Fix mask write register (#961)

* Fix mask write register.

According to the spec the "Mask Write Register" function ands the
or-mask with ~and-mask.

See section 6.16 of the specification (v1.1b3):
    Result = (Current Contents AND And_Mask) OR (Or_Mask AND (NOT And_Mask))

Co-authored-by: jan iversen <jancasacondor@gmail.com>

Author: floitsch

pymodbus/register_write_message.py

@@ -313,7 +313,7 @@ def execute(self, context):
         if not context.validate(self.function_code, self.address, 1):
             return self.doException(merror.IllegalAddress)
         values = context.getValues(self.function_code, self.address, 1)[0]
-        values = (values & self.and_mask) | self.or_mask
+        values = (values & self.and_mask) | (self.or_mask & ~self.and_mask)
         context.setValues(self.function_code, self.address, [values])
         return MaskWriteRegisterResponse(self.address, self.and_mask, self.or_mask)
 

test/modbus_mocks.py

@@ -30,6 +30,28 @@ def setValues(self, fx, address, values):
         """Set values."""
 
 
+class MockLastValuesContext(IModbusSlaveContext):
+    """Mock context."""
+
+    def __init__(self, valid=False, default=True):
+        """Initialize."""
+        self.valid = valid
+        self.default = default
+        self.last_values = []
+
+    def validate(self, fx, address, count=0):
+        """Validate values."""
+        return self.valid
+
+    def getValues(self, fx, address, count=0):
+        """Get values."""
+        return [self.default] * count
+
+    def setValues(self, fx, address, values):
+        """Set values."""
+        self.last_values = values
+
+
 class FakeList:
     """Todo, replace with magic mock."""
 

test/test_register_write_messages.py

@@ -13,7 +13,7 @@
     WriteSingleRegisterResponse,
 )
 
-from .modbus_mocks import MockContext
+from .modbus_mocks import MockContext, MockLastValuesContext
 
 # ---------------------------------------------------------------------------#
 #  Fixture
@@ -138,10 +138,17 @@ def test_mask_write_register_request_decode(self):
 
     def test_mask_write_register_request_execute(self):
         """Test write register request valid execution"""
-        context = MockContext(valid=True, default=0x0000)
-        handle = MaskWriteRegisterRequest(0x0000, 0x0101, 0x1010)
+        # The test uses the 4 nibbles of the 16-bit values to test
+        # the combinations:
+        #     and_mask=0, or_mask=0
+        #     and_mask=F, or_mask=0
+        #     and_mask=0, or_mask=F
+        #     and_mask=F, or_mask=F
+        context = MockLastValuesContext(valid=True, default=0xAA55)
+        handle = MaskWriteRegisterRequest(0x0000, 0x0F0F, 0x00FF)
         result = handle.execute(context)
         self.assertTrue(isinstance(result, MaskWriteRegisterResponse))
+        self.assertEqual([0x0AF5], context.last_values)
 
     def test_mask_write_register_request_invalid_execute(self):
         """Test write register request execute with invalid data"""