Modbus slave sensors not available when master sensor is deactivated

[pstzh]

The problem

It seems, that since core-2025.6.0 my slave sensors of a multiple modbus sensor configuration (slave_count = 20) are not available because I had a deactivated master sensor. I currently do not need the value of the master sensor but probably want to have it in future. After Activation of the master sensor in 2025.7.4 all of the slave sensors were available again.

What version of Home Assistant Core has the issue?

core-2.7.4

What was the last working version of Home Assistant Core?

core-2025.5.3

What type of installation are you running?

Home Assistant OS

Integration causing the issue

modbus

Link to integration documentation on our website

No response

Diagnostics information

No response

Example YAML snippet

# part of the entry of my modbus config (kibernetik heatpump)
  sensors:
    - name: rego_hol1
      unique_id: rego_hol1
      slave: 1
      address: 4
      input_type: holding
      slave_count: 20
      data_type: int16
      scale: 0.1
      precision: 1
      scan_interval: 300
      unit_of_measurement: °C

Anything in the logs that might be useful for us?

Additional information

Activation of master entity "rego_hol1" and set it to invisible solves the problem for me. The behavior just changed since core-2.6.0.

[home-assistant]

modbus documentation
modbus source

[crug80]

The issue is not clear.
Slaves are created as the sons of a first sensor. It is only a way to optimize the requests over the bus for adjacent registers.
You could create 20 separated sensors instead of slaves, or you could create slaves beginning from address 5, instead of 4 in your example..
So, what do you mean with "deactivated master sensor"?

[pstzh]

Exactly - I try to optimize the number of requests over the modbus, also for the future. Therefor I read all the defined adjacent 20 registers with one request. Currently I dont need the first sensor and my solution was to deactivate sensor on address 4 in the entity GUI. When I need it later, I don't have to make a second request for address 4 later on, as all names would change when I change the startadress from 5 to 4. Anyhow, when I deactivate in the homeassistent entity GUI one of the 20 entities, there should be no sideeffect to the other 19 sensors, even if it is the master, which is finally only the first address of adjacent registers. This was exactly the case until 2025.5.3. but changed to invalidate all the sons on deactivation of the master.

[crug80]

I'm currently running 2025.7.4 and I tested your scenario. I have 1 sensor with 4 slaves. Disabling the first sensor on the gui doesn't affect the slaves and their states are correctly updated.
I think that your problem is elsewhere.
Please, post the full configuration (and log, if you can) giving you the problem.

[pstzh]

I reverified it also. You are right that just disabling the master does not invalidate the sons. But after restart without enabling, at least in my case, the sons are invalid. Did you also restart the homeassistent after setting the master sensor do disabled?

[crug80]

Just tested a full reboot. The son restores its previous state without polling for a new state.
Honestly speaking, the previous bahviour was totally unexpected instead of this one, but after reading the HA docs it is clear. To really disable/enable a sensor you need a system restart, so the previous scenario was a false positive, but it is also a good workaround in your case.
You just need to create an automation to enable the sensor before a system/config restart and then another automation to disable it after its first update.
Anyway I don't know if this could be a real issue of modbus.. I'll debug a little bit the case in the next days

[crug80]

Going deep into the source code, the explaination is clear: HA doesn't call the update for disabled sensors and because the sons are updated using the data read from the father, if the system doesn't raise the update event for it, the sons are not updated as well.
As I said before, I don't know if this could be a real bug, it seems a design choice and it make sense IMHO.
Anyway, you can use the workaround I suggested to overcome the your problem or, an easier way could be to ignore the father, I mean, to hide that sensor in your wall and to use only the sons.

[pstzh]

Thank you for your valuable research and the suggested workaround. The current behaviour is technical explainable and understandable. But still I think that this is not consistent from a user point of view. As menioned above, reading of multiple adjacent registers is primary a way for optimising access to modbus interface. The behaver should still be the same as if I read each of the registers separatly, which is not the case in the current implementation. Disabling one of the son sensors does not affect the other son sensors, wheras disabling of the master sensor invalidates all of the son sensors.
Still, for me it is absolutly OK if this is the finally the defined behaviour. The suggested workaround is an acceptale way for me.