Beispiel #1
0
    def uploadblock(self,data, final):
        finalmarker = ETB
        if final:
            finalmarker = ETX

        crc = dexcrc16.crcStr(data  + finalmarker)
        print "Writing block:", data.rstrip(),"CRC=",crc
        self.ser.flushInput()
        sleep(0.01)
        self.ser.write(DLE)
        self.ser.write(STX)
        self.ser.write(data)
        self.ser.write(DLE)
        self.ser.write(finalmarker)
        
        # Write checksum
        self.ser.write(chr(crc & 0xFF))
        self.ser.write(chr(crc >> 8))
        
        receivedData = ""
        while True:
            x = self.ser.read()
            if len(x) > 0:
                self.printReceivedData(x)
                receivedData += x
                if len(receivedData) >= 2 and \
                        receivedData[-2] == DLE and \
                        (receivedData[-1] == '0' or receivedData[-1]=='1'):
                    print "received confirmation of block" 
                    return
Beispiel #2
0
    def uploadblock(self, data, final):
        finalmarker = ETB
        if final:
            finalmarker = ETX

        crc = dexcrc16.crcStr(data + finalmarker)
        print "Writing block:", data.rstrip(), "CRC=", crc
        self.ser.flushInput()
        sleep(0.01)
        self.ser.write(DLE)
        self.ser.write(STX)
        self.ser.write(data)
        self.ser.write(DLE)
        self.ser.write(finalmarker)

        # Write checksum
        self.ser.write(chr(crc & 0xFF))
        self.ser.write(chr(crc >> 8))

        receivedData = ""
        while True:
            x = self.ser.read()
            if len(x) > 0:
                self.printReceivedData(x)
                receivedData += x
                if len(receivedData) >= 2 and \
                        receivedData[-2] == DLE and \
                        (receivedData[-1] == '0' or receivedData[-1]=='1'):
                    print "received confirmation of block"
                    return
Beispiel #3
0
 def masterHandshake(self):
     print "Entering Master Handshake"
     
     state = 0
     retries = 5
     self.ser.flushInput()
     self.ser.write(ENQ)
     while retries > 0:
         x = self.ser.read()
         if len(x) > 0 or state == 3:
             self.printReceivedData(x)
             retries = 5
             
             if state == 0:
                 print "State 0 - waiting for command"
                 if x == DLE:
                     print "Got DLE"
                     state = 1
                 else:
                     print "Got something else. Sending ENQ"
                     self.ser.write(ENQ)
                     self.ser.flush()
                     sleep(0.1)
             elif state == 1:
                 if x == '0':
                     print "Got second half of DLE"
                     state = 3
                 else:
                     print "Something wrong"
                     self.ser.write(ENQ)
                     self.ser.flush()
                     sleep(0.1)
                     state = 0
             elif state == 3:
                 print "State 3 - Sending master key"
                 self.ser.write(DLE)
                 sleep(0.015)
                 self.ser.write(SOH)
                 sleep(0.015)
                 for char in self.key:
                     self.ser.write(char)
                     sleep(0.015)
                 self.ser.write(DLE)
                 self.ser.write(ETX)
                 crc = dexcrc16.crcStr(self.key + ETX)
                 self.ser.write(chr(crc & 0xFF))
                 self.ser.write(chr(crc >> 8))
                 state = 4
             elif state == 4:
                 print "State 4 - Waiting for confirmation"
                 if x == DLE:
                     print "Got DLE first half of confirmation"
                     state = 5
                 else:
                     print "Got something other than DLE. Bad"
                     return False
             elif state == 5:
                 if x == '0' or x == '1':
                     print "Got second half of confirmation. All good"
                     self.ser.write(EOT)
                     self.ser.flush()
                     print "End of master handshake"
                     return True
                 else:
                     print "Got something wrong. Sending NAK"
                     self.ser.write(NAK)
                     state = 0                        
         else:
             retries = retries - 1
             sleep(0.01)
     print "gave up"  
Beispiel #4
0
 def exchangeData(self):
     print "Exchanging data"
     
     receivedData = ""
     block = ""
     
     blockNumber = 1
     state = 0
     retries = 5
     self.ser.flushInput()
     
     while retries > 0:
         x = self.ser.read()
         if len(x) > 0:
             self.printReceivedData(x)
             retries = 5
             if state == 0:
                 print "State 0 - waiting for command"
                 if x == ENQ:
                     print "Got ENQ, replying"
                     self.ser.write(DLE)
                     sleep(0.015)
                     self.ser.write('0')
                     sleep(0.015)
                     self.ser.flush()
                 elif x == DLE:
                     print "Got DLE, start of block.."
                     state = 1
             elif state == 1:
                 print "State 1 - waiting for STX"
                 if x == STX:
                     print "Got STX"
                     state = 2
                     receivedData = ""
                     block = ""
             elif state == 2:
                 print "State 2 - receiving data"
                 if x == DLE:
                     print "Got DLE, end of data"
                     state = 3
                 else:
                     receivedData += x
                     block += x
             elif state == 3:
                 print "State 3 - waiting for end of block"
                 if x == ETB:
                     print "Got ETB, end of current block"
                     receivedData += x
                     state = 4
                 elif x == ETX:
                     print "Got ETX, end of last block"
                     receivedData += x
                     state = 6
                 else:
                     print "Got something other than end of block"
                     self.ser.write(NAK)
                     state = 0
             elif state == 4:
                 print "State 4 - Waiting for first half of CRC"
                 receivedData += x
                 state = 5
             elif state == 5:
                 print "State 5 - Waiting for second half of CRC"
                 receivedData += x
                 crc = dexcrc16.crcStr(receivedData)
                 print "Got all data, crc=",crc
                 if crc == 0:
                     print "CRC is good"
                     self.content += block
                     self.ser.write(DLE)
                     self.ser.write('0')
                     state = 0  
                 else:
                     print "CRC failed"
                     self.ser.write(NAK)
                     state = 0 
             elif state == 6:
                 print "State 6 - Waiting for first half of CRC"
                 receivedData += x
                 state = 7
             elif state == 7:
                 print "State 7 - Waiting for second half of CRC"
                 receivedData += x
                 crc = dexcrc16.crcStr(receivedData)
                 print "Got all data, crc=",crc
                 if crc == 0:
                     print "CRC is good"
                     self.content += block
                     self.ser.write(DLE)
                     self.ser.write('0')
                     state = 8   
                 else:
                     print "CRC failed"
                     self.ser.write(NAK)
                     state = 0 
             elif state == 8:
                 print "State 8 - waiting for EOT"
                 if x == EOT:
                     print "Got EOT, End of data exchange"
                     return True
                 else:
                     print "Got something else."
                     self.ser.write(NAK)
                     state = 0
         else:
             retries = retries-1
     print "Gave up"
     return False    
Beispiel #5
0
 def slaveHandshake(self):
     print "Entering slave handshake"
     self.ser.flushInput()
     retries = 5
     state = 0
     receivedData = ""
     while retries > 0:
         x = self.ser.read()
         if len(x) > 0:
             self.printReceivedData(x)
             retries = 5
             
             if state == 0:
                 print "State 0 - waiting for command"
                 if x == EOT:
                     print "Got EOT. Ending now"
                     return True
                 elif x == ENQ:
                     print "Got ENQ. Replying with DLE"
                     self.ser.write(DLE)
                     self.ser.write('0')
                     self.ser.flush()
                 elif x == DLE:
                     print "Got DLE. Will receive data."
                     state = 1                        
             elif state == 1:
                 print "State 1 - waiting for SOH"
                 if x == SOH:
                     print "Got SOH"
                     receivedData = ""
                     state = 2
             elif state == 2:
                 print "State 2 - receiving data"
                 if x == DLE:
                     print "Got DLE. End of data."
                     state = 3
                 else:
                     receivedData += x
             elif state == 3:
                 print "State 3 - Waiting for ETX"
                 if x == ETX:
                     print "Got ETX"
                     receivedData += x
                     state = 4
                 else:
                     print "Got something was not ETX. Resetting"
                     state = 0
             elif state == 4:
                 print "State 4 - Receiving first crc byte"
                 receivedData += x
                 state = 5
             elif state == 5:
                 print "State 5 - Receiving second crc byte"
                 receivedData += x
                 crc = dexcrc16.crcStr(receivedData)
                 print "Calculated crc=",crc
                 if crc == 0:
                     print "CRC is good. Sending DLE,1"
                     self.ser.write(DLE)
                     self.ser.write('1')
                     self.ser.flush()
                 else:
                     print "CRC failed. Sending NAK"
                     self.ser.write(NAK)
                     self.ser.flush()
                 state = 0
         else:
             retries = retries - 1
             
     print "end of slave handshake"
Beispiel #6
0
    def masterHandshake(self):
        print "Entering Master Handshake"

        state = 0
        retries = 5
        self.ser.flushInput()
        self.ser.write(ENQ)
        while retries > 0:
            x = self.ser.read()
            if len(x) > 0 or state == 3:
                self.printReceivedData(x)
                retries = 5

                if state == 0:
                    print "State 0 - waiting for command"
                    if x == DLE:
                        print "Got DLE"
                        state = 1
                    else:
                        print "Got something else. Sending ENQ"
                        self.ser.write(ENQ)
                        self.ser.flush()
                        sleep(0.1)
                elif state == 1:
                    if x == '0':
                        print "Got second half of DLE"
                        state = 3
                    else:
                        print "Something wrong"
                        self.ser.write(ENQ)
                        self.ser.flush()
                        sleep(0.1)
                        state = 0
                elif state == 3:
                    print "State 3 - Sending master key"
                    self.ser.write(DLE)
                    sleep(0.015)
                    self.ser.write(SOH)
                    sleep(0.015)
                    for char in self.key:
                        self.ser.write(char)
                        sleep(0.015)
                    self.ser.write(DLE)
                    self.ser.write(ETX)
                    crc = dexcrc16.crcStr(self.key + ETX)
                    self.ser.write(chr(crc & 0xFF))
                    self.ser.write(chr(crc >> 8))
                    state = 4
                elif state == 4:
                    print "State 4 - Waiting for confirmation"
                    if x == DLE:
                        print "Got DLE first half of confirmation"
                        state = 5
                    else:
                        print "Got something other than DLE. Bad"
                        return False
                elif state == 5:
                    if x == '0' or x == '1':
                        print "Got second half of confirmation. All good"
                        self.ser.write(EOT)
                        self.ser.flush()
                        print "End of master handshake"
                        return True
                    else:
                        print "Got something wrong. Sending NAK"
                        self.ser.write(NAK)
                        state = 0
            else:
                retries = retries - 1
                sleep(0.01)
        print "gave up"
Beispiel #7
0
    def exchangeData(self):
        print "Exchanging data"

        receivedData = ""
        block = ""

        blockNumber = 1
        state = 0
        retries = 5
        self.ser.flushInput()

        while retries > 0:
            x = self.ser.read()
            if len(x) > 0:
                self.printReceivedData(x)
                retries = 5
                if state == 0:
                    print "State 0 - waiting for command"
                    if x == ENQ:
                        print "Got ENQ, replying"
                        self.ser.write(DLE)
                        sleep(0.015)
                        self.ser.write('0')
                        sleep(0.015)
                        self.ser.flush()
                    elif x == DLE:
                        print "Got DLE, start of block.."
                        state = 1
                elif state == 1:
                    print "State 1 - waiting for STX"
                    if x == STX:
                        print "Got STX"
                        state = 2
                        receivedData = ""
                        block = ""
                elif state == 2:
                    print "State 2 - receiving data"
                    if x == DLE:
                        print "Got DLE, end of data"
                        state = 3
                    else:
                        receivedData += x
                        block += x
                elif state == 3:
                    print "State 3 - waiting for end of block"
                    if x == ETB:
                        print "Got ETB, end of current block"
                        receivedData += x
                        state = 4
                    elif x == ETX:
                        print "Got ETX, end of last block"
                        receivedData += x
                        state = 6
                    else:
                        print "Got something other than end of block"
                        self.ser.write(NAK)
                        state = 0
                elif state == 4:
                    print "State 4 - Waiting for first half of CRC"
                    receivedData += x
                    state = 5
                elif state == 5:
                    print "State 5 - Waiting for second half of CRC"
                    receivedData += x
                    crc = dexcrc16.crcStr(receivedData)
                    print "Got all data, crc=", crc
                    if crc == 0:
                        print "CRC is good"
                        self.content += block
                        self.ser.write(DLE)
                        self.ser.write('0')
                        state = 0
                    else:
                        print "CRC failed"
                        self.ser.write(NAK)
                        state = 0
                elif state == 6:
                    print "State 6 - Waiting for first half of CRC"
                    receivedData += x
                    state = 7
                elif state == 7:
                    print "State 7 - Waiting for second half of CRC"
                    receivedData += x
                    crc = dexcrc16.crcStr(receivedData)
                    print "Got all data, crc=", crc
                    if crc == 0:
                        print "CRC is good"
                        self.content += block
                        self.ser.write(DLE)
                        self.ser.write('0')
                        state = 8
                    else:
                        print "CRC failed"
                        self.ser.write(NAK)
                        state = 0
                elif state == 8:
                    print "State 8 - waiting for EOT"
                    if x == EOT:
                        print "Got EOT, End of data exchange"
                        return True
                    else:
                        print "Got something else."
                        self.ser.write(NAK)
                        state = 0
            else:
                retries = retries - 1
        print "Gave up"
        return False
Beispiel #8
0
    def slaveHandshake(self):
        print "Entering slave handshake"
        self.ser.flushInput()
        retries = 5
        state = 0
        receivedData = ""
        while retries > 0:
            x = self.ser.read()
            if len(x) > 0:
                self.printReceivedData(x)
                retries = 5

                if state == 0:
                    print "State 0 - waiting for command"
                    if x == EOT:
                        print "Got EOT. Ending now"
                        return True
                    elif x == ENQ:
                        print "Got ENQ. Replying with DLE"
                        self.ser.write(DLE)
                        self.ser.write('0')
                        self.ser.flush()
                    elif x == DLE:
                        print "Got DLE. Will receive data."
                        state = 1
                elif state == 1:
                    print "State 1 - waiting for SOH"
                    if x == SOH:
                        print "Got SOH"
                        receivedData = ""
                        state = 2
                elif state == 2:
                    print "State 2 - receiving data"
                    if x == DLE:
                        print "Got DLE. End of data."
                        state = 3
                    else:
                        receivedData += x
                elif state == 3:
                    print "State 3 - Waiting for ETX"
                    if x == ETX:
                        print "Got ETX"
                        receivedData += x
                        state = 4
                    else:
                        print "Got something was not ETX. Resetting"
                        state = 0
                elif state == 4:
                    print "State 4 - Receiving first crc byte"
                    receivedData += x
                    state = 5
                elif state == 5:
                    print "State 5 - Receiving second crc byte"
                    receivedData += x
                    crc = dexcrc16.crcStr(receivedData)
                    print "Calculated crc=", crc
                    if crc == 0:
                        print "CRC is good. Sending DLE,1"
                        self.ser.write(DLE)
                        self.ser.write('1')
                        self.ser.flush()
                    else:
                        print "CRC failed. Sending NAK"
                        self.ser.write(NAK)
                        self.ser.flush()
                    state = 0
            else:
                retries = retries - 1

        print "end of slave handshake"
Beispiel #9
0
 def firstHandshakeDCMaster(self):
     print "Entering First Handshake DC as Master"
     state = 0
     retries = 5
     self.ser.flushInput()
     self.ser.write(ENQ)
     self.ser.flush()
     while retries > 0:
         x = self.ser.read()
         if len(x) > 0:
             printReceivedData(x)
             retries = 5
             if state == 0:
                 print "State 0: Expecting DLE"
                 if x == DLE:
                     print "Got DLE"
                     state = 1
                 elif x == ENQ:
                     print "Got ENQ. TODO: slave handshake"
                     # start slaveHandshake
                     sleep(0.01)
                     self.ser.write(NAK)
                     self.ser.flush()
                 else:
                     print "Got something else. Sending ENQ to restart master handshake"
                     sleep(0.01)
                     self.ser.write(ENQ)
                     self.ser.flush()
             elif state == 1:
                 print "State 1: Expecting '0'"
                 if x == '0':
                     print "Got second half of DLE"
                     print "Sending DLE"
                     sleep(0.01)
                     self.ser.write(DLE)
                     #print "Sending SOH"
                     self.ser.write(SOH)
                     # print "Sending communication ID"
                     for char in self.communicationID:
                         self.ser.write(char)
                     #print "Sending DLE"
                     self.ser.write(DLE)
                     #print "Sending ETX"
                     self.ser.write(ETX)
                     print self.communicationID
                     crc = dexcrc16.crcStr(self.communicationID + ETX)
                     print crc
                     self.ser.write(chr(crc & 0xFF))
                     self.ser.write(chr(crc >> 8))
                     self.ser.flush()
                     sleep(0.2)
                     state = 2
                 else:
                     #print "Something Wrong. Sending ENQ to restart master handshake"
                     sleep(0.01)
                     self.ser.write(ENQ)
                     self.ser.flush()
                     state = 0
             elif state == 2:
                 print "State 2: Expecting DLE"
                 if x == DLE:
                     print "Got DLE"
                     state = 3
                 else:
                     print "Got something other than DLE. Bad"
                     print "Received data:",x,"=",x.encode('hex')
                     return False
             elif state == 3:
                 print "State 3: Expecting '1' or '0'"
                 if x == '1' or x == '0':
                     print "Got second half of DLE. Sending EOT"
                     sleep(0.01)
                     self.ser.write(EOT)
                     self.ser.flush()
                     print "First Handshake DC as Master completed"
                     return True
                 else:
                     #print "Got something wrong. Sending NAK"
                     sleep(0.01)
                     self.ser.write(NAK)
                     self.ser.flush()
                     state = 0
         else:
             retries = retries - 1
             print "***** trying again *****"
     print "First Handshake DC as Master gave up"
     return False
Beispiel #10
0
 def DC2VMDExchange(self, content):
     print "Exchanging data DC to VMD"
     blocks = chunkstring(content, BLOCK_SIZE)
     blockIterator = -1
     state = 0
     retries = 5
     self.ser.flushInput()
     self.ser.write(ENQ)
     self.ser.flush()
     while retries > 0:
         x = self.ser.read()
         if len(x) > 0 or state == 2 or state == 3:
             printReceivedData(x)
             retries = 5
             if state == 0:
                 print "State 0: Expecting DLE"
                 if x == DLE:
                     print "Got DLE"
                     state = 1
                 else:
                     print "Got something else. Sending ENQ to restart data exchange"
                     sleep(0.01)
                     self.ser.write(ENQ)
                     self.ser.flush()
             elif state == 1:
                 print "State 1: Expecting second half of DLE"
                 if x == '0' or x == '1':
                     if blockIterator == len(blocks) - 1:
                         #print "Reached end of content"
                         sleep(0.01)
                         self.ser.write(EOT)
                         self.ser.flush()
                         print "Data exchange DC to VMD completed"
                         return True
                     else:
                         print "Got second half of DLE. Sending next block"
                         blockIterator += 1
                 else:
                     print "Got something else. Resending block"
                 sleep(0.01)
                 self.ser.write(DLE)
                 self.ser.write(STX)
                 for char in blocks[blockIterator]:
                     self.ser.write(char)
                 if blockIterator < (len(blocks) - 1):
                     state == 2
                 elif blockIterator == (len(blocks) - 1):
                     state == 3
             elif state == 2:
                 print "State 2: Sending block end"
                 self.ser.write(DLE)
                 self.ser.write(ETB)
                 crc = dexcrc16.crcStr(blocks[blockIterator] + ETB)
                 self.ser.write(chr(crc & 0xFF))
                 self.ser.write(chr(crc >> 8))
                 self.flush()
                 state = 0
             elif state == 3:
                 print "State 3: Sending content end"
                 self.ser.write(DLE)
                 self.ser.write(ETX)
                 crc = dexcrc16.crcStr(blocks[blockIterator] + ETX)
                 self.ser.write(chr(crc & 0xFF))
                 self.ser.write(chr(crc >> 8))
                 self.flush()
                 state = 0
         else:
             retries = retries - 1
             sleep(0.01)
             print "tring again"
     print "Exchanging data DC to VMD Gave Up"
     return False
Beispiel #11
0
    def VMD2DCExchange(self):
        #print "Exchanging data VMD to DC"
        receivedData = ""
        block = ""
        state = 0
        retries = 5
        currentAck = ACK0
        self.ser.flushInput()

        while retries > 0:
            x = self.ser.read()
            printReceivedData(x)
            if len(x) > 0:
                retries = 5
                if state == 0:
                    #print "State 0: Expecting ENQ"
                    if x == ENQ:
                        #print "Got ENQ. Replying ACK"
                        sleep(0.01)
                        self.ser.write(DLE)
                        self.ser.write(currentAck)
                        self.ser.flush()
                        if currentAck == ACK0:
                            currentAck = ACK1
                        elif currentAck == ACK1:
                            currentAck = ACK0
                    elif x == DLE:
                        #print "Got DLE. Start of block"
                        state = 1
                elif state == 1:
                    #print "State 1: Expecting STX"
                    if x == STX:
                        #print "Got STX. Start block receiving"
                        state = 2
                        receivedData = ""
                        block = ""
                elif state == 2:
                    #print "State 2: Receiving data block"
                    if x == DLE:
                        #print "Got DLE. end of data"
                        state = 3
                    else:
                        receivedData += x
                        block += x
                elif state == 3:
                    #print "State 3: waiting for end of block"
                    if x == ETB:
                        #print "Got ETB, end of current block"
                        receivedData += x
                        state = 4
                    elif x == ETX:
                        #print "Got ETX, end of last block"
                        receivedData += x
                        state = 6
                    else:
                        #print "Got something other than end of block"
                        self.ser.write(NAK)
                        state = 0
                elif state == 4:
                    #print "State 4 - Waiting for first half of CRC"
                    receivedData += x
                    state = 5
                elif state == 5:
                    #print "State 5 - Waiting for second half of CRC"
                    receivedData += x
                    crc = dexcrc16.crcStr(receivedData)
                    #print "Got all data, crc=",crc
                    if crc == 0:
                        #print "CRC is good"
                        self.content += block
                        sleep(0.01)
                        self.ser.write(DLE)
                        self.ser.write(currentAck)
                        self.ser.flush()
                        if currentAck == ACK0:
                            currentAck = ACK1
                        elif currentAck == ACK1:
                            currentAck = ACK0
                        state = 0
                    else:
                        #print "CRC failed"
                        sleep(0.01)
                        self.ser.write(NAK)
                        self.ser.flush()
                        state = 0
                elif state == 6:
                    #print "State 6 - Waiting for first half of CRC"
                    receivedData += x
                    state = 7
                elif state == 7:
                    #print "State 7 - Waiting for second half of CRC"
                    receivedData += x
                    crc = dexcrc16.crcStr(receivedData)
                    #print "Got all data, crc=",crc
                    if crc == 0:
                        #print "CRC is good"
                        self.content += block
                        self.ser.write(DLE)
                        self.ser.write(currentAck)
                        state = 8
                        self.ser.flush()
                        if currentAck == ACK0:
                            currentAck = ACK1
                        elif currentAck == ACK1:
                            currentAck = ACK0
                    else:
                        #print "CRC failed"
                        self.ser.write(NAK)
                        state = 0
                elif state == 8:
                    #print "State 8 - waiting for EOT"
                    if x == EOT:
                        #print "Got EOT, End of data exchange"
                        return self.content
                    else:
                        #print "Got something else."
                        sleep(0.01)
                        self.ser.write(NAK)
                        self.ser.flush()
                        state = 0
            else:
                retries = retries - 1
                sleep(0.01)
                #print "trying again"
        #print "Exchanging data VMD to DC Gave Up"
        return False
Beispiel #12
0
 def secondHandshakeVMDMaster(self):
     #print "Entering Second Handshake VMD as Master"
     self.ser.flushInput()
     state = 0
     retries = 5
     receivedData = ""
     while retries > 0:
         x = self.ser.read()
         if len(x) > 0:
             printReceivedData(x)
             retries = 5
             if state == 0:
                 #print "State 0: Waiting for Command"
                 if x == ENQ:
                     #print "Got ENQ. Replying with DLE '0'"
                     sleep(0.01)
                     self.ser.write(DLE)
                     self.ser.write('0')
                     self.ser.flush()
                 elif x == EOT:
                     #print "Second Handshake VMD as Master Completed"
                     return True
                 elif x == DLE:
                     #print "Got DLE. Receiving data now"
                     state = 1
             elif state == 1:
                 #print "State 1: Expecting SOH"
                 if x == SOH:
                     #print "Got SOH"
                     receivedData = ""
                     state = 2
             elif state == 2:
                 #print "State 2: Receiving data"
                 if x == DLE:
                     #print "Got DLE. End of Data"
                     state = 3
                 else:
                     receivedData += x
             elif state == 3:
                 #print "State 3: Expecting ETX"
                 if x == ETX:
                     #print "Got ETX"
                     receivedData += x
                     state = 4
                 else:
                     #print "Got Something else. Resetting"
                     state = 0
             elif state == 4:
                 #print "State 4: Receiving first CRC byte"
                 receivedData += x
                 state = 5
             elif state == 5:
                 #print "State 5: Receiving second CRC byte"
                 receivedData += x
                 crc = dexcrc16.crcStr(receivedData)
                 #print "Calculated crc=",crc
                 if crc == 0:
                     #print "CRC is good. Sending DLE,1"
                     sleep(0.01)
                     self.ser.write(DLE)
                     self.ser.write('1')
                     self.ser.flush()
                 else:
                     #print "CRC failed. Sending NAK"
                     self.ser.write(NAK)
                     self.ser.flush()
                 state = 0
         else:
             retries = retries - 1
             sleep(0.01)
             #print "trying again"
     #print "Second Handshake VMD as Master Gave up"
     return False