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docs-standalone/docs/connection-modes.md

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+# Connection Modes
+
+The rewrite supports two panel input/output modes.
+
+## 1) MQTT mode (default)
+
+Enabled when:
+
+- `mqtt_server` is configured
+- `use_ha_api` is not present
+
+Behavior:
+
+- Subscribes to every panel `panelRecvTopic`
+- Expects JSON payload containing `CustomRecv`
+- Publishes commands to `panelSendTopic`
+
+Example receive payload:
+
+```json
+{"CustomRecv":"event,startup,54,eu"}
+```
+
+## 2) Home Assistant API mode (`use_ha_api`)
+
+Enabled when key `use_ha_api` exists in config.
+
+Behavior:
+
+- Subscribes to HA event `esphome.nspanel.data`
+- Routes events by `device_id` (must match configured `panelRecvTopic`)
+- Sends panel commands by calling Home Assistant service:
+  - `<panelSendTopic>_nspanelui_api_call`
+
+Service payload shape:
+
+```yaml
+data: "...panel command..."
+command: 2
+```
+
+## Common to both modes
+
+- Home Assistant websocket connection is used for entity state cache and service calls.
+- UI actions (button presses, sliders, mode selects) are translated to Home Assistant service calls.