Problems with RS422/RS485 port on Apalis Evaluation board

I have not been able to get the RS422/485 port to work on the Apalis Evaluation Board Rev1.1. Has anyone else had success with this port?

I am assigning DMA_LPUART3 to the cortex M4_0 on the I.MX8, and using the M4_0 to process the UART data. Everything works fine when I use the RS232 upper port on X28, but when I use port X55 nothing happens. My blocking read hangs, and if I continuously read the DMA_LPUART3 status register, nothing changes (always read 0x00c000c0 - Receive data register is not full).

I am setting the jumpers as specified in the apalis_arm_evaluation_board_v1.1_datasheet.pdf:

Here are my X5, X6, X7 jumper connections:
Connect the M4_0 debug UART for my printfs:
MMX3_110 (M40.UART.RX) connected to UART1_RXD.
MMX3_120 (M40.UART.TX) connected to UART1_TXD.
Connect DMA_LPUART3 to eval board UART2:
MMX3_126 (DMA.UART3.TX) connected to UART2_TXD.
MMX3_132 (DMA.UART3.RX) connected to UART2_RXD.

My internal IO-muxing is working, because if I switch jumper JP11 to open, and send my serial data in through RS232 upper port on X28, everything works fine (can send and receive data through DMA_LPUART3 to the M4_0 core). I also confirmed with SCU reads that the pin-muxing for pins 126/132 is set to the DMA_LPUART3.

One change between the RS232 test and the RS422/485 test is that I use a USB to RS422/485 converter. I have confirmed that the cable works by doing a loopback test on the end of the cable. I have checked the RS422/485 (X55) pin mapping that is listed in section (I tried flipping RX/TX connections in case I had them backwards).

Any other ideas, or confirmation that this port does indeed work would be greatly appreciated.

1 Like

Is your USB to RS422/485 converter configured for half-duplex or full-duplex?


Greetings @dlm080!

Did you connect any RTS/CTS signals? I haven’t tested a configuration such as yours - routing the M4_0 UART to the RS422/485 interface - but I believe these controls signals are needed so messages are exchanged.

I got the same problem!

Hi @xavier . Could you create a new ticket with full information about your case?