"TXD" stands for "Transmit Data" and it is a signal used in serial communication between two devices.
In a typical serial communication setup, one device acts as the transmitter and the other device acts as the receiver. The transmitter sends data to the receiver using a serial data line, while the receiver sends acknowledgement signals to the transmitter using control lines such as CTS and RTS.
The TXD signal is used by the transmitter to send the serial data to the receiver. When the transmitter sends data, it is transmitted one bit at a time on the serial data line, and the receiver reads the bits and reassembles them into a complete message.
The TXD signal is usually an output signal from the transmitter, which means that it is connected to a pin or port on the transmitter's microcontroller or other processing device. The specific characteristics of the TXD signal, such as its voltage level and timing requirements, can vary depending on the specific communication protocol being used.
TXD is often paired with a "Receive Data" (RXD) signal, which is used by the receiver to receive data from the transmitter. Together, TXD and RXD enable two-way communication between the devices.