Add a command get_fan_drive_max_frequency.

python/lsst/ts/vent/controller/config.py

@@ -30,7 +30,7 @@ class Config:
     device_id = 1
     """The default modbus device ID for the variable frequency drive."""
 
-    max_freq = 50.0
+    max_freq = 25.0
     """Default maximum frequency for the dome fans."""
 
     megaind_bus = 1

python/lsst/ts/vent/controller/controller.py

@@ -200,6 +200,14 @@ async def get_fan_frequency(self) -> float:
         output_frequency *= 0.1  # RFR register holds frequency in units of 0.1 Hz
         return output_frequency
 
+    def get_max_frequency(self) -> float:
+        """Returns the maximum allowed frequency.
+
+        Calls to `set_fan_frequency` may not have an argument exceeding
+        this value.
+        """
+        return self.config.max_freq
+
     async def set_fan_frequency(self, frequency: float) -> None:
         """Sets the target frequency for the dome exhaust fan. The frequency
         must be between zero and MAX_FREQ.

python/lsst/ts/vent/controller/dispatcher.py

@@ -89,6 +89,7 @@ def __init__(
         self.dispatch_dict: Final[dict[str, list[type]]] = {
             "close_vent_gate": [int, int, int, int],
             "open_vent_gate": [int, int, int, int],
+            "get_fan_drive_max_frequency": [],
             "reset_extraction_fan_drive": [],
             "set_extraction_fan_drive_freq": [float],
             "set_extraction_fan_manual_control_mode": [bool],
@@ -164,12 +165,13 @@ async def read_and_dispatch(self) -> None:
             # Convert the arguments to their expected type.
             args = [cast_string_to_type(t, arg) for t, arg in zip(types, args)]
             # Call the method with the specified arguments.
-            await getattr(self, command)(*args)
+            return_value = await getattr(self, command)(*args)
             # Send back a success response.
             await self.respond(
                 json.dumps(
                     dict(
                         command=command,
+                        return_value=return_value,
                         error=0,
                         exception_name="",
                         message="",
@@ -211,6 +213,9 @@ async def open_vent_gate(
                 if gate != -1:
                     raise ValueError(f"Invalid vent ({gate}) must be between 0 and 3.")
 
+    async def get_fan_drive_max_frequency(self) -> float:
+        return self.controller.get_max_frequency()
+
     async def reset_extraction_fan_drive(self) -> None:
         await self.controller.vfd_fault_reset()