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[ The PC Guide | Troubleshooting and Repair Guide | The Troubleshooting Expert | Troubleshooting Specific Components | Troubleshooting Modems | Errors and Download Problems ]

I am having overrun problems with a high-speed external modem and suspect it is because my COM ports do not have a 16550 UART. I need to find out if they do or not, and if not, what can be done about this

Explanation: You are experiencing overrun errors on an external modem, especially a faster modem attached to an older PC. This has been attributed to the fact that your PC's serial (COM) ports do not have a modern 16550 UART, which includes a 16-byte buffer to reduce the possibility of overruns. You need to find out what sort of UART you have and if you don't have a 16550, explore options for getting around this shortcoming in your system.

Diagnosis: Most newer PCs do have a 16550 UART, although many older ones do not. Using a high-speed modem on an older PC can depend on getting the newer UART so that the overrun problems are avoided. Fortunately, this is relatively straight-forward to do.


  • 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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