How can i extract a specific sequence from serial data - python-3.x

Question

I am trying to read some serial data from raspberry pi 3 using pyserial api, I need the data in a particular sequence e.g 3xxx7xxxBxxxExxx, however, sometimes the data stream starts from Exxx3xxx7xxxBxxx, how can I ensure that I always read the right sequence irrespective of how the data streams in.
This is for reading data from a control card used in a special pump application.
import serial
import time
ser = serial.Serial(
port = '/dev/ttyACM0',
baudrate = 9600,
parity = serial.PARITY_NONE,
stopbits = seial.STOPBITS_ONE,
bytesize = serial.EIGHTBITS,
timeout = 1
)
time.sleep = 1
try:
while 1:
if ser.inWaiting() > 0:
data = ser.read(8)
print(data.hex())
this gives me the result in 3xxx7xxxBxxxExxx sometimes and other times it starts from 7xxx or Exxx, however, I always want to be able to extract the sequence 3xxx7xxxBxxxExxx irrespective of where the stream starts from.

Answer 1

Assuming that the marker which you use to determine the start of the sequence is a full byte 0x30 you can try the following code. It will read one byte at a time until the start marker is found. Then it will read another 7 bytes and append them:
import serial
import time
ser = serial.Serial(
port = '/dev/ttyACM0',
baudrate = 9600,
parity = serial.PARITY_NONE,
stopbits = seial.STOPBITS_ONE,
bytesize = serial.EIGHTBITS,
timeout = 1
)
time.sleep = 1
start_marker = b'\xE0'
try:
while 1:
if ser.inWaiting() > 0:
data = ser.read(1)
if data != start_marker:
continue
data += ser.read(7)
print(data.hex())
EDIT
Try this code to match the most significant 4 bits of each bytes pair:
import serial
import time
ser = serial.Serial(
port = '/dev/ttyACM0',
baudrate = 9600,
parity = serial.PARITY_NONE,
stopbits = seial.STOPBITS_ONE,
bytesize = serial.EIGHTBITS,
timeout = 1
)
time.sleep = 1
data = bytes()
try:
while 1:
if ser.inWaiting() > 0:
data += ser.read(1)
if len(data) < 8:
continue
if len(data) > 8:
data = data[-8:]
if not (
data[0] >> 4 == 0x3 and
data[2] >> 4 == 0x7 and
data[4] >> 4 == 0xB and
data[6] >> 4 == 0xE
):
continue
print(data.hex())