I have a ZigBee network for sensors controlled by an XBee S2C module with ZHA component.
I also have another XBee S2C device that controls a relay. In order to open or close the relay I must send arbitrary bytes to it from the XBee module over UART, i.e. 0x65 to open the relay and 0x6F to close it.
According to the XBee specs (https://www.digi.com/resources/documentation/digidocs/pdfs/90002002.pdf) I must issue an API command (0x10 Transmit Request frame) from the coordinator device (which is controlled by HA).
My problem is that I don’t know how to do it from HA, and even don’t know where start looking.
I guess it is not supported out-of-the-box and I probably need to write a custom module providing the service and probably customize zigpy or zha-quirks or whatever, but I need your advice on where to start, because I am not really familiar with all these stuff yet.