Merged in cbillington/labscript/bugfix (pull request labscript-suite#39)

Static DO fix

Author: chrisjbillington

labscript.py

@@ -1610,12 +1610,14 @@ def __init__(self, *args, **kwargs):
         self._static_value = None
 
     def go_high(self):
-        if self._static_value is not None:
+        if self._static_value is None:
             self.add_instruction(0,1)
             self._static_value = 1
+        else:
+            raise LabscriptError('%s %s has already been set to %s. It cannot also be set to %s.'%(self.description, self.name, self.instruction_to_string[self._static_value], self.instruction_to_string[value]))
 
     def go_low(self):
-        if self._static_value is not None:
+        if self._static_value is None:
             self.add_instruction(0,0) 
             self._static_value = 0
         else: