The Baud rate (baudrate) specifies the speed of serial communication in symbols per second. A symbol is a distinct change in the signal state (e.g., a voltage level transition) on the communication line. For simple binary communication where each symbol represents a single bit (either a 0 or a 1), the baud rate is effectively the same as bps (bits per second).
This is the case for common serial interfaces like RS-232C, where the terms are often used interchangeably. For instance, setting a COM port to "9600 baud" means it will transmit and receive data at a rate of 9600 bits per second. It is a critical parameter for asynchronous serial communication because there is no separate clock signal shared between the transmitter and receiver. Instead, both devices must agree on the timing beforehand. Therefore, for two devices (e.g., a computer and a programmable power supply connected via RS-232C) to communicate successfully, their baud rates must be set to the exact same value. If there is a mismatch (e.g., one is set to 9600 and the other to 19200), the receiver will misinterpret the timing of the bits, resulting in garbled, nonsensical data and a complete failure of communication.
Common baud rate values include 2400, 4800, 9600, 19200, 38400, 57600, and 115200. While modern interfaces like USB and Ethernet have replaced RS-232C in many applications, it remains a simple and reliable interface for many industrial and lab instruments.