Is there anything that could be blocking their receive that I haven't tried? Once again, I'm assuming something is configured differently since the first port has expected behavior (other than the auto loopback I didn't expect), but I haven't found any rules applied to any of the ports. I'm not sure why that would happen.Īfter testing the other ports that were showing 0 on transmit and receive, I noticed that I actually am seeing the transmit value increase, but they are unable to receive. So, I can cat that file any time before I run the python script, as many times as I want, but immediately after I run that Python, I can't cat the file, it just immediately returns with nothing. There's a few more serial ports, but they all have 0 tx and rx, and on port 03F8, the tx and tx increase when I send to the serial port ttyS0Īnother problem/symptom that could be related is that after I run the python script, I'm no longer able to cat /dev/ttys0 There is no change to this output when the echo command to the port is running with or without the cable connected.Ĭheck that the transmit and receive numbers change Output: /dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4ĬTS and similar pins change when signal applied If I run without sudo I get a blank line returned Output: Error I will have to copy in later. Output: crw-rw-, 1 user group 4, Date /dev/ttyS0 Like I said, I get the expected output from both tests, but I get the same output whether the cable is connected or not.Ĭhecking if interface is really a character device ![]() Take a metal paperclip or wire and cross pins T- with R- and T+ with R+. You can do this by taking a female/female cable or gender changer and plugging it into the serial port. Convert your serial port to be female if it is not already. The same thing happens when I use the much shorter and simple serial test shown below To check whether or not a RS-422 serial port is working, perform a RS-422 loopback test. Ser = serial.Serial(COM_PORT, BAUD_RATE, timeout=1, parity=serial.PARITY_NONE, rtscts=False, dsrdtr=False) In case it's useful information I'm using pySerial shown below (I had to type some of it from memory so I apologize if there are errors, but my question isn't about the code as much as it is how these ports actually work) import serial Is there some kind of software loopback that already exists for ports in Linux? I have something in python that tests the port, but I've found that the program returns the expected values even when my loopback isn't attached. ![]() This is when the receiver will report to the transmitter - stop because my buffer is about to be filled ! Then the transmitter waits till the receiver digests the information that was received and then allows the transmitter to continue to send more data.I'm new to using serial ports, and I have a loopback cable for serial on a Centos 7 machine. Often hardware flow control is used to fix such issues. ![]() Otherwise you will face a buffer overrun (where the buffer is unable to hold all received data due to limited space inside the UART). Keep in mind that the buffer depth is limited so only test with a few characters at a time. You should be able to see what you transmitted inside your receive buffer. Try a simple write to the UART with a few char string like "Hello" -> be sure that TX & RX are shorted for your UART pins -> check the receive buffer after this code run. If you wish to see what you transmit then TX & RX pins must be shorted so that the OUTPUT from the UART (TX) is mated with the INPUT to the UART (RX).Īlso review this thread for an improved solution to using the serial ports: Change the value of Data to 'b' and then again to 'c' to see the results.
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