serialstruct

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:

structPacket {
intsensor1;
intsensor2;
}

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.

structPacket {
charheader[5]; // Any sequence without a '\0'intsensor1;
charpad1; // '\0'intsensor2;
charpad2; // '\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.

importstructimportserialimportserialstructimporttimePACKET_STRUCT=struct.Struct("<IxIx")
classMyPacket(serialstruct.StructuredPacket):
DATA_SIZE=10# Excluding header: 4+1+4+1def__init__(self):
super(MyPacket, self).__init__(self.DATA_SIZE)
defhandle_packet(self, packet):
print(PACKET_STRUCT.unpack(packet))
defsend_packet(self, packet):
self.transport.write(self.HEADER)
self.transport.write(packet)
ser=serial.serial_for_url("loop://", baudrate=115200, timeout=1)
withserial.threaded.ReaderThread(ser, MyPacket) asprotocol:
# unsigned int, pad, unsigned int, pad; with no alignmentpacket=PACKET_STRUCT.pack(1, 2)
protocol.send_packet(packet)
time.sleep(1)

Prints:

(1, 2)