The ELM (Expected Lighting Module) is a controller function to maintain a list of expected light levels for each light on the bus. When a lighting command is sent by or through the controller, the controller updates the expected light level for any affected devices.
In some cases, the ELM must revert back into an unknown state (an example being the command GO TO LAST ACTIVE LEVEL, which the controller does not store). In this case, the light will be polled soon thereafter and the expected state is set by what the device responds with.
If you see an ELM mismatch in the monitor or are witnessing lights going to unexpected levels, this may indicate that the devices you are monitoring are changing state as a result of outside factors but does not necessarily imply that they are definitely misbehaving (though this is frequently the case).
Possible explanations for this are:
- They are controlled by a relay and have been recently turned off / on and are now at power on level.
- They have been turned on / off in general and thus, are now at power on level.
- System failure level has been triggered
- Lamp fault
- Misinterpreting commands on the bus (highly prevalent in pre Dali 2 certified devices)
Some things to test:
- Set the power on level and system failure levels to unique values (ie, 254 for power on, 253 for system failure level.
- Run a monitor and filter for the term ELM Mismatch. Once found, you can also filter for lighting commands to see if there was anything directed at this device.
If there are individual lights in a group going to incorrect levels and the controller is configured to only speak to them through group commands, it is highly likely the devices/wiring is at fault.
In the case of wireless controllers, the ELM will act on any disparity. For wired controllers, we currently only report.
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