Editing Serial port (section)

===Speed===
Serial ports use two-level (binary) signaling, so the data rate in bits per second is equal to the symbol rate in [[baud]]. The total speed includes bits for framing (stop bits, parity, etc.) and so the effective data rate is lower than the bit transmission rate. For example, with 8-N-1 character framing, only 80% of the bits are available for data; for every eight bits of data, two more framing bits are sent.

A standard series of rates is based on multiples of the rates for electromechanical [[teleprinter]]s; some serial ports allow many arbitrary rates to be selected, but the speeds on both sides of the connection must match for data to be received correctly. Bit rates commonly supported include 75, 110, 300, 1200, 2400, 4800, 9600, 19200, 38400, 57600 and {{nowrap|115200 bit/s}}.<ref name="winbase.h"/> Many of these standard modem bit rates are multiples of either {{nowrap|1.2 kbit/s}} (e.g., 19200, 38400, 76800) or {{nowrap|0.9 kbit/s}} (e.g., 57600, 115200).<ref>{{cite web |url=https://cdn.hackaday.io/files/1614916909230944/SX1272_DS_V4.pdf |title=SX1272/73 - 860 MHz to 1020 MHz Low Power Long Range Transceiver Datasheet |publisher=[[Semtech]] |date=January 2019 }}</ref> [[Crystal oscillator]]s with a frequency of 1.843200&nbsp;MHz are sold specifically for this purpose. This is 16 times the fastest bit rate, and the serial port circuit can easily divide this down to lower frequencies as required.

The capability to set a bit rate does not imply that a working connection will result. Not all bit rates are possible with all serial ports. Some special-purpose protocols such as [[MIDI]] for musical instrument control, use serial data rates other than the teleprinter standards. Some serial port implementations can automatically choose a bit rate by observing what a connected device is sending and synchronizing to it.