The receiver then starts sampling the data line in order to determine if its a 0 or a 1. Instead, transmitter and receiver are supposed to agree beforehand about which bitrate to use.Ī start bit on the data line signals the start of each "character" (typically a byte, but with start/stop/parity bits framing it).
#Smart serial data cable speed rate serial#
In response to the comment about clocking (with a caveat: I'm a software guy): asynchronous serial communication doesn't transmit the clock (that's the asynchronous part right there) along with the data. I regularly use a RS232 link running at 460,800 bps, with a USB-based adapter. If you use a USB-based serial adapter, there's no 16550 involved and the limit is instead set by the specific chip(s) used in the adapter, of course. I believe the "classical" PC UART (the 16550) in modern implementations can handle at least 1.5 Mbps. The maximum speed is limited by the specs of the UART hardware.
#Smart serial data cable speed rate driver#
Whether or not the driver accepts 115200 as the maximum value is a driver implementation detail, they usually accept higher values. They do accept a baudrate selection but just ignore it for the USB bus itself, it only applies to the last half-inch in the dongle you plug in the device.
![smart serial data cable speed rate smart serial data cable speed rate](https://usbwiringdiagram.com/wp-content/uploads/2019/02/db9-serial-to-usb-wiring-diagram-wiring-diagram-usb-to-serial-adapter-wiring-diagram.jpg)
Which have a custom driver to emulate a serial port. True serial hardware based on a UART chip like 16550 have been disappearing rapidly and replaced by USB emulators.
![smart serial data cable speed rate smart serial data cable speed rate](https://www.techadvisor.com/cmsdata/slideshow/3671060/best-ethernet-cables-2021_thumb800.jpg)
This is all ancient history and doesn't exactly apply to modern hardware anymore. To go further you need a different approach, like RS-422's differential signals. At 115200 only very short cables will do in practice. The maximum recommended cable length at 9600 baud is only 50 feet. The electrical standard is very sensitive to noise, there is no attempt at canceling induced noise and no attempt at creating an impedance-matched transmission line. Getting real RS-232 hardware running at 115200 baud reliably is a significant challenge. The point of asynchronous signalling, the A in UART, the start bit always re-synchronizes the receiver. But close enough, the clock rate doesn't have to be exact. So the highest baudrate you can get is by setting the divider to the smallest value, 2.
![smart serial data cable speed rate smart serial data cable speed rate](https://cdn.thewirecutter.com/wp-content/uploads/2020/03/lightningcables-lowres-8522.jpg)
The UART itself then divides it by 16 to generate the sub-sampling clock for the data line. From there, it first went through a programmable divider, the one you change to set the baudrate. A crystal made to run an oscillator circuit at the color burst frequency in the NTSC television standard. And turned to crystals that were widely in use at the time, used in any color TV in the USA. The engineers that designed it needed a cheap way to generate a stable frequency.