Format of Data Communication Messages
For a simple asynchronous system such as RS-232, it is common practice to send one character at a time. The format of a typical character frame is indicated in Figure 1.
Figure 1. Format of a
Typical Serial Asynchronous Data Character
Initially the data communications link is in the idle state: the line is in the mark state, held to a constant negative voltage.
The parity bit included at the end of the character is effectively a fingerprint of the character to enable the receiver to identify whether any errors have occurred in the transmission. For example, even parity means that the total number of logic 1 bits in the data together with the associated parity bit must be an even number.
In summary, the optional settings for asynchronous transmission of characters are:
Start Bits - 1
Data Bits - 5, 6, 7, 8
Parity Bits - even, odd, mark, space or none
Stop Bits - 1, 1 1/2 or 2