improve HasRamp

Change-Id: I0ddabb8b2681712637a18c392da5424d30a05b4b

frappy_psi/parmod.py

@@ -22,13 +22,16 @@
 
 """modules to access parameters"""
 
-from frappy.core import Drivable, IDLE, Attached, StringType, Property, Proxy
-from frappy.mixins import HasRamp
+from frappy.core import Drivable, IDLE, Attached, StringType, Property, \
+    Parameter, FloatRange
 from frappy.errors import ConfigError
 from frappy_psi.convergence import HasConvergence
+from frappy_psi.mixins import HasRamp
 
 
 class Driv(Drivable):
+    value = Parameter(datatype=FloatRange(unit='$'))
+    target = Parameter(datatype=FloatRange(unit='$'))
     read = Attached(description='<module>.<parameter> for read')
     write = Attached(description='<module>.<parameter> for read')
     unit = Property('main unit', StringType())
@@ -40,8 +43,7 @@ class Driv(Drivable):
         super().setProperty(key, value)
 
     def checkProperties(self):
-        self.parameters['value'].setProperty('unit', self.unit)
-        self.parameters['target'].setProperty('unit', self.unit)
+        self.applyMainUnit(self.unit)
         if self.read == self.name or self.write == self.name:
             raise ConfigError('illegal recursive read/write module')
         super().checkProperties()