mjhjmtu

I'm new to using serial ports, and I have a loopback cable for serial on a Centos 7 machine. 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. Is there some kind of software loopback that already exists for ports in Linux?

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

BAUD_RATE = 9600

def main():
 tx = b"A"

 COM_PORT='/dev/ttyS0'
 ser = serial.Serial(COM_PORT, BAUD_RATE, timeout=1, parity=serial.PARITY_NONE, rtscts=False, dsrdtr=False)
 ser.setDTR(False)
 ser.flush()
 print("Port " + COM_PORT)
 for y in range (0,10):
 print("Sent: " + str(tx))
 ser.write(tx)
 rx = ser.read(1)
 print("Received: " + str(rx))
 if(len(rx) > 0):
 print(COM_PORT + " - " + str(rx))
 ser.close()

if __name__ == "__main__":
 main()

The same thing happens when I use the much shorter and simple serial test shown below

Terminal 1

cat /dev/ttyS0

Terminal 2

echo "test" > /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.

Output From Tests

• 
  

  Checking if interface is really a character device

  
  
  

  ls -l /dev/ttyS0

  
  

output: crw-rw----, 1 user group 4, Date /dev/ttyS0

• 
  

  Check no other processes have it open

  
  
  

  sudo lsof /dev/ttyS0

  
  

output: Error I will have to copy in later. If I run without sudo I get a blank line returned

• 
  

  Checking for UART

  
  
  

  sudo setserial -v /dev/ttyS0

  
  

output: /dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4

• 
  

  CTS and similar pins change when signal applied

  
  
  

  sudo statserial /dev/ttyS0

  
  

output: /dev/ttyS0, UART: 16550A, Port: 0x03f8, IRQ: 4

There is no change to this output when the echo command to the port is running with or without the cable connected.

• 
  

  Check that the transmit and receive numbers change

  
  
  

  sudo cat /proc/tty/driver/serial

  
  

output:

0: uart:16550A port:000003F8 irq:4 tx:8335 rx:8301
1: uart:16550A port:000002F8 irq:3 tx:0 rx:0
//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 ttyS0

Another problem/symptom that could be related is that after I run the python script, I'm no longer able to cat /dev/ttys0

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. I checked and the file isn't still open. I'm not sure why that would happen.

Update

After 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. 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.