It you are just using qbasic then try qb64 so you can run it natively.

Have you checked the dosbox.conf file to see if the serial interface is configured to use the physical connection? Never had issues with one of these USB to serial adapters (mine has a FTDI controller btw) when playing Hexen in a co-op session.

Nevertheless, I second DosFreak's advice of running it natively if that's possible.

Thanks, DosFreak, I've tried now, but at the output time (i.e. command "PRINT #2, a$;"), I'm getting the response "Unhandled Error #54: Bad file mode".

I'll try now the suggestion of Gmlb256

Hurray, great, thanks to Gmlb256! The interface is up and running now. Perfect - you saved my day & week 😀

Next thing I'll try is to get my programs compiled by qb64 running as well - because I've seen in the wiki that soundfiles can be embedded there (we'll see how more or less easily that works...). I'll keep you informed.

Ok, here's the status after several different approaches:
- the program runs nicely in the DOS-Box with qb11, i.e. interpreter-mode, and the USB-RS232 communication as "COM2" works
- the same program compiled with qb64 results in a "bad file mode" error, and the interface isn't working
- still the test by hterm works well under Windows11, i.e. the manual entry

Are there any properties I need to assign to qb64 or to the compiled program? Or what else could be the reason for the error?

Thanks in advance for your suggestions!

Mabye print is formatted differently in qb64 or doesnt recognize what you are giving it? There should be a debug capability in qb64.

Thanks for the hint - but the error isn't at the print command. It happens already at the opening attempt, i.e.
Open "com2:9600,n,8,2,cs,ds,rs,cd" For Output As #2

Also trying only
Open "com2:9600,n,8,2" For Output As #2
results in the same error.

Are you aware of another syntax for com-Ports in qb11 versus qb64?

I have an idea:
For qb11 I've inserted the following line in the "qb11.conf" (in the subdirectory "_config"):
serial2=directserial realport:COM6
I guess I need to do a similar thing in the config for qb64.
Now the question is: Where is the respective config file for qb64? Is this "setup_win"? or "config" in the subdirectory "internal"? Or anywhere else?

That is the DOSBox configuration file. That line tells DOSBox to use an emulated COM2 port based on real COM6.
Your problem is you are telling qb64 to use real COM2 which probably doesn't exist - use COM6 instead.

Thanks jmarsh!

I've tried, but still confusing results:

- Opening works for Com6 (open "com6:9600,n,8,2..."), i.e. no error is reported at runtime, no matter if I open as random, input or output
- There's indeed a physical connection to the device, because the system behaves differently, if the device is connected (i.e. seemingly ok) versus not connected ("Device unavailable")
- Reading (or likeweise writing) results in "bad file name" or "bad file mode" at different places in the program, depending on the first opening the port (or 2nd after closing)

Where do I find the DOSBox config file? Ie in which of the subdirectories?
What might cause this confusion?

Thanks!!!