Installation
$ pip install serialstructMotivation
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)1 maintainer
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