UART2 and UART3 Not Responding on SparkFun Quadband GNSS RTK Breakout

[y4my4m]

(I initially had planned to embed a few pictures of the problem into this post, but this forum lets new users only embed one picture. I have screenshots of the QGNSS UART settings and of the oscilloscope screen.)
I have the SparkFun Quadband GNSS RTK Breakout and was testing the UART communication on my board.
Communication over UART1, which is connected to the board’s USB port, works as expected.

However, when I try the same setup using UART2 or UART3, I receive no response.

Here is the UART console, where you can see there’s no response from the board:

The first command in the console is a reset command, which the chip seems to recognize — the PPS LED stops blinking while the chip resets.

Initially, I suspected the USB-to-Serial adapter might be the issue, but I tried a second adapter of the same type and observed the same problem.

I then connected my oscilloscope while testing UART3 communication again.
I confirmed that the command is correctly sent and that there is no response from the board.

Could it be that UART2 and UART3 are non-functional on my board? Or is there a setting I may have overlooked to enable these UART ports?

[PaulZC]

Hi @y4my4m ,

Welcome!

Please tell is more about your USB-to-Serial adapter. Is it 3.3V? Have you crossed TX->RX and RX->TX correctly? Have you also attached GND-GND?

How are you powering the board while you are using the USB-to-Serial adapter? Via the USB connector?

Please connect on UART1 and send $PQTMVERNO*58. This will tell you what version of firmware the module is running. Please post that information here too.

Best wishes,
Paul

[y4my4m]

Hello Paul,

Thanks for the quick answer.

Here is a picture of the setup I was using:

As you can see TX and RX are crossed, GND’s are connected and it is a 3.3V adapter.

I am powering the board via the adapter and JST connector.

This is the response to the version number command:
[11:11:26.882 Uart Rx:0054 B] $PQTMVERNO,LG290P03AANR01A05S,2025/05/08,16:23:00*13

[y4my4m]

Here is a screenshot of the oscilloscope screen, confirming the voltage levels and the missing response from the board:

[bamarcant]

[PaulZC]

Hi @y4my4m ,

OK. Thank you.

Please try sending these three commands on UART1 to restore and reset the GNSS:

$PQTMRESTOREPAR*13
$PQTMSAVEPAR*5A
$PQTMSRR*4B

I hope this helps,
Paul

[PaulZC]

Hi Marco (@bamarcant ),

By default the 4-pin VCC pin is 3.3V. With care, it can be used for input or output.

There is a jumper on the back of the board which can be changed to select 5V input.

Best wishes,
Paul

[y4my4m]

@bamarcant, the oscilloscope measurement was done from the PTH’s.

@PaulZC, here is the output from the reset commands:
[12:30:47.335 Uart Rx:0023 B] $PQTMRESTOREPAR,OK*3B
[12:32:09.143 Uart Rx:0020 B] $PQTMSAVEPAR,OK*72

I had already done that before and the outcome was sadly the same as now, no change:

[PaulZC]

@y4my4m

Please send those three commands from UART1. It is important that they are acknowledged.

When the GNSS has been reset, you should see NMEA on all TX pins at 460800 baud.

Best,
Paul

[PaulZC]

Please also check you have Ascii, NMEA and CRLF selected in the “Advance” option:

[y4my4m]

@PaulZC

Thanks for the quick answer!

The three commands were sent from UART1, without a reset between the commands.
I just copied the command confirmations in here.
This is what the console looks like with UART1:

[PaulZC]

Hi @y4my4m ,

OK. Thank you.

I am almost out of ideas.

Maybe it is the CH340 driver on your computer? Your USB-UART Adapter has a CH340 on it. Please check you are using the correct driver. Please see the link below.

If you install a Terminal Emulator like Tera Term, or Realterm, you can check the CH340 by performing a loop-back test. Disconnect the GNSS and use a jumper wire or jumper link to link TX to RX on the Adapter. Open the COM port. Whatever you type in the terminal window should be echoed back again. Remove the link, make sure the echo stops. Replace the link, make sure the echo restarts.

I’m afraid that’s all I can suggest. You can Return the board if needed.

Best wishes,
Paul

How to Install CH340 Drivers - SparkFun Learn

[y4my4m]

@PaulZC

Thank you again for your help. I just tested the adapter and it works:

Sadly, I already assumed the interfaces were non functional.

I didn’t purchase the board through Sparkfun, but from a german reseller. I will try to return the product with them.

Best wishes,
Mirko

[PaulZC]

If you have “local echo” turned on, you will see what you type. It is important that you try with and without the link. Without the link, the echo should stop.

Faults like this are very rare, but they do happen. I’m suspicious that both UART2 and UART3 appear to have failed, but not UART1. It is unlikely to be a “dry” solder joint, because you see 3.3V on the TX pin with your oscilloscope…

But, I have no other ideas. Sorry!

Apologies for the inconvenience,
Paul