/ttyS0

The echo -e command enables the interpretation of backslash escapes.

On has to make sure of course that (i) the serial settings are correct and (ii) the serial device is not unblocking.

All you have to do is open two terminals. In the first terminal you cat everything from the device, e.g.

cat /dev/ttyS0

in the other terminal, you can send arbitrary hex characters and text to the terminal e.g. as follows:

echo -e "\x7E\x03\xD0\xAF und normaler Text" > /dev/ttyS0

The echo -e command enables the interpretation of backslash escapes.

One has to make sure of course that (i) the serial settings (speed, word length, flow ctrl, etc) are correct and (ii) the serial device (on the other end) is not blocking.