KB 43

COM Port Overruns in Windows NT 4

Article:43
Created:1999-09-01 16:39:54
Categories:
  Windows NT 4

Question or Symptom

How do I stop COM Port Overruns in Windows NT 4?

Resolution

      In Windows 95 there is a slider in the Modem control panel which
      lets you modify the FIFO triggers for your COM Ports to improve
      performance. There is no such control under NT 4.0, so you have to
      edit the registry to accomplish the same goal.
      If you're seeing COM port overruns in NT4, try the following.
      WARNING: Editing the Registry in NT or 95 is a dangerous
      undertaking! Be sure to back up your Registry before editing it.
      Beware that if you screw this up your computer may become
      unbootable! Be careful!
 
        1. Run the registry editor, go to
           HKEY_LOCAL_MACHINE/SYSTEM/CurrentControlSet/Service/Serial
        2. Change RxFIFO RegDWORD from 8 to 2
 
      This changes the FIFO to trigger after 2 bytes are available to
      send instead of waiting for 8, which reduces the chance that the
      FIFO will fill up and overflow before your machine is able to
      transmit the bytes. A setting of 4 may be acceptable as well, and
      result in slightly better performance. Some experimentation may be
      necessary.