![]() Compiled it uses just 62 bytes of flash, and does not require any RAM (there is no buffering). I've written the serial code to work as an Arduino library. With no base current, Q1 will be turned off, and the Rx line on the serial adapter will be disconnected from the transmission. When the serial adapter is transmitting a 0, D1 will allow it to pull the AVR pin low. When the AVR transmits a 1, Rx will no longer be pulled low, and Rx will return to high state. R1 ensures current flow through the base of Q1 is kept below 1mA. When the AVR transmits a 0, with Q1 on, Rx will get pulled low indicating a 0. When the TTL serial adapter is not transmitting, the voltage from the Tx pin keeps Q1 turned on, and the AVR pin (Tx/Rx) will sense a high voltage, indicating idle state. ![]() ![]() ![]() To understand the circuit, a tutorial on TTL serial communication may help.
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