How to "trigger" an mqtt topic to read another topic

[rlkoshak (Rich Koshak)]

@tillykeats

My “Generic Thing TRV” has a number of MQTT topics where a command topic (no payload) must be sent in order to read the corresponding state topic. For example, to obtain the VOLTAGE topic , one must send a null payload to topic /energenie/eTRV/Command/VOLTAGE/8008 (8008=manufactuer deviceID) and then the eTRV will publish the voltage value, once but the value is persistent, to the topic /energenie/eTRV.Report/Voltage/8008, for example 3.012 (an asci string for the voltage).

This is not how MQTT is supposed to work. The device should just publish the voltage value. It shouldn’t require the subscribers to do anything to get the value.

@tillykeats

but I’m not sure what Items I need and how to set these up or if indeed Items are the right thing to do? And do I have do do something else with the Channels?

If you want to do anything with the Channels you must link them to Items.

@tillykeats

I have also noted the ability to setup a “Publish Trigger” on the MQTT Broker thing, is something I need?

Not in this case I think.

@tillykeats

It is this refresh request that I’m not sure how to model in OH.

On first thought, assuming you have more than one of these devices:

• Create a Generic Thing for each device. Have a Channel for the refresh topic, the value topic, and any other topic provided/used by that device.
• Link Items to all the Channels you care about. The refresh Channel should probably be a Switch type. then you can sendCommand(ON) to it and it will publish the empty message to the refresh topic in response.
• Add all the Items linked to the refresh topic to a Group:Switch.
• Create a Time cron triggered Rule that periodically sendComman(ON) on the Group. The command will be forwarded to all the members of the Group, which will result in the message being sent to the refresh topic and in turn the devices will publish the value to the state topic which will be received by OH and the Items updated. I believe this trigger type works in the NGRE PaperUI Rules UI.

@tillykeats

i am stuck on what channels and items I need , and maybe things?

Step back a second. You are trying to run before you can walk. Review How to get started (there is no step-by-step tutorial). Your struggles are because you lack an understanding of how OH works. In particular, the relationships between Things, Items and Rules.