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I am working with a SOM mounted on a carrier board running Ubuntu 14.04 with the generic 3.13 kernel.

While testing out the peripherals, I hit a problem with serial communication.

Basically, I can transmit data from the custom platform to an external Linux machine, but I can not properly receive data from the external Linux machine to the custom platform.

Through my research I have messed with all sorts of BIOS settings, baud rates, hardware flow control, parity, etc. Nothing has worked. Most info I have found online just says "Make sure your baud rates and other settings match", and they do. It is not my first time working with Linux serial ports. But it is my first time encountering a problem like this.

Does anyone have any suggestions, recommendations, or has anyone ever seen an issue like this before?

More info: We are running a quad-core Intel Atom micro with a custom serial breakout interface. The serial port is at /dev/ttyS0.

To reiterate: I can properly send data FROM the custom board TO the external Linux box, but not the other way around.

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From your description the issue can be in two places, Tx pin on Linux box can be demaged, or Rx pin on custom board.

I sugest to check serial port on Linux box and on custom board, by connecting Rx and Tx pin together on the same platform creating echo. This can show where problem is, on Linux box or on custom board.

Have you tried using other wire, maybe wire is broken?

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    I traced it down to the TI MAX232 part on the custom board. I put a scope on the Tx and Rx lines coming from the Linux box and the levels looked fine. I then looked at the Tx and Rx lines from the MAX232 part to the custom SOM and the Rx line stayed low. So basically the MAX232 part was not properly translating the signal level on the Rx line to the custom board. – RScottCarson Jul 15 '14 at 13:32

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