Installation

$ pip install serialstruct

Motivation

When sending a structured binary packet over Serial, the only way (that I’m aware of) to guarantee packet alignment with arbitrary data is to send a header that’s larger than any of the elements and add padding between each element. Here’s an example:

struct Packet {
        int sensor1;
        int sensor2;
}

If we send this over the wire and start reading at an arbitrary time, it’s impossible to know what byte of the packet we’re reading. To mitigate this we can add a header and some padding.

struct Packet {
        char header[5]; // Any sequence without a '\0'
        int sensor1;
        char pad1;      // '\0'
        int sensor2;
        char pad2;      // '\0'
}

Now we just need to wait until the header sequence is read and we can consume the rest of the packet without worrying about alignment.

pySerial only implements a FramedPacket which expects a unicode sequence that starts with a ‘(’ (0x28) and ends with a ‘)’ (0x29). This means the bytes 0x28 and 0x29 cannot appear anywhere in the binary data.

Usage

Subclass serialstruct.StructuredPacket to specify the data size in the packet and to implement the handle_packet() callback function.

import struct

import serial
import serialstruct
import time


PACKET_STRUCT = struct.Struct("<IxIx")

class MyPacket(serialstruct.StructuredPacket):

    DATA_SIZE = 10 # Excluding header: 4+1+4+1

    def __init__(self):
        super(MyPacket, self).__init__(self.DATA_SIZE)

    def handle_packet(self, packet):
        print(PACKET_STRUCT.unpack(packet))

    def send_packet(self, packet):
        self.transport.write(self.HEADER)
        self.transport.write(packet)


ser = serial.serial_for_url("loop://", baudrate=115200, timeout=1)
with serial.threaded.ReaderThread(ser, MyPacket) as protocol:
    # unsigned int, pad, unsigned int, pad; with no alignment
    packet = PACKET_STRUCT.pack(1, 2)
    protocol.send_packet(packet)
    time.sleep(1)

Prints:

(1, 2)