Implement getResponseSize()

getResponseSize() can be used to know in advance how many
bytes needs to be read after request. Using this information it
is not needed to wait for serial port timeout during receive.

pymodbus/pdu.py

@@ -106,6 +106,13 @@ def doException(self, exception):
                 (self.function_code, exception))
         return ExceptionResponse(self.function_code, exception)
 
+    def getResponseSize(self):
+        ''' Returns expected packet size of response for this request
+
+        :raises: A not implemented exception
+        '''
+        raise NotImplementedException()
+
 
 class ModbusResponse(ModbusPDU):
     ''' Base class for a modbus response PDU

pymodbus/register_read_message.py

@@ -38,6 +38,13 @@ def decode(self, data):
         '''
         self.address, self.count = struct.unpack('>HH', data)
 
+    def getResponseSize(self):
+        ''' Returns expected packet size of response for this request
+
+        :returns: The expected packet size
+        '''
+        return 1 + 2 * self.count
+
     def __str__(self):
         ''' Returns a string representation of the instance
 

pymodbus/register_write_message.py

@@ -61,6 +61,13 @@ def execute(self, context):
         values = context.getValues(self.function_code, self.address, 1)
         return WriteSingleRegisterResponse(self.address, values[0])
 
+    def getResponseSize(self):
+        ''' Returns expected packet size of response for this request
+
+        :returns: The expected packet size
+        '''
+        return 2 + 2
+
     def __str__(self):
         ''' Returns a string representation of the instance
 

pymodbus/transaction.py

@@ -6,7 +6,7 @@
 import socket
 from binascii import b2a_hex, a2b_hex
 
-from pymodbus.exceptions import ModbusIOException
+from pymodbus.exceptions import ModbusIOException, NotImplementedException
 from pymodbus.constants  import Defaults
 from pymodbus.interfaces import IModbusFramer
 from pymodbus.utilities  import checkCRC, computeCRC
@@ -62,10 +62,19 @@ def execute(self, request):
             try:
                 self.client.connect()
                 self.client._send(self.client.framer.buildPacket(request))
-                # I need to fix this to read the header and the result size,
-                # as this may not read the full result set, but right now
-                # it should be fine...
-                result = self.client._recv(1024)
+                try:
+                    recvsize = self.client.framer.getResponseSize(request)
+                except NotImplementedException:
+                    recvsize = 0;
+
+                # TODO: read first only as much as needed to check Modbus
+                # exception and then all remaining bytes. Otherwise
+                # exception always takes _timeout_ seconds.
+                if recvsize:
+                    result = self.client._recv(recvsize)
+                else:
+                    result = self.client._recv(1024)
+
                 if not result and self.retry_on_empty:
                     retries -= 1
                     continue
@@ -553,6 +562,14 @@ def buildPacket(self, message):
         packet += struct.pack(">H", computeCRC(packet))
         return packet
 
+    def getResponseSize(self, message):
+        ''' Returns expected packet size of response for request
+
+        :param message: Request message
+        :returns: The expected packet size
+        '''
+        return 1 + 1 + message.getResponseSize() + 2
+
 
 #---------------------------------------------------------------------------#
 # Modbus ASCII Message