RS-232 Communication Issue on Jetson (JetPack 5.1.2) – No Signal on TX Pin

[SS.nb]

Dear NVIDIA Support,

I’m currently testing RS-232 communication on a custom Jetson carrier board running JetPack 5.1.2. However, I am not observing any voltage change on the TX line (SODIMM Pin 99) when sending data. Below are the details of my environment and what I’ve tried so far:

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■ Environment

• JetPack version: 5.1.2
• Board: Custom carrier board with Jetson module
• Pin in use: SODIMM Pin 99 (UART0_TXD)

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■ What I’ve done

1. Added user to dialout group:

   sudo adduser $USER dialout  
   

2. Disabled nvgetty:

   sudo systemctl disable nvgetty.service  
   

3. Verified active UARTs with:

   sudo dmesg | grep -i tty  
   

   → Confirmed ttyTHS0 is listed and corresponds to UART0

4. Sent test string to UART0 TX:

   echo "hello" > /dev/ttyTHS0  
   

5. Confirmed that GPIO level on Pin 99 can be toggled using gpioset (so the pin is physically responsive)

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■ Pinmux Configuration
The pinmux settings for UART0_TXD and UART0_RXD are default (UB3_TXD, UB3_RXD), as shown in the attached screenshot and pinmux.dtsi. The UART0 function appears to be correctly assigned.

■ Problem
Despite all the above steps, there is no signal change or voltage output observed on SODIMM Pin 99 when sending data via /dev/ttyTHS0.

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■ Question
Is there any additional step required in JetPack 5.1.2 or pinmux configuration to fully enable UART0 for RS-232 level communication?
Do I need to check anything else in the device tree or kernel configuration?

Thank you very much for your support.
Orin-nabel-pinmux.txt (65.8 KB)

[KevinFFF]

Hi SS.nb,

@SS.nb

→ Confirmed ttyTHS0 is listed and corresponds to UART0

It seems you are using serial interface from M.2 Key E port.
Have you verified UART loopback test for this interface?

As my understanding, SODIMM Pin 99 (UART0_TXD) should be uart-b, which maps to 3110000.serial.
Please enable this node(serial@3110000) in device tree.

[SS.nb]

Thank you for your response.

Sorry for the confusion — I’m not trying to use the serial interface from the M.2 Key E port.

My goal is to use the UART for external RS-232 communication with another device, not for internal debugging or M.2.

Instead of a loopback test, I am observing the TX pin (SODIMM Pin 99) with an oscilloscope to confirm whether there is any voltage change during transmission.

I have configured the pinmux settings using the official spreadsheet tool as previously shown (UB3_TXD / UB3_RXD), but I’m wondering if this alone is not sufficient.

Should I also enable a specific serial node (e.g. serial@3100000 or serial@3110000) in the device tree explicitly for this to work?

Any advice would be appreciated.

[SS.nb]

Thank you for your previous responses.

I was able to get the UART TX pin working by explicitly modifying the Device Tree as follows:

Environment

• JetPack Version: 5.1.2
• Board: Custom carrier board for Jetson Orin

1. Device Tree modification

File location:
/Linux_for_Tegra/source/public/hardware/nvidia/soc/t23x/kernel-dts/tegra234-soc/tegra234-soc-uart.dtsi

I changed the serial@3110000 node (UARTB, /dev/ttyTHS1) to:

serial@3110000 {
    status = "okay";
};

Then I rebuilt and flashed the kernel. After this change, the TX pin output started working as expected.

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However, I couldn’t find this requirement explicitly documented in the Jetson Linux Developer Guide or official pinmux/DT overlay workflows.

• Is there any official documentation or reference that describes this step?
• Or is there a more standard/clean way to enable UARTB for external RS-232 communication?

Any guidance or clarification would be greatly appreciated.

[KevinFFF]

The serial interface and the mapping may differ due to the releases.
You can refer to 9.7 Universal Asynchronous Receiver/Transmitter(UART) in official Technical Reference Manual document for details, Or https://elinux.org/Jetson/L4T/peripheral/#Mapping_3 for the mappings.

We would suggest also checking the dmesg and device tree for the current status.