Why can’t I change to 76800 baud rate on PC?
A common question during serial communications troubleshooting is why a PC cannot be configured to use a baud rate of 76800, even though some industrial devices appear to support it. This limitation is not related to the operating system or application software, but rather to how standard PC serial hardware is designed.
Most PCs rely on a traditional UART (Universal Asynchronous Receiver/Transmitter) architecture derived from the 16C550 family. These UARTs are designed to operate using fixed clock divisors that only support specific baud rates derived from a base clock frequency.
UART Clocking and Standard Baud Rates
Standard PC UARTs generate baud rates by dividing a fixed clock frequency. Because of this, only baud rates that are clean integer divisions of the UART clock are supported. Common baud rates such as 9600, 19200, and 38400 are standard because they align cleanly with the internal clock timing.
A baud rate of 76800 does not align with the standard UART clock divisors used by most PC serial controllers. As a result, the UART cannot reliably sample or transmit data at that speed. Attempting to force this rate typically results in framing errors, dropped characters, or complete communication failure.
This limitation is hardware-based. Even if an operating system allows a custom baud rate to be entered, the underlying UART may still be unable to generate or detect the required timing accurately.
Why Some Devices Support 76800 Baud
Many embedded and industrial devices do not rely on fixed PC-style UART implementations. Instead, they often use microcontrollers or programmable serial interfaces that allow the bit clock to be configured directly.
Because these devices can customize their clock generation, they may support non-standard baud rates such as 76800. This flexibility is common in controllers, gateways, and field devices where communication timing can be tuned to match specific system requirements.
Possible Workarounds
In most cases, there is no direct way to force a standard PC serial port to operate reliably at 76800 baud. However, there are a few potential workarounds depending on the application.
One option is to use a specialized serial interface or expansion card that provides a programmable baud rate generator. These cards allow the UART bit clock to be adjusted beyond the limitations of standard PC hardware.
Another approach is to use a protocol converter or serial gateway that can translate between 76800 baud on one side and a standard baud rate on the PC side. This allows the PC to communicate using a supported speed while the converter handles the non-standard timing.
Standard PC Baud Rates
The following baud rates are commonly supported by standard PC UARTs and are considered reliable for serial communications:
- 150
- 300
- 1200
- 2400
- 4800
- 9600
- 19200
- 38400
- 115200
FAQ
Is 76800 baud supported by most PCs?
No. Most PCs use UART hardware that cannot reliably generate or sample this baud rate.
Is this a software or hardware limitation?
It is a hardware limitation of the standard PC UART design.
Why do some devices allow 76800 baud?
Many devices use programmable serial hardware that is not constrained by fixed clock divisors.
Is there a direct fix?
No. Workarounds include using programmable serial cards or protocol converters.