Reading 'Weird' Serial Communication GPIO

Question

I'm trying to read communication from a Curtis motor controller to my Pi 2. Serial communication is normally Idle=High and the start bit is low. The motor controller however has a Idle low, and a high start bit. So using pyserial to read just gives random characters. The output of the motor controller is being stepped down to 3.3V, the encoding is ascii.

-0 4P!3@"7 %^=!      

-^!\8( 4P!7 @!3 %.   

- K@"

How would I be able to read in the data successfully?

Answer 1

The following untested code may work. Untested as I don't have a device which inverts the logic.

Change the baud rate to suit.

#!/usr/bin/env python

# serial_invert.py
# 2016-03-18
# Public Domain

import time
import pigpio # http://abyz.me.uk/rpi/pigpio/python.html

RXD=15

pi = pigpio.pi()

if not pi.connected:
   exit(0)

pigpio.exceptions = False # Ignore error if already set as bit bang read.

pi.bb_serial_read_open(RXD, 9600) # Set baud rate here.

pigpio.exceptions = True

pi.bb_serial_invert(RXD, 1) # Invert line logic.

stop = time.time() + 60.0

while time.time() < stop:

   (count, data) = pi.bb_serial_read(RXD)
   if count:
      print(data)

   time.sleep(0.2)

pi.bb_serial_read_close(RXD)

pi.stop()

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EDITED TO ADD

I added an inverter between the TXD and RXD GPIO so I could test the code.

The following shows RXD is the inverse of TXD.

The received data is interpreted as gibberish until the line logic is inverted (slri command)

$ pigs slro 15 9600 8 # Open RXD for bit bang serial read.
$ echo "Hello" >/dev/ttyAMA0
$ pigs slr 15 100
14 171 211 210 82 42 235 0 171 211 210 82 42 235 0 # Gibberish
$ echo "Hello" >/dev/ttyAMA0
$ echo "Hello" >/dev/ttyAMA0
$ pigs -a slr 15 100
21 \xab\xd3\xd2R*\xeb\x00\xab\xd3\xd2R*\xeb\x00\xab\xd3 # Gibberish
$ pigs slri 15 1 # Now invert line logic.
$ echo "Hello" >/dev/ttyAMA0
$ pigs -a slr 15 100
7 Hello\x0d\x0a
$ echo "The lazy brown fox jumps over the sleeping dog" >/dev/ttyAMA0
$ pigs -a slr 15 100
48 The lazy brown fox jumps over the sleeping dog\x0d\x0a

Answer 2

Possible problems:

Ground Connection

---

As well as TX and RX, you will need a common ground between the two devices.

Baudrate

---

The Pi's standard baudrate is 115200, while most modules are 9600, and some are different, make sure you are using the correct baudrate.