161 lines
3.5 KiB
Python
161 lines
3.5 KiB
Python
#!/usr/bin/env python2
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import serial
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import random
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import time
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from crcmod.predefined import PredefinedCrc
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# Timings in bit times
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TIMEOUT = 48
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CLEARCHANNEL = 2 * TIMEOUT
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class SSS7Bus(object):
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def __init__(self, port, baudrate=9600):
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random.seed()
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self._bit_time = 1.0 / baudrate
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self._serial = serial.Serial(port, baudrate, timeout=self._bit_time * TIMEOUT)
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self._buffer = []
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def _debug(self, msg):
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print "[SSS7Bus] %s" % msg
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def _flush_input_buffer(self):
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while self._serial.inWaiting() > 0:
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self._debug("Flushing input buffer")
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self._read_frame()
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def _can_send(self):
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self._flush_input_buffer()
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self._serial.timeout = self._bit_time * CLEARCHANNEL
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self._debug("Checking if bus is idle for %f seconds" % (self._serial.timeout))
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first_byte = self._serial.read(1)
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if len(first_byte) == 0:
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self._debug("Bus seems idle")
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return True
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self._debug("Bus is not idle, reading frames")
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self._read_frame_rest(first_byte)
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return False
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def _read_frame(self):
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self._debug("Trying to read a frame from the bus")
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self._serial.timeout = timeout=self._bit_time * TIMEOUT
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first_byte = self._serial.read(1)
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if len(first_byte) == 0:
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self._debug("Timeout reading first byte")
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return False
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return self._read_frame_rest(first_byte)
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def _read_frame_rest(self, first_byte):
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if ord(first_byte) <> 0xAA:
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self._debug("Wrong first byte: %s" % hex(ord(first_byte)))
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return False
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second_byte = self._serial.read(1)
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if len(second_byte) == 0:
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self._debug("Timeout reading second byte")
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return False
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if ord(second_byte) <> 0xFE:
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self._debug("Wrong second byte")
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return False
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length_byte = self._serial.read(1)
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if len(length_byte) == 0:
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self._debug("Timeout reading length byte")
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return False
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length = ord(length_byte) + 2 # Payload length + 2 byte CRC16
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self._serial.timeout = timeout=self._bit_time * TIMEOUT
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self._debug("Trying to read remaing %d bytes of frame" % length)
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data = self._serial.read(length)
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# if read returns less then length, no new byte has been received for timeout seconds
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if len(data) != length:
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self._debug("Timeout reading frame payload")
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return False
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crc = data[-2:]
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payload = data[:-2]
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crc16 = PredefinedCrc('crc16')
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crc16.update(payload)
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real_crc = crc16.digest()
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if real_crc != crc:
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self._debug("Wrong crc for frame payload")
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return True
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self._debug("Sucessfully read rest of frame")
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self._buffer.insert(0,payload)
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return True
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def _send_byte(self, byte):
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self._serial.write(byte)
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return byte == self._serial.read(1)
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def _send_frame(self, frame):
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while not self._can_send():
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pass
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for byte in frame:
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result = self._send_byte(byte)
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if not result:
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self._debug("Sending frame failed with collision")
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return False
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self._debug("Successfully send frame")
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return True
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def send_message(self, msg, priority=5):
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if len(msg) > 253:
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raise ValueError("Message length can not exceed 253 byte")
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crc16 = PredefinedCrc('crc16')
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crc16.update(msg)
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msg_crc = crc16.digest()
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frame = chr(0xAA) + chr(0xFE) + chr(len(msg)) + msg + msg_crc
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result = self._send_frame(frame)
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while not result:
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backoff = TIMEOUT * self._bit_time * (4 + priority + random.randint(1,5))
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self._debug("Collision occured backing off %f" % backoff)
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time.sleep(backoff)
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result = self._send_frame(frame)
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def has_message(self):
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return len(self._buffer) > 0
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def read_message(self):
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while not self.has_message():
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self._read_frame()
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return self._buffer.pop()
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