RS485 Direction Control by UART RTS Signal

Issue Overview

Users are experiencing difficulties with the RS485 transceiver control via the RTS signal of UART0 on the Nvidia Jetson Orin Nano Dev board. The main symptoms include:

• Inability to receive messages: Users can send messages from the Jetson board to a PC but cannot receive messages sent back from the PC.
• RTS Signal Behavior: The RTS signal remains in a high state continuously, preventing the UART from switching to receive mode.
• Driver Modification Concerns: Users suspect that modifications in the kernel driver (specifically in kernel/drivers/tty/serial/) may be necessary to control the RTS signal properly.

The problem occurs during setup and operation of UART0, specifically when using the serial device /dev/ttyTHS1 with minicom. The issue has been consistently reported, indicating a potential systemic problem rather than isolated incidents. This affects user experience significantly, as it limits the functionality of RS485 communication, which relies on proper control of the RTS signal for sending and receiving data.

Possible Causes

• Hardware Configuration: The custom board’s hardware setup may not be properly configured to handle RTS signals for RS485 communication.

• Software Bugs or Conflicts: There may be bugs in the UART driver that prevent it from managing the RTS signal correctly.

• Configuration Errors: Incorrect settings in minicom or other serial applications might lead to improper flow control settings.

• Driver Issues: The current driver may not support dynamic control of RTS states as required for RS485 communication.

• User Misconfigurations: Users may not have disabled hardware flow control in their serial applications, which is necessary for RS485 communication.

Troubleshooting Steps, Solutions & Fixes

1. Check Jetpack Version:

   • Confirm you are using Jetpack version 35.3.1 or later, as earlier versions may have unresolved issues with UART drivers.

2. Verify Hardware Flow Control Settings:

   • Open minicom and ensure that hardware flow control is disabled for /dev/ttyTHS1.
   • Use the following command in minicom settings:

     Ctrl+A then O (to access configuration)
     Navigate to Serial Port Setup and ensure HW Flow Control is set to 'No'.
     

3. Test Communication without Hardware Flow Control:

   • Attempt sending and receiving data with hardware flow control disabled. If successful, this confirms that flow control settings were causing the issue.

4. Modify Driver for RTS Control:

   • If you need to implement dynamic control of the RTS signal, consider modifying the UART driver located at:

     kernel/kernel-5.10/drivers/tty/serial/serial-tegra.c
     

   • Look for sections handling RTS signaling and implement logic to toggle its state based on your application needs.

5. Consult Documentation and Community Resources:

   • Refer to Nvidia’s official documentation and community forums for additional insights on UART configurations and RS485 communication specifics.

6. Testing with Different Configurations:

   • If possible, test with another UART interface or transceiver to isolate whether the issue lies within the specific hardware or configuration used.

7. Best Practices for Future Prevention:

   • Always verify flow control settings before initiating communication.
   • Keep your system updated with the latest Jetpack versions and patches that might address known issues with UART drivers.

8. Unresolved Aspects:

   • Further investigation may be needed regarding specific driver modifications and their impact on overall system stability.
   • Users are encouraged to document their changes and share outcomes within community forums for collective troubleshooting efforts.