US What baud rate means
Baud rate is the number of symbols a communication channel transmits each second, where a symbol is one distinct state of the signal: a voltage level, a tone or a phase. It is measured in baud (Bd), named after Émile Baudot, the French engineer whose five-bit telegraph code of the 1870s was an early forerunner of modern character codes.
On a serial port the signal has only two states, so each symbol carries one bit and the baud rate equals the bit rate. That is why “9600 baud” and “9600 bps” are used interchangeably for RS-232 and RS-485. The baud rate fixes how long each bit lasts: at 9600 baud a bit is about 104 microseconds, and at 115200 baud it is under 9 microseconds.
There is no clock wire on an asynchronous serial link. The transmitter and receiver each time the bits from their own clock, which is why both must be set to the same rate before they can understand each other.
Baud rate vs bit rate
Baud and bits per second only diverge when one symbol carries more than one bit. Dial-up modems are the classic example: a V.32 modem signals at 2,400 baud but uses 16 combinations of phase and amplitude, so each symbol carries four bits and the link runs at 9,600 bits per second. Ethernet, Wi-Fi and cellular radios go much further, packing many bits into every symbol.
Bit rate and useful data rate also differ. A UART adds framing bits to every byte, so the throughput is lower than the baud rate suggests.
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Start bit. One bit that marks the start of each character and lets the receiver lock on to its timing.
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Data bits. Usually eight, sometimes seven for older text protocols.
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Parity bit. Optional: none, even or odd, for a simple error check. Modbus RTU uses even parity by default.
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Stop bits. One or two idle bits that end the character.
Common baud rates and where they are used
Serial settings are written as a rate and a frame, such as 9600 8N1: 9600 baud, eight data bits, no parity and one stop bit. With 8N1, each byte takes ten bits on the wire, so dividing the baud rate by ten gives the bytes per second.
| Baud rate | Bytes per second at 8N1 | Typical use |
|---|---|---|
| 1200 | Around 120 | Legacy modems, some meters and telemetry radios |
| 4800 | Around 480 | GPS receivers on NMEA 0183 |
| 9600 | Around 960 | Default for many instruments, Modbus RTU devices and network console ports |
| 19200 | Around 1,920 | Modbus RTU default in the Modbus serial line specification |
| 38400 | Around 3,840 | Faster instruments, BACnet MS/TP and high-speed NMEA |
| 57600 | Around 5,760 | Older modems and some microcontroller boards |
| 115200 | Around 11,520 | Linux and bootloader consoles on embedded boards, microcontroller debugging |
| 230400–921600 | Around 23,040–92,160 | High-speed RS-232/422/485 cards and short, clean links |
The standard rates descend from early teleprinter and modem speeds and from UART clock dividers. A 1.8432 MHz crystal divided by 16 gives 115200 baud, and dividing it further gives each standard rate below it. Higher rates are possible: one of the 8-port PC/104-Plus serial I/O modules we list runs at up to 921.6 kbps in RS-232 mode and 1.8432 Mbps on RS-422 or RS-485, and the 8-port serial MezIO modules for Neousys computers run each port from 50 bps to 921.6 kbps.
How baud rate affects cable length
The faster the rate, the shorter each bit, and the less cable capacitance and noise a link can tolerate before bits blur together. For RS-485, the usual guidance from TIA-485 application notes is around 1,200 m at up to about 100 kbps, falling to roughly 12 m at 10 Mbps. RS-232 was specified for about 15 m at its original rates, with shorter runs at 115200 baud. Twisted-pair cable, proper termination and lower rates all extend the reach.
On an RS-485 field bus the rate is a trade-off between polling speed and reach. Many Modbus RTU networks run at 9600 or 19200 baud because the extra speed is rarely needed and the slower rate is more tolerant of long, noisy cable runs.
When baud rates do not match
A mismatch does not stop a link outright; it corrupts it. The receiver samples each bit in the middle of where it expects it to be, so a wrong rate gives random characters, framing errors or nothing at all. The usual checks are simple.
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Confirm the full frame. Rate, data bits, parity and stop bits must all match, and so must flow control if it is used.
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Try the common rates. Most devices default to 9600, 19200 or 115200 baud, so trying those in turn usually finds the answer.
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Check clock accuracy. UART timing tolerates only a few per cent of error between the two ends, so non-standard rates or poor clocks can fail even when the settings match.
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Watch for autobaud. Some devices detect the rate from the first character they receive, which can lock on to the wrong rate if noise arrives first.
Quick answers on baud rate
Baud rate: common questions
What is 9600 baud?
9,600 symbols per second. On a serial link that is 9,600 bits per second, or around 960 bytes per second with 8N1 framing. It is the default rate for many instruments, PLC ports and network equipment consoles.
Is 115200 a baud rate?
Yes. 115200 baud is a standard rate, the highest on the original PC serial port, and the usual console speed for embedded Linux boards and bootloaders. It moves about 11.5 kB of data per second at 8N1.
What baud rate should I use?
The one the device expects. Check its manual or configuration. If you can choose, use the highest rate the cable length and noise allow; for Modbus RTU on RS-485, 9600 or 19200 is the usual choice.
How do I find the baud rate of a serial port?
Check the port settings. On Windows, Device Manager shows the COM port’s default rate, and on Linux the stty command reports it. The rate the far device uses has to be found in its own settings or by trying the common rates.
Does CAN bus use a baud rate?
It uses a bit rate, often called baud. CAN networks run at fixed rates such as 125, 250, 500 or 1,000 kbps, and every node must match. J1939 vehicle networks typically run at 250 or 500 kbps.
We fit serial cards and modules in industrial and embedded computers as standard, and set each port’s mode, termination and drivers before the system ships. Tell us how many RS-232, RS-422 and RS-485 devices you need to connect, at what rates and cable lengths, and we will specify the ports to match.