Modbus: Failed to update item 'xxx' because '15.7' could not be converted to the item unit '°C'

Add-ons Bindings
1

I am trying to read a value via modbus and convert it into an Item. The modbus-tcp binding is working, as is the ‘modbus poller’ and the ‘modbus data’.

What fails is the conversion to an Item:

[WARN ] [openhab.core.library.items.NumberItem] - Failed to update item 'OutsideTemperature' because '15.7' could not be converted to the item unit '°C'

It seems to me that ‘15.7’ is a Number (it is also a plausible outside temperature value), so why does the conversion to a temperature (Number:Temperature) fail?

Here is the modbus data definition created through the UI:

UID: modbus:data:outside_temperature:outside_temperature
label: Modbus Data Outside Temperature
thingTypeUID: modbus:data
configuration:
  readValueType: int16
  readTransform: default
  writeTransform: default
  readStart: "30009"
  updateUnchangedValuesEveryMillis: 1000
  writeMultipleEvenWithSingleRegisterOrCoil: false
  writeMaxTries: 3
bridgeUID: modbus:poller:source:outside_temperature
location: outside
channels:
  - id: number
    channelTypeUID: modbus:number-type
    label: Value as Number
    description: Number item channel
    configuration: {}
  - id: switch
    channelTypeUID: modbus:switch-type
    label: Value as Switch
    description: Switch item channel
    configuration: {}
  - id: contact
    channelTypeUID: modbus:contact-type
    label: Value as Contact
    description: Contact item channel
    configuration: {}
  - id: dimmer
    channelTypeUID: modbus:dimmer-type
    label: Value as Dimmer
    description: Dimmer item channel
    configuration: {}
  - id: datetime
    channelTypeUID: modbus:datetime-type
    label: Value as DateTime
    description: DateTime item channel
    configuration: {}
  - id: string
    channelTypeUID: modbus:string-type
    label: Value as String
    description: String item channel
    configuration: {}
  - id: rollershutter
    channelTypeUID: modbus:rollershutter-type
    label: Value as Rollershutter
    description: Rollershutter item channel
    configuration: {}
  - id: lastReadSuccess
    channelTypeUID: modbus:last-successful-read-type
    label: Last Successful Read
    description: Date of last read
    configuration: {}
  - id: lastReadError
    channelTypeUID: modbus:last-erroring-read-type
    label: Last Erroring Read
    description: Date of last read error
    configuration: {}
  - id: lastWriteSuccess
    channelTypeUID: modbus:last-successful-write-type
    label: Last Successful Write
    description: Date of last write
    configuration: {}
  - id: lastWriteError
    channelTypeUID: modbus:last-erroring-write-type
    label: Last Erroring Write
    description: Date of last write error
    configuration: {}

I then defined an Item:

label: Outside Temperature
type: Number:Temperature
category: ""
groupNames:
  - outside
tags:
  - Temperature
  - Point

I linked this Item to the channel modbus:data:outside_temperature:outside_temperature:number using a Gain-Offset Correction with offset 0 and gain 0.1 to convert the modbus integer value (e.g. 157) to a float 15.7.

openHAB OutsideTemperature
openHAB OutsideTemperature2578×1682 303 KB

(Sorry, could not find the YAML describing this in the UI anywhere.)

Since I see 15.7 displayed in the log, I am assuming that the gain-offset correction has worked.

How come the conversion to a temperature fails?

2

What version of OH are you running?

what makes if you set the unit on the gain like it describes?

3

Maybe add the unit to the gain factor.
Have a look at this post, here I did this for a power value read from Modbus and it is working for me.

4

You can directly use 0.1 [°C] as Gain.

5

I set the gain to “0.1 [°C]” and now it works. :grinning:

Thank you for all the helpful advice!