At the moment, the availability of a RS485 device is equal to the availability of the PLC device as they use the same availability topic.
However, ideally, each configured RS485 device has it's own separate availability topic allowing an indication of device read failures via it's availability topic.
Home-Assistant docs indicate that multiple availability topics for an entity are supported, meaning that the PLC availability topic and RS485 individual device availability topic can be combined to reflect the entity availability.
At the moment, the availability of a RS485 device is equal to the availability of the PLC device as they use the same availability topic.
However, ideally, each configured RS485 device has it's own separate availability topic allowing an indication of device read failures via it's availability topic.
Home-Assistant docs indicate that multiple availability topics for an entity are supported, meaning that the PLC availability topic and RS485 individual device availability topic can be combined to reflect the entity availability.