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14 July 2020

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Interfacing a Garmin GPS Receiver

Garmin GPSWindmill software lets you collect data from Garmin GPS Receivers, and other instruments with an RS232 serial port, on a PC running Windows. Subscribers to the Monitor newsletter (ISSN 1472-0221) can download the software for free, but with no technical support.

This page helps you get started with Windmill 7 Software.

  1. Run the Windmill ComDebug program and enter your Garmin's communication settings.
  2. Run the Windmill DDE Panel or Logger program to save and display data from your GPS.

After you have entered your configuration settings with ComDebug, you don't need to use this again and can go straight to the logging and display programs. More details on entering the configuration settings are below.

Run the Windmill ComDebug program

Follow the instructions in the overview of using Windmill ComDebug. When you come entering specific settings for the Garmin (point 3 onwards), return to this page and see our suggestions below. You might also find our Connecting a GPS Receiver to a Laptop PC page useful.

COM Port Settings

These must be the same as your GPS uses. If in doubt start with...

Comm Port: Select the port to which your instrument is connected.
Baud Rate: 4800
Data Bits: 8
Parity: None
Flow Control: None

Instrument Timing

Read: in Background
Timeout: 5000 ms
Instrument Idle or Wait Time: 250 ms
Data Persistence Time: 5000 ms

Data Format

You should now see data from the GPS receiver in ComDebug's Terminal or Message Screen (see Trouble-Shooting below if not). Click the Parse button and enter instructions to extract the data from the GPS's message string.

Prompt command: Not generally needed
Example data string: $GPRMC,hhmmss,A,llll.ll,N,yyyyy.yy,W,kk.k,tt.t,ddmmyy,mm.m,W*CS
where lllll.ll is latitude and yyyyy.yy is longitude
Data format: ASCII
Example parsing to extract latitude: Search for "$GPRMC"
Ignore 10 characters
Extract Until comma
Example parsing to extract longitude: Search for "N,"
Extract Until comma

Depth data is contained in a different data string, prefixed by $SDDPT


xxx.x is depth in metres
yyy.y is keel offset in metres
*CS is the check sum

To extract depth, in ComDebug's Parser window you could: Search for "$SDDPT," Extract until the next comma.

Further Information:
Getting Started with ComDebug
Serial Settings
Connecting a GPS Receiver to a Laptop PC
Data Logging from NMEA Devices


Now open Windmill DDE Panel

When you've finished with ComDebug use DDE Panel to show your data.

  1. From the File menu select Load Hardware Setup and choose the *.ims file you just saved.
  2. Connect your GPS channels. You should see the correct weight in DDE Panel.
  3. Proceed similarly for the Logger and Chart programs.

Getting the data into Excel or Mapping Programs

You can use the Windmill Logger program to collect data, and after collection has finished import it into Excel or your mapping program. Alternatively, you can collect data with Excel in real-time by using an Excel macro to read data from the Windmill DDE Panel. For more details see our Data Logging with Excel page.

mapped data from Windmill
Depth and position data collected by Windmill and mapped using Surfer software.

Need More Help?

If you don't see any data...

  1. Make sure that the GPS receiver is switched on and plugged into the PC's com (RS232) port.
  2. Check that the communication settings you entered in ComDebug match those currently in use by the GPS: COM port, baud rate, parity, etc.
  3. If using a USB-RS232 converter, its settings must also match those used by the GPS - read the USB-Serial converter tips page for details, especially point 7.
  4. If you still have problems then if possible try swapping the cable between the GPS and the PC for another to see if that is the problem.
  5. If still no luck see our Testing Serial Port Communication page.

If you still have problems and you have followed the above instructions, fill in this from with as many details as possible. Include the steps you have taken so far and any error messages that you have received.







Need More Information?

Try these pages.