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• First of all, if necessary, identify what sort of UARTs are in your system. To do this you normally need to use some sort of software diagnostic tool; the cheapest and simplest to use for this task is Microsoft Diagnostics, or MSD. If you do have 16550 UART then the rest of this section does not apply to you, and your overrun problems likely have a different cause.

Warning: Do not attempt to discern what the UART is by running MSD (Microsoft Diagnostics) from within a DOS session under Windows. You can get incorrect results.

• The easiest solution for getting a high-speed modem to work on a system without a 16550 UART is to use an internal modem instead of an external one. The external modem uses the PC's serial ports, but an internal modem plugs right into the system bus and comes with its own UART. If this is an option, it's the easiest way to go.
• You can add an upgrade I/O controller that includes serial ports that have 16550 UARTs, to replace the older ones that do not. Note that on older PCs that use an I/O controller in an expansion slot, you should be able to replace the old controller with a new one. On newer PCs that have built-in serial ports, you will need to disable them (through BIOS settings) to ensure that the old and new do not conflict.
• You could upgrade your motherboard to a newer model that includes built-in serial ports that have 16550 UARTs, but obviously, this is a bit of an extreme measure that will not appeal to most people.

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