Serial protocol design in Raspberry Pi - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to open a serial port on Raspberry Pi.

Raspberry Pi

import serial
ser = serial.Serial([1], 9600)

Drag options to blanks, or click blank then click option'

A"/dev/ttyS0"

B"/dev/ttyUSB0"

C"COM1"

D"/dev/serial0"

Attempts:

3 left

💡 Hint

Common Mistakes

Using Windows COM port names like COM1.

Using USB serial device names when not connected via USB.

On Raspberry Pi, the primary serial port is usually accessed via /dev/serial0.

2fill in blank

medium

Complete the code to send the string 'Hello' over the serial port.

Raspberry Pi

ser.write([1])

Drag options to blanks, or click blank then click option'

Ab'Hello'

B"Hello"

C'Hello'

DHello

Attempts:

3 left

💡 Hint

Common Mistakes

Sending a string without converting to bytes causes errors.

Forgetting the byte prefix b.

The write method requires bytes, so prefix the string with b.

3fill in blank

hard

Fix the error in reading one byte from the serial port.

Raspberry Pi

data = ser.read([1])

Drag options to blanks, or click blank then click option'

A-1

B1

C10

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Passing 0 or negative numbers causes no data or errors.

Passing a larger number reads more bytes than needed.

To read one byte, pass 1 as the argument to read.

4fill in blank

hard

Fill both blanks to create a dictionary mapping bytes to their ASCII codes for letters only.

Raspberry Pi

ascii_map = {chr([1]): [2] for [1] in range(65, 91)}

Drag options to blanks, or click blank then click option'

Ai

Bord(i)

Cchr(i)

Di+32

Attempts:

3 left

💡 Hint

Common Mistakes

Using ord(i) causes a TypeError because i is an integer, not a string or character.

Using the wrong variable name in the comprehension.

Use i as the loop variable and i as the value since range(65, 91) generates the ASCII codes for uppercase letters directly.

5fill in blank

hard

Fill all three blanks to filter and map serial data bytes greater than 100.

Raspberry Pi

filtered = [[1] for [2] in data if [2] [3] 100]

Drag options to blanks, or click blank then click option'

Abyte

Bb

C>

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using the same name for output and loop variable causes confusion.

Using < instead of > changes the filter condition.

Use byte as the output, b as the loop variable, and > to filter bytes greater than 100.

Practice

1. What is the main purpose of using start and end bytes in a serial protocol?

easy

A. To increase the speed of data transfer

B. To mark the beginning and end of a message

C. To encrypt the message data

D. To compress the message content

Solution

1. Step 1: Understand message framing

   Start and end bytes are special markers that show where a message begins and ends in a data stream.

2. Step 2: Identify their role in communication

   They help the receiver know exactly which bytes belong to one message, avoiding confusion with other data.

3. Final Answer:

   To mark the beginning and end of a message -> Option B

4. Quick Check:

   Start/end bytes = message boundaries [OK]

Hint: Start/end bytes frame messages clearly [OK]

Common Mistakes:

• Thinking start/end bytes encrypt data
• Confusing start/end bytes with speed control
• Assuming they compress data

2. Which of the following is the correct way to represent a start byte in a serial protocol in Python code?

easy

A. start_byte = 02

B. start_byte = '0x02'

C. start_byte = b'\x02'

D. start_byte = "\x02"

Solution

1. Step 1: Understand byte representation in Python

   Bytes are represented with a b prefix and escape sequences like \x02 inside single or double quotes.

2. Step 2: Check each option

   start_byte = b'\x02' uses b'\x02' which is a bytes object with hex value 0x02. start_byte = '0x02' is a string, not bytes. start_byte = 02 is an integer. start_byte = "\x02" is a string, not bytes.

3. Final Answer:

   start_byte = b'\x02' -> Option C

4. Quick Check:

   Bytes need b prefix and hex escape [OK]

Hint: Bytes start with b and use \x for hex [OK]

Common Mistakes:

• Using strings instead of bytes
• Missing the b prefix
• Using decimal numbers instead of bytes

3. Given this Python snippet for a simple serial message:

message = b'\x02HELLO\x03'
start_byte = b'\x02'
end_byte = b'\x03'
if message.startswith(start_byte) and message.endswith(end_byte):
    content = message[1:-1]
else:
    content = b''
print(content)

What will be printed?

medium

A. b'HELLO'

B. b'\x02HELLO\x03'

C. b''

D. HELLO

Solution

1. Step 1: Check message start and end bytes

   The message starts with b'\x02' and ends with b'\x03', so the condition is True.

2. Step 2: Extract content between start and end bytes

   content = message[1:-1] removes the first and last byte, leaving b'HELLO'.

3. Final Answer:

   b'HELLO' -> Option A

4. Quick Check:

   Strip start/end bytes = b'HELLO' [OK]

Hint: Slice message to remove start/end bytes [OK]

Common Mistakes:

• Printing the whole message without slicing
• Forgetting bytes notation in output
• Confusing string and bytes output

4. This code is intended to check if a serial message has a valid checksum byte at the end:

def check_checksum(msg):
    data = msg[:-1]
    checksum = msg[-1]
    calculated = sum(data) & 0xFF
    return checksum == calculated

message = b'\x02ABC\x06'
print(check_checksum(message))

What is the error in this code?

medium

A. It includes the start byte in checksum calculation

B. It does not convert checksum to integer

C. It uses bitwise AND instead of modulo

D. It compares checksum with data instead of sum

Solution

1. Step 1: Analyze checksum calculation

   The code sums all bytes except the last one, but this includes the start byte (b'\x02'), which usually should be excluded.

2. Step 2: Identify correct checksum range

   Typically, checksum is calculated only on the data bytes, excluding start and end bytes. Here, start byte is wrongly included.

3. Final Answer:

   It includes the start byte in checksum calculation -> Option A

4. Quick Check:

   Exclude start byte from checksum [OK]

Hint: Exclude framing bytes from checksum sum [OK]

Common Mistakes:

• Including start/end bytes in checksum
• Confusing bitwise AND with modulo
• Not converting bytes to int before sum

5. You want to design a serial protocol message with this format: start byte (0x7E), payload (ASCII text), checksum (sum of payload bytes mod 256), and end byte (0x7F). Which Python code correctly builds this message for payload = b'HELLO'?

hard

A. payload = b'HELLO'
start = b'\x7E'
end = b'\x7F'
checksum = sum(payload) & 0xFF
message = start + payload + checksum + end

B. payload = b'HELLO'
start = b'\x7E'
end = b'\x7F'
checksum = sum(payload) % 256
message = start + payload + checksum + end

C. payload = b'HELLO'
start = b'\x7E'
end = b'\x7F'
checksum = bytes([sum(payload)])
message = start + payload + checksum + end

D. payload = b'HELLO'
start = b'\x7E'
end = b'\x7F'
checksum = bytes([sum(payload) % 256])
message = start + payload + checksum + end

Solution

1. Step 1: Calculate checksum correctly

   Checksum is sum of payload bytes modulo 256, so sum(payload) % 256 is correct.

2. Step 2: Convert checksum to bytes and build message

   Checksum must be a single byte, so bytes([value]) converts it properly. Then concatenate start, payload, checksum, and end bytes.

3. Final Answer:

   payload = b'HELLO' start = b'\x7E' end = b'\x7F' checksum = bytes([sum(payload) % 256]) message = start + payload + checksum + end -> Option D

4. Quick Check:

   Checksum as bytes and modulo 256 [OK]

Hint: Use bytes([sum(payload) % 256]) for checksum [OK]

Common Mistakes:

• Not converting checksum to bytes
• Using bitwise AND instead of modulo
• Concatenating int with bytes directly