fix bugs with persistent_limit parameter

take into account that reading the switch heater might be delayed

secop_psi/ips_mercury.py

@@ -119,16 +119,27 @@ class Field(MercuryChannel, Magfield):
             return current / self.atob
         return 0
 
-    def start_ramp_to_field(self, state):
-        self.change('PSU:SIG:FSET', self.persistent_field)
+    def set_and_go(self, value):
+        self.change('PSU:SIG:FSET', value)
         assert self.write_action('hold') == 'hold'
         assert self.write_action('run_to_set') == 'run_to_set'
+
+    def start_ramp_to_field(self, state):
+        try:
+            self.set_and_go(self.persistent_field)
+        except (HardwareError, AssertionError):
+            state.switch_undef = self.switch_on_time or state.now
+            return self.wait_for_switch
         return self.ramp_to_field
 
+    def wait_for_switch(self, state):
+        if self.now - self.switch_undef < self.wait_switch_on:
+            return Retry()
+        self.set_and_go(self.persistent_field)
+        return self.ramp_to_field
+        
     def start_ramp_to_target(self, state):
-        self.change('PSU:SIG:FSET', self.target)
-        assert self.write_action('hold') == 'hold'
-        assert self.write_action('run_to_set') == 'run_to_set'
+        self.set_and_go(self.target)
         return self.ramp_to_target
 
     def start_ramp_to_zero(self, state):