Fix for issue #21

pymodbus/constants.py

@@ -20,6 +20,12 @@ class Defaults(Singleton):
        The default number of times a client should retry the given
        request before failing (3)
 
+    .. attribute:: RetryOnEmpty
+
+       A flag indicating if a transaction should be retried in the
+       case that an empty response is received. This is useful for
+       slow clients that may need more time to process a requst.
+
     .. attribute:: Timeout
 
        The default amount of time a client should wait for a request
@@ -73,6 +79,7 @@ class Defaults(Singleton):
     '''
     Port          = 502
     Retries       = 3
+    RetryOnEmpty  = False
     Timeout       = 3
     Reconnects    = 0
     TransactionId = 0

pymodbus/transaction.py

@@ -38,13 +38,15 @@ class ModbusTransactionManager(object):
     This module helps to abstract this away from the framer and protocol.
     '''
 
-    def __init__(self, client):
+    def __init__(self, client, **kwargs):
         ''' Initializes an instance of the ModbusTransactionManager
 
         :param client: The client socket wrapper
+        :param retry_on_empty: Should the client retry on empty
         '''
         self.tid = Defaults.TransactionId
         self.client = client
+        self.retry_on_empty = kwargs.get('retry_on_empty', Defaults.RetryOnEmpty)
 
     def execute(self, request):
         ''' Starts the producer to send the next request to
@@ -62,6 +64,9 @@ def execute(self, request):
                 # as this may not read the full result set, but right now
                 # it should be fine...
                 result = self.client._recv(1024)
+                if not result and self.retry_on_empty:
+                    retries -= 1
+                    continue
                 self.client.framer.processIncomingPacket(result, self.addTransaction)
                 break;
             except socket.error, msg:
@@ -132,7 +137,7 @@ def __iter__(self):
 
         :returns: An iterator of the managed transactions
         '''
-        return self.transactions.iterkeys()
+        return iter(self.transactions.keys())
 
     def addTransaction(self, request, tid=None):
         ''' Adds a transaction to the handler