baud

A baud is a unit of signalling speed, measuring how many discrete signal changes occur per second on a transmission line. Named after Émile Baudot, the 19th-century telegraph engineer, it quantifies the rate at which a modem, serial interface, or communication channel switches between signal states, regardless of how many data bits each state represents.

Baud differs fundamentally from bit rate, a distinction that confuses many technicians. A 9600 baud modem does not necessarily transmit 9600 bits per second. If the modem uses four-phase modulation, where each signal state encodes two bits, then 9600 baud equals 19,200 bits per second. The relationship depends entirely on the encoding scheme: bits per second equals baud multiplied by bits per symbol.

In serial communications, particularly RS-232 and similar interfaces, baud rate and bit rate often align because each symbol typically carries one bit. Common rates include 1200, 2400, 9600, 19,200, 57,600, and 115,200 baud. The term persists in software and hardware documentation even where symbol and bit rates are identical, creating a vocabulary overlap that remains standard in the field.

Practical complications arise when baud rates mismatch between devices. A terminal expecting 9600 baud connected to equipment transmitting at 19,200 baud receives corrupted data; characters appear as gibberish or the connection drops. Configuration errors, particularly when swapping equipment or adding serial devices, commonly stem from baud rate disagreement rather than physical faults. Determining the correct rate requires consulting equipment documentation or attempting common speeds systematically.

Higher baud rates reduce transmission time but increase susceptibility to noise and signal degradation. Longer cables and electrically noisy environments constrain practical baud rates, especially for unshielded serial lines. Modern USB and Ethernet have made baud-rate discussions less critical in new designs, but legacy industrial equipment, programmable logic controllers, and instrumentation still operate at specific baud rates that cannot be changed. Understanding the distinction between baud and bit rate becomes essential when designing protocol converters or troubleshooting communication failures in mixed-age systems.