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You can write back on a generic MQTT topic from a custom namespace tag as long as the Edge device sending the generic MQTT data into MQTT Engine supports command writes.

There are some additional steps to configure at MQTT Engine and within Ignition to allow this to happen.

These Here are the requirements:

1. By default, tags created by the MQTT Engine Custom Namespace are read-only. You will need to configured the Custom Namespace to create writable tags.
   1. Make sure to restart MQTT Engine after making this configuration change
   2. This will only make the tags writable but will not result in outgoing MQTT messages.
2. Create a tag change script on each custom namespace tag that calls engine.publish() to publish the appropriate an MQTT message back to the Edge device on the proper topic with the proper payload.
   1. See MQTT Publishing via MQTT Engine for examples
3. The Edge device must support writes on the published topic.

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