Typically you can open a filename "COM1", this should open the Serial Port as configured in Win device manager. It's a synonym like LPT for parallel a.s.o.

I don't know much about modern basic, but you should be able to
print# and get# chars or bytes then from there.

In C it would be CreateFile , ReadFile and WriteFile...

If you really want to stick to Windows you might want to have a look at the Win32 SDK and there ComCTL , DCB a.s.o....

A USB to RS232 adapter is typically mapped to f.e. COM3 by a wrapper USB driver.

I forgot, you can still access the uart in hardware from the dosbox, all accesses will be mapped through.
16550 manual: http://www-s.ti.com/sc/ds/tl16c550c.pdf

Works with any other 16550 too including auto flow.
Only other thing is that you have to set the PIC if you use a PC.

You can take this part from some example, there are alot available.

I will look at the code and try to post a snippet where it does the I/O

I solved the problem. I bought Microsoft Virtual PC 2004. Works like a charm!!

Good to hear....and thanks for reporting back. 😀

I wonder if the free Microsoft Virtual Server 2005 (not as easy to use as VPC but compatibility-wise should be the same), the free Vmware Server Beta 2 or the OpenSource Qemu would have solved your problem though. 😉

Quite possible Qemu may have worked, but the other two products are considerably more complex to use. The PITA factor would more than balance out the cost, being a customer application. Using the VPC is pretty much a no-brainer - it was dead easy to set up, and the 45day free trial made it risk free. I was able to try it and prove it worked without spending any cash, and it's cheaper for the customer than re-coding the program.