An implementation of Eberspacher K-Line diagnostics adapter modchip, as described in https://t4-wiki.de/wiki/Diagnoseinterface_fuer_Standheizungen and https://www.drive2.ru/b/493589462480060493/
I used ATtiny13A microcontroller; the code can be compiled with avr-gcc 10 and make.
In short: this chip is meant to pull K-Line to ground for a certain amount of time, when RTS serial port line is asserted (seconds, depends on the number of RTS assertions). I used an external NPN transistor as a current buffer.