RS232/RS485 Communication Issues on Colibri iMX6S

Technical Support
1

Hello,

I’m trying to configure UART1 on a Colibri iMX6S module to support both RS232 and RS485 communication, but I’m experiencing issues with both modes.

Hardware Configuration

  • Module: Colibri iMX6S
  • Carrier Board: Custom carrier board with SODIMM 200 connector
  • Pin Mapping:
    • UART1_RX: SODIMM 33
    • UART1_TX: SODIMM 35
    • RS485/RS232 mode switch: SODIMM 23
    • RS485 direction control: SODIMM 27

Uart Section of the Device Tree Configuration

&uart1 {
    linux,rs485-enable-at-boot-time;
    dma-names = "rx","tx";
    status = "okay";
};

Issues

RS232 Mode

  • TX: The TX LED is flashing when sending data, but the receiver side is not getting any data.:cross_mark:
  • RX: The RX LED remains off, and no data is received when another device sends data to the interface.:cross_mark:

RS485 Mode

  • TX: The TX LED is flashing when sending data, and the receiver side is getting the correct data.:white_check_mark:
  • RX: The RX LED remains off, and no data is received when another device sends data to the interface.:cross_mark:

Additional Observations

  • Manually switching SODIMM pins 23 or 27 (using gpioset to high or low) has no effect on either mode.

Questions

  1. Are there any additional device tree properties required for proper RS232/RS485 operation on the iMX6S UART?
  2. Should SODIMM pin 27 be explicitly configured in the device tree for RS485 direction control?

The serial cable and USB adapter for testing are not the problem. I have already tested it with another device.

tdx-info

Software summary

Bootloader: U-Boot
Kernel version: 6.6.108-7.4.0-00057-g451f1e35226f #1-Torizon SMP PREEMPT Mon Jun 30 14:11:55 UTC 2025
Kernel command line: root=LABEL=otaroot rootfstype=ext4 quiet logo.nologo vt.global_cursor_default=0 plymouth.ignore-serial-consoles splash fbcon=map:3 ostree=/ostree/boot.1/torizon/893cd76ca21a93ca4fab8d9e3f8f5d218ffee158fe1910c24657918130c0bef6/0
Distro name: NAME=“Torizon OS Upstream”
Distro version: VERSION_ID=7.4.0-build.28
Distro variant: VARIANT=“Docker”
Hostname: colibri-imx6-11510521

Hardware info

HW model: Toradex Colibri iMX6DL/S
Toradex version: 0077 V1.1A
Serial number: 11510521
Processor arch: armv7l

Any guidance would be greatly appreciated!

Thanks in advance.

2

Hi, I’m the Toradex AI Assistant.

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RS485/RS232 Configuration Issues on Colibri iMX6S

Based on your configuration and the available documentation, I can help you troubleshoot the UART1 RS485/RS232 issues on your Colibri iMX6S module.

Device Tree Configuration Issues

Your current device tree configuration has a syntax error that needs correction (1). The property should be:

&uart1 {
    linux,rs485-enabled-at-boot-time;
    dma-names = "rx","tx";
    status = "okay";
};

(1)

Note the correction: linux,rs485-enabled-at-boot-time (not linux,rs485-enable-at-boot-time).

Additional RS485 Properties Required

For proper RS485 operation, you need additional device tree properties (2):

&uart1 {
    status = "okay";
    linux,rs485-enabled-at-boot-time;
    rs485-rts-active-low;
    // Remove rs485-rx-during-tx to suppress echo
};

(2)

The rs485-rts-active-low property controls the RTS signal polarity, which is crucial for proper direction control in RS485 mode (2).

RTS Control for RS485 Direction

For RS485 operation on iMX6-based modules like your Colibri iMX6S, the RTS signal is used for direction control (3). The documentation shows that for i.MX6 modules, you can configure RS485 through registry entries (3):

[HKLM\Driver\BuiltIn\COMX]
"RTSToggleAlwaysOn" = "dword:1"
"RTSPol" = "dword:1"
"RTSPin" = "dword:1b" ; module SODIMM/MXM3 pin number

(3)

However, this is for Windows CE. For Linux, the device tree approach should handle RTS control automatically when properly configured.

Troubleshooting Steps

  1. Verify RTS Signal Connection: Ensure SODIMM pin 27 (your RS485 direction control) is properly connected to the RS485 transceiver’s direction enable pin.

  2. Check RTS Polarity: The rs485-rts-active-low property may need adjustment based on your transceiver circuit design (2).

  3. Remove Echo Configuration: Avoid using rs485-rx-during-tx in your device tree as this can cause echo issues (2).

  4. Test RTS Toggle: You can verify if the RTS signal is being toggled during transmission by monitoring the pin with an oscilloscope or logic analyzer (2).

Recommended Device Tree Configuration

&uart1 {
    status = "okay";
    linux,rs485-enabled-at-boot-time;
    rs485-rts-active-low;  // Adjust polarity as needed for your hardware
    dma-names = "rx","tx";
};

(1), (2)

Additional Considerations

  • The RX issues you’re experiencing in both RS232 and RS485 modes suggest a potential hardware connection or configuration problem rather than just a software issue.
  • Verify that your custom carrier board’s UART1 connections match the expected Colibri pinout.
  • Check if any other device tree overlays might be conflicting with your UART1 configuration.

The fact that TX works in RS485 mode but RX doesn’t work in either mode points to a potential issue with the receive path configuration or hardware connections.

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