GPS clock and Event Logger
External GPS hardware unit keeps PC clock accurate to 1 mS.
PC software also provides accurate event timing and logging.
This type of GPS clock can be found all over the NET. So what's different with my design ?
Not much, except the ability to log external events with an accuracy of 1 mS.
Very large Time display which is handy for viewing from a distance.
External events are recorded and can be viewed immediately and/or saved to a csv file.
The event is NOT sent by RS-232, that would make this system very unstable.
As with many other designs I use the 1 pps to interrupt the PC, and the event is also hardwired into the PC thereby eliminating any RS-232 delays.
In case you are wondering: "Will it work with a USB or RS-232 adapter!" The simple answer is: Not as accurately as it does with real hardware RS-232 ports. USB to RS-232 adapters insert delays for all connections that are normally hard wired, lines such as DSR and CTS. The delay can also be variable. For best accuracy I recommend a real UART based RS-232 COM port.
Most reasonable GPS receivers have a 1 pps output which can be accurate to 1 microsecond, there are GPS receivers with 1 pps accuracy to 50 nano-seconds.
The RS232 link is used for transferring NMEA strings from the GPS to the PC. The time and date data in these NMEA strings are used in conjunction with the 1 pps to synchronize the PC.
After the PC has been synchronized, all external events are logged against the PC clock. A normal PC clock is not very stable, however it is stable enough for 1 mS accurate event logging, and as the PC will be synchronized automatically every minute, so accuracy is maintained.
This is the program I wrote to automatically synchronize the PC and log external events. The events shown here in this image are manually triggered random events.
If there is enough interest in this design perhaps I will publish the electronic schematic and/or provide a kit.
Here is a sample file. The external event source was a free running highly stable (2ppm) 1 second oscillator.
email: hans at this web site
Copyrightę 2005 Hans Wedemeyer, Houston, Texas, U.S.A. All Rights Reserved.