Can I make my USB to serial adaptor imitate a null modem cable with a straight cable?
No probably not. As discussed above the USB->UART part of the converter is not as much of an issue, as is the RS-232 transceiver chip. Since the RS-232 transceiver has dedicated TX and RX there is no way to switch it in firmware. I agree such a device would be useful in a way similar to Ethernet cards which can auto-sense if you are using cross-over cable or strait cable.The best solution to your problem would probably be to just buy or build your own null modem adapter.