Communication between two serial ports with Linux

Question

I hope someone can help me. I am trying to make a serial ports comunication in an ARM (I am working with Linux). The thing is that I am using program called terminal in windows which I use for see the output. I have two cable connected from my ARM to the USB of my computer. What I do is open two windows of terminal program, and I connect with the respectives serial port, until here everything ok. Now in one of the windows I have the ARM console (this is the ttymxc1 serial port) in which I write for example "echo hello > /dev/ttymxc2" (the ttymxc2 is the other port in the other windows). When I executed it somenthing appear, somenthing like this : "<0><0><0><0><0><0><0>". I was reading and I found out that it usually is a baud rate problem, so I change the baud rate of the "ttymxc2" to 115200 like the "ttymxc1" both with the same speed. Once I did that and I tried to send again the same echo, appear this: <0>©::*ë<0> so, somenthig was changing. I get all the params of both serial ports…

Accepted Answer

Based on the positive results of the /proc/tty/driver/IMX-uart tests and the garbage data received, it seems safe to assume that Linux on your ARM board is sending output when you issue the shell command `echo hello > /dev/ttymxc2`. And we know that you have a good serial link between the host PC and /dev/ttymxc2, the serial console. PART Ia Normally the first item to check is the output of imx-uart.2 at its connector on the board. You would need an oscilloscope, and could confirm that the logic levels or RS-232 voltages seemed okay. This check is to also confirm that you're connected on the correct pin. PART Ib In the setup and the results (so far) that you have described, you have one working serial link and another serial link of unknown condition. The working serial link is: ``` ----------(cable_A)----------- ``` and the other serial link consists of: ``` ----------(cable_B)----------- ``` This testing assumes that all four serial ports are configured for 115200 8n1. By selective substitution into the working link, you can verify that the components of the second link are functional. For instance, the first substitution could be replacing the cable: ``` ----------(cable_B)----------- ``` If the link to the console still works, then the next substitution could be swapping the the PC port connection: ``` ----------(cable_B)----------- ``` And then the other serial link would now consist of ``` ----------(cable_A)----------- ``` So now the second serial link is composed of previously-known working components and just one unknown, the port on the ARM board. Redo your tests. PART II IF you still cannot get positive results from the swapping, then you need to provide more details of your setup. You mention something about USB. So apparently PC COM port_A and PC COM port_B are each a combination of USB port and somekind of RS-232-to-USB adapter? Please provide details of the board connections for imx-uart.1 and imx-uart.1. Are theses interfaces at RS-232 voltages or at logic level? If logic level, then what are you using to convert to RS-232? If you're using a FTDI USB-to-TTL-serial "cable", then that could be an issue. I had a problem with the FTDI "cable" connected to one SoC that had a slightly-slow baud rate and slow slew rates. The issue was that the received data was always total garbage characters. Replacing the FTDI "cable" with a USB-to-RS232 adapter and a MAX3232 converter (RS-232 to 3.3V logic) solved that connection problem. Compared to the Sparkfun board and the FTDI "cable" (and its variants and knockoffs), I would recommend the use of a real TTL-to-RS232 converter (using a MAX3232 or similar chip) for reliability. Use the Sparkfun board or the FTDI "cable" only after you or someone has verified that the setup actually works. In that situation I had, one serial port (on the SoC) was fixed at 115200 baud. But you have full control of your situation. So when you are receiving garbage data, one test to try is to reduce the baud rate (at both ends of the serial link) to 9600 or even 1200.

Communication between two serial ports with Linux

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