Legacy PLC software and USB (slightly OT)

Really a software topic, but relevant to control:
I hear of problems with old instrumentation software using PC COM ports when
it's used with a newer PC which emulates the port using a USB adaptor. Without any direct experience, I guess that the problem may be that the newer arrangement uses a driver that make the port acceessible through Windoze API calls (ie. looks like a file) rather than as a proper UART with standard I/O port mapping. No doubt a lot of the older software would expect a 'real' UART, accessing it via direct port read/write.
Is anyone aware of a robust solution to this problem, ie. a USB-based solution that presents on the OS side as a dinky-di COM port, that can be accessed like a real one? I've done a fair amount of googling and checking of product specs, nothing is particularly specific on the matter. Suppliers haven't been any help. TIA
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Situations like this generally require new software (a minor re-write with the COM port calls changed to suit the OS) and can then run in Windoze and similar 'multi-tasking' environments with few problems. USB allows the COM port to be located elsewhere, but still doesn't present you with 'real' port access because you're still restricted to the Windows API.
Fortunately it's really only data analyser software that needs to reliably track individual data bytes that requires dedicated hardware. As I see it, you have two options:
1. Replace the entire system (hardware and software) with something new. Hey, if it's *that* old they are probably due for an upgrade anyway.. ;-)
2. Install dedicated hardware to suit the software. There are still manufacturers of single-board computers out there (like Advantech) that can supply DOS-compatible hardware with a standard COM port, an Ethernet port and a 3.5" floppy. We've used this set-up to provide a gateway between the old gear (Enraf tank gauges) and the new (DCS) calling it a "communications interface". Didn't even need a hard-drive..
Your subject mentioned PLC software: We keep on old Toshiba laptop installed with Windows 98SE in one corner of the office is loaded with the old Mitsubishi, Modicon and Allen-Bradley software we need to support our legacy clients. It doesn't get used very often and the battery was stuffed long ago, but it doesn't take up much space either.
HTH, Cameron:-)
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