[ntp:questions] Setting the COM port for the NMEA driver

Todd Glassey CISM CIFI tglassey at earthlink.net
Sat May 9 14:21:27 UTC 2009

Dave Hart wrote:
>> Does NTP even allow COM4?
> I don't think so.  The unit numbers are limited to 0-3, I believe, but
> I don't have any concept of why.  Windows never has a COM0:, so
> arguably it was a poor choice to map unit number to com port number
> without adding 1, but it's far too late to change that.  At the cost
> of additional documentation complexity, ntpd could easily enough
> interpret unit 0 as COM4:
> Two solutions come to mind, either of which would probably work.
> Somewhere in the registry there lurks a mapping, obviously named by
> someone very young and self-impressed, known as the "COM port
> database".  I got your database right here, buddy.  There's a tiny
> little API used by serial port drivers to claim COM port numbers, but
> I suspect you can ignore the API and fiddle the registry directly.
> Quite possibly easiest would be to disable or renumber COM1:, COM2: or
> COM3:.  I think there might even be an easy mousy way to choose the
> COMx: assignment via device manager properties dialogs.  Try changing
> an unused 1-3 to a higher port and your USB adapter will likely claim
> the lowest-numbered available port next boot.
There is the possibility of having COM ports up into two digit numbers 
too when you install Bluetooth and other USB controllers.

In windows the com ports acan be easily re-numbered  - See 

The best solution in UNIX is to create a link to the proper device as 
/devv/mnea or some such exactly as UUCP or the TIP interface uses com 
ports for like /dev/cua0 or like the mouse /dev/mouse etc etc etc.

> Good luck,
> Dave Hart
> P.S.  I predict you'd be able to get better time over the internet
> than over USB, but I'd love to be wrong.   But then, you might end up
> the first to have tried PPS over serial USB and get much better
> results than your predecessors.  Thanks for taking the time to
> investigate this.
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