diff --git a/tests/system/responses/test_succesful_responses.py b/tests/system/responses/test_succesful_responses.py
index 9a51541..99ceaeb 100644
--- a/tests/system/responses/test_succesful_responses.py
+++ b/tests/system/responses/test_succesful_responses.py
@@ -1,8 +1,21 @@
 import pytest
 
+from umodbus import conf
 from umodbus.client import tcp
 
 
+@pytest.fixture(scope='module', autouse=True)
+def enable_signed_values(request):
+    """ Use signed values when running tests it this module. """
+    tmp = conf.SIGNED_VALUES
+    conf.SIGNED_VALUES = True
+
+    def fin():
+        conf.SIGNED_VALUES = tmp
+
+    request.addfinalizer(fin)
+
+
 @pytest.mark.parametrize('function', [
     tcp.read_coils,
     tcp.read_discrete_inputs,
@@ -28,21 +41,21 @@ def test_response_on_multi_bit_value_read_requests(sock, function):
     slave_id, starting_address, quantity = (1, 0, 10)
     req_adu = function(slave_id, starting_address, quantity)
 
-    assert tcp.send_message(req_adu, sock) == [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
+    assert tcp.send_message(req_adu, sock) == [0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
 
 
-@pytest.mark.parametrize('function', [
-    tcp.write_single_coil,
-    tcp.write_single_register,
+@pytest.mark.parametrize('function, value', [
+    (tcp.write_single_coil, 0),
+    (tcp.write_single_register, -1337),
 ])
-def test_response_single_value_write_request(sock, function):
+def test_response_single_value_write_request(sock, function, value):
     """ Validate responde of succesful Read Single Coil and Read Single
     Register request.
     """
-    slave_id, starting_address, quantity = (1, 0, 0)
+    slave_id, starting_address, quantity = (1, 0, value)
     req_adu = function(slave_id, starting_address, quantity)
 
-    assert tcp.send_message(req_adu, sock) == 0
+    assert tcp.send_message(req_adu, sock) == value
 
 
 @pytest.mark.parametrize('function, values', [
diff --git a/tests/system/server.py b/tests/system/server.py
index 02e4ef2..c063679 100644
--- a/tests/system/server.py
+++ b/tests/system/server.py
@@ -3,7 +3,9 @@
 except ImportError:
     from SocketServer import TCPServer
 
-from umodbus import get_server, RequestHandler
+from umodbus import get_server, RequestHandler, conf
+
+conf.SIGNED_VALUES = True
 
 app = get_server(TCPServer, ('localhost', 0), RequestHandler)
 
@@ -15,7 +17,7 @@ def read_status(slave_id, address):
 
 @app.route(slave_ids=[1], function_codes=[3, 4], addresses=list(range(0, 10)))
 def read_register(slave_id, address):
-    return address
+    return -address
 
 
 @app.route(slave_ids=[1], function_codes=[5, 15], addresses=list(range(0, 10)))
diff --git a/tests/unit/conftest.py b/tests/unit/conftest.py
index 8ce551d..66b730b 100644
--- a/tests/unit/conftest.py
+++ b/tests/unit/conftest.py
@@ -1,8 +1,21 @@
 import pytest
 
+from umodbus import conf
 from umodbus.config import Config
 
 
+@pytest.fixture(scope='module', autouse=True)
+def enable_signed_values(request):
+    """ Use signed values when running tests it this module. """
+    tmp = conf.SIGNED_VALUES
+    conf.SIGNED_VALUES = False
+
+    def fin():
+        conf.SIGNED_VALUES = tmp
+
+    request.addfinalizer(fin)
+
+
 @pytest.fixture
 def config():
     return Config()
diff --git a/umodbus/_functions/__init__.py b/umodbus/_functions/__init__.py
index fba65c5..91206ee 100644
--- a/umodbus/_functions/__init__.py
+++ b/umodbus/_functions/__init__.py
@@ -744,12 +744,18 @@ def value(self):
 
     @value.setter
     def value(self, value):
-        """ Data must be value between and including 0 and 0xFFFF. """
-        if 0 <= value <= 0xFFFF:
-            self._value = value
-        else:
+        """ Value to be written on register.
+
+        :param value: An integer.
+        :raises: IllegalDataValueError when value isn't in range.
+        """
+        try:
+            struct.pack('>' + conf.TYPE_CHAR, value)
+        except struct.error:
             raise IllegalDataValueError
 
+        self._value = value
+
     @property
     def request_pdu(self):
         """ Build request PDU to write single register.