Create growatt_rs232.markdown

This is the help for the new growatt_rs232 integration I'm going to PR soon.

source/_integrations/growatt_rs232.markdown

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+---
+title: Growatt RS232
+description: Instructions on how to integrate your Growatt solar inverter within Home Assistant via the RS232 connection.
+ha_category:
+  - Sensor
+  - Energy
+ha_release: 0.113
+ha_iot_class: Local Polling
+ha_quality_scale: No score
+ha_config_flow: true
+ha_codeowners:
+  - "@ArdescoConsulting"
+ha_domain: growatt_rs232
+---
+
+These are sensors to collect information from your Growatt inverters using the RS232 connection.
+
+Instead of using the Growatt cloud service, it communicates directly via Modbus over the serial RS232 connector. Good for your privacy and security.
+
+The only thing required is a simple usb to serial RS232 adapter with db9 connector (costing around 15€). Before buying, check if the adapter is compatible with Linux/Windows without having to install extra drivers.
+
+The RS232 db9 connector is un the underside of the inverter. It's under a small cover plate requiring you to remove 2 screws.
+
+## Configuration
+
+Only interactive integration is supported (i.e. no yaml).
+
+Go to Configuration / Integrations / + (in the bottom right corner).
+
+You need to define
+
+1. The usb port your RS232 adapter is using. In Linux normally /dev/ttyUSB0.
+2. The Modbus slave address of the inverter. Between 1-247 but normally 1.
+
+## Remarks
+
+Depending on the firmware version of your inverter, not all entities may have data. Just disable those in Configuration / Entities.
+
+When the solar panels generate insufficient power at night, the inverter shuts down completely and hence doesn't provide any data. The entities then show up as unavailable.
+
+Some (very) old inverter versions only provide a proprietary RS232 serial protocol without Modbus. This integration is not compatible with that.