I use also the COM1 under Windows XP in a DOS-Box and it works great.

The main problem is, that older DOS-programs not expected such damned fast CPUs (e.g. 3GHZ) and thus e.g a timeout counter expired long time before the connection was established.

If you have the source code, then it is easy to adapt the DOS program in this way.

XP simulate a full working COM, which can be accessed by inportb() and outportb() directly, even if the COM was connected over an USB/RS-232 converter.

But then an additional delay occur and also a bigger FIFO (128 Byte) was implemented.
Also you should only select COM1...COM4, since COM5...COM255 are not known under DOS.

If you watch these changes, then you can still use DOS programs even under XP.


Peter