def sendPacket( self, messageType, data = "" ):
packet = struct.pack( '=BBBBIBHB',
0xAA, # SAH
len(data)+11, # data length + compliment
255-len(data)-11,
PROTOCOL_VERSION, # version (in doc is used version 1, but sample code sends 0)
self.timestamp, # unique number for ACK
0, # flags
messageType, # 16bit
0x55 ) # STX - data start
packet += data
packet += struct.pack("H", checksum( [ord(x) for x in packet] ))
# print repr(packet)
self.timestamp += 1
self.com.write( packet )
def _readPacket( self ):
b = self.com.read(1)
while b != chr(0xAA):
print "skipping", hex(ord(b))
b = self.com.read(1)
length = ord(self.com.read(1))
notLength = ord(self.com.read(1))
assert length+notLength == 0xFF, (length, notLength)
ret = ""
for i in xrange(length):
b = self.com.read(1)
ret += b
assert checksum( [0xAA, length, notLength] + [ord(x) for x in ret[:-2]] ) == ord(ret[-2]) + 256*ord(ret[-1])
version, timestamp, flags, msgType, stx = struct.unpack("=BIBHB", ret[:9])
assert version == 0x00, version # receiving ver0, but sending ver1
assert flags == 0, flags
assert stx == 0x55, stx
return timestamp, msgType, ret[9:-2]
