The scenario
A 5V MCU UART must send and receive data on an automotive 12V K-Line bus without a dedicated transceiver IC.
What is in this drawing
Read the decisions behind it.
01
Open-collector NPN Q1 pulls the K-Line low when TXD is high, with R2 providing the recessive pull-up to 12V.
02
RX path divides the K-Line voltage through R3 and level-shifts it with Q2, restoring a logic-level signal at RXD.
03
External terminals TXD, RXD and KLINE keep the diagram reusable as a system block.
Use this pattern for bench interfaces, ECUs, or diagnostics where a single-wire bidirectional automotive bus must connect to logic-level UART.
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