On Sat, 2 Jan 2021 11:08:27 +0100, RobertoA 
declaimed the following:

>No particular protocol is required, the final goal is to be able to use
>a Hyperterminal / Realterm  type program on a Windows PC to read/write
>on any of the 6 Raspberry4 uart boards
>

 Since you imply plain text I/O, the simplest route would probably be to
SSH into the R-Pi, and then use a command line terminal program on the R-Pi
to connect to the specific UART(s) -- rather than try to create a terminal
server on the R-Pi exporting virtual com ports on Windows..

https://www.cyberciti.biz/hardware/5-linux-unix-commands-for-connecting-to-the-
serial-console/
{NOTE: subject says "serial console" but you just want raw serial}

https://linux.die.net/man/1/cu
"""
cu takes a single argument, besides the options. If the argument is the
string "dir" cu will make a direct connection to the port. This may only be
used by users with write access to the port, as it permits reprogramming
the modem.
"""

 The "screen" command, alone, doesn't seem that helpful -- at least,
most examples seem to assume one is just toggling shell sessions on the
device; I've only found one example claiming to open connected to a serial
port.... Ah, wait
https://www.cyberciti.biz/faq/unix-linux-apple-osx-bsd-screen-set-baud-rate/
"""
 If a tty name (e.g. “/dev/ttyS0”) is specified as the first parameter to
the screen command, the window is directly connected to this device.
"""

 Similar process with "minicom"... SSH from host to R-Pi, start minicom,
connect to desired UART/serial port.

 If using something like PuTTY on the Windows box, you could maybe even
provide a start-up command to be run when the connection is made -- that
command could be to start one of the above commands automatically (you'd
have a PuTTY configuration defined for each target serial port, so clicking
on the configuration in PuTTY session control menu would connect to the
R-Pi, run the appropriate connection command for the port, ...





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