Troubleshooting Serial Connection

This section explains how to troubleshoot NMEA 0183 serial connections when connecting instruments (GPS, AIS, depth sounder, heading sensor, etc.) to a Windows computer using a serial or USB-to-serial adapter. NMEA 0183 instruments communicate through a COM port (physical or virtual). Most issues are related to drivers, incorrect COM port selection, baud rate mismatches, or wiring polarity.

Verify the Virtual COM Port in Windows

  • Right-click the Windows Start button and select Device Manager.

  • Expand Ports (COM & LPT).

  • Confirm that your serial adapter appears as a COM port (for example: USB Serial Port (COM3)).

If the COM port does not appear or shows a warning icon:

  • Disconnect the USB-to-serial adapter.

  • Install the latest driver from the adapter manufacturer.

  • Reconnect the adapter and check Device Manager again.

  • If Windows still does not recognize the device, refer to the USB Drivers Troubleshooting section of this guide.

Tips: Unplugging and reconnecting the adapter should make the COM port disappear and reappear in Device Manager. This is a quick way to confirm which COM port belongs to which adapter.

Write down the COM port number: you will need it later.

Check Data Using the Port Monitor

  • Open TimeZero in Navigation mode

  • Open the Connection Wizard, select "Port Monitor"

  • Use the Port Monitor to view incoming data on each available COM port and look for valid NMEA sentences such as $GPGGA or $GPRMC for GPS, $SDDBT for Depth, $HDT or $HDG for Heading, !AIVDM for AIS

If the Port Monitor displays unreadable or “garbage” characters, the most common cause is an incorrect baud rate. Most GPS, depth sounders, and heading sensors use a baud rate of 4800, while AIS receivers and multiplexers typically use 38400. Some devices may use other speeds such as 9600. Change the baud rate and observe the data again. If unreadable characters continue to appear at all baud rates, reverse the data wire polarity and verify that the wiring standard (RS-422 versus RS-232) is compatible between the instrument and the adapter.

If no data appears at all, check the wiring and confirm that the instrument is powered on and actively outputting NMEA 0183 data.

Configure the Connection in the Software

Open the Connection Wizard and use the Auto Detection feature first to allow the software to configure the connection automatically. If Auto Detection fails, use the Manual Configuration option, select the correct COM port, and set the appropriate baud rate. Complete the wizard when valid NMEA data is detected. Repeat this process for each additional instrument connected to the computer. Be aware that the COM port number assigned by Windows may change if the adapter is plugged into a different USB port

AIS-Specific Configuration

When an AIS receiver is connected, it is important not to use the AIS COM port as the primary source for GPS position, COG, SOG, or heading data. AIS position updates are slower and should not be used as the primary source ("Master") for GPS or Heading. Use the Port Priority of the Data Source selection page to assign GPS and heading data to their dedicated COM ports, or disable Position and Heading data coming from the AIS port.