AnalogInput: write mode only when needed

- this does probably influence the input channel for some short time
+ issue a warning when read value is -1. this may indicate an error!

frappy_psi/ionopimax.py

@@ -38,7 +38,6 @@ class Base:
 
     def initModule(self):
         super().initModule()
-        self.log.info('initModule %r', self.name)
         candidates = list(Path('/sys/class').glob(f'ionopi*/*/{self.addr}'))
         if not candidates:
             raise ConfigError(f'can not find path for {self.addr}')
@@ -49,10 +48,12 @@ class Base:
 
     def read(self, addr, scale=None):
         with open(self._devpath / addr) as f:
-            result = f.read()
+            result = f.read().strip()
+        if result == '-1':
+            self.log.warning('read %r (error?) %s', result, self._devpath / addr)
         if scale:
             return float(result) / scale
-        return result.strip()
+        return result
 
     def write(self, addr, value, scale=None):
         value = str(round(value * scale)) if scale else str(value)
@@ -135,7 +136,9 @@ class VoltageInput(AnalogInput):
 
     def initModule(self):
         super().initModule()
-        self.write(f'{self.addr}_mode','U')
+        if self.read(f'{self.addr}_mode') != 'U':
+            # change mode only if needed, as this disturbs the input for some time
+            self.write(f'{self.addr}_mode', 'U')
 
 
 class LogVoltageInput(VoltageInput):
@@ -157,7 +160,10 @@ class CurrentInput(AnalogInput):
 
     def initModule(self):
         super().initModule()
-        self.write(f'{self.addr}_mode', 'U')
+        if self.read(f'{self.addr}_mode') != 'U':
+            # change mode only if needed, as this disturbs the input for some time
+            self.log.warning('set mode to U')
+            self.write(f'{self.addr}_mode', 'U')
 
 
 class AnalogOutput(AnalogInput, Writable):