Serial Port Basics (continued)
More Asynchronous Serial Parameters
In most cases, the data rate in bytes/second can be approximated by dividing the baud rate (in bits/second) by 10. So, if a byte consists of 8 bits, why divide by 10?
To transfer data asynchronously, the UART frames a byte's 8 data bits between a stop bit and a start bit. The start bit is always a zero, while the stop bit is always a one. So, a byte of data sent serially is made up of 10 bits instead of the usual 8.
Asynchronous serial devices can communicate using 7 or 8 data bits, and 1, 1½, or 2 stop bits. To further complicate matters, devices can also employ a parity bit instead of an eighth data bit to check for errors. Even parity systems transmit a one when the sum of the seven bits is an even number, while odd parity systems transmit a one when the sum is odd. Still more exotic systems may specify "mark" or "space" parity, where the parity bit is always a one or zero, respectively.
What does all of this mean? Device vendors usually specify their data rate and format using statements like "9600, 8N1", which translates to 9600 bps, 8 data bits, no parity, and 1 stop bit or "19200, 7E1", which translates to 19200 bps, 7 data bits, even parity, and 1 stop bit.
NOTE: Most serial devices format of eight data bits, no parity, and one stop bit (8N1).
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