Unable to send/receive data via HC-12/UART in Python

Question

I've written some code to communicate between two Raspberry Pi's, using identical HC-12 433Mhz transceivers. I was able to successfully echo between the two Pi's using a direct serial connection and echo/cat, however am unable to replicate this using HC-12s, which theoretically work by a similar principal. I'm using the port ttyAMA0 on both for this example, but ttyS0 is also available and have tried every combination of these ports.

The following code is common to both the sending and receiving, just writing once for sake of brevity:

import serial
import time

ser = serial.Serial(
    port = "/dev/ttyAMA0",
    baudrate = 9600,
    parity = serial.PARITY_NONE,
    stopbits = serial.STOPBITS_ONE,
    bytesize = serial.EIGHTBITS
)
print("Serial status: " + str(ser.isOpen()))

This is the sending program:

while True:
    print("Sending...")
    ser.write("hello\n".encode())
    time.sleep(1)

And the receiving program:

while True:
    print("Receiving...")
    data = ser.readlines()
    print(data.decode())

The sending program simply loops as expected, but the receiver prints "Receiving...", and then nothing.

When I keyboard interrupt the receiving program at that point, it says it is currently up to data = ser.readlines().

Any help would be much appreciated - I've spent the better part of the last week trawling and exhausting forums and READMEs to no avail, and this is literally my last option. Am close to insanity on this one!

Accessible

.readlines(), with no timeout specified, is going to keep reading forever. Try .readline() instead. – Ahearn

@Ahearn thanks for the suggestion, but no luck - same result! – Accessible

Answer 1

The pyserial readlines() function relies on the timeout parameter to know when end-of-file is reached - this is warned about in the doco. So with no timeout, the end never occurs, so it keeps buffering all lines read forever.

So you can just add a timeout to the serial port open, and your existing code will begin to work.

ser = serial.Serial(
    port     = "/dev/ttyAMA0",
    baudrate = 9600,
    parity   = serial.PARITY_NONE,
    stopbits = serial.STOPBITS_ONE,
    bytesize = serial.EIGHTBITS,
    timeout  = 2                        # seconds     # <-- HERE
)

A better approach might be to use readline() (note singular, no 's'), for each line in turn:

print( "Receiving..." )
while True:
    try:
        data = ser.readline()
        print( data.decode() )
        # TODO - something with data
    except:
        print( "Error reading from port" )
        break

As that will allow the code to act on the input line-by-line.

Physicality

Answer 2

Use Serial.read_until method. The default termination character is \n.

For example,

data = ser.read_until()
print(data)

Pictish