USB to RS-232 Cables...
Many laptops today do not include RS-232 ports or even parallel ports...They have all moved to USB ports. That means, the USB-(insert obsolete port here) market has grown for backward compatibility. A USB port is much more complicated than RS-232 because it includes the plug-n-play protocol on top of the simple serial interface. This means, the device identifies what it is when you plug in a USB cable from a gizmo (digital camera, flash drive, etc.). The old RS-232 port has no such protocol built into any OS (that I know of).
It is true that USB provides 5 volts at some nominal current sourcing level (?) and that RS-232 uses +/- 12 volts as its signalling level. However, USB-to-RS232 conversion cables (perhaps with a module) include built in level converters (charge pumpts, DC/DC converters) which understand the RS-232 voltage levels. And...when you plug this cable into a USB port, it will identify itself (ala firmare actually in the cable IC) that it is a "COM port" device. You may need drivers (from included CD) to be installed.
The problem may be more recognition of the COM port name/number by the software. Many times, USB ports are numbered around 10, 11, etc. rather than starting at 1 (COM1
. The software must know which "virtual" com port it is opening and talking to. So see if the USB driver supports a given range of com port numbers and make sure it matches the application port number.