User`s manual

Overview of the Serial Port
8-13
1 The start bit is transmitted with a value of 0.
2 The data bits are transmitted. The first data bit corresponds to the least
significant bit (LSB), while the last data bit corresponds to the most
significant bit (MSB).
3 The parity bit (if defined) is transmitted.
4 One or two stop bits are transmitted, each with a value of 1.
The number of bits transferred per second is given by the baud rate. The
transferred bits include the start bit, the data bits, the parity bit (if defined),
and the stop bits.
Start and Stop Bits
As described in “Synchronous and Asynchronous Communication” on page
8-12, most serial ports operate asynchronously. This means that the
transmitted byte must be identified by start and stop bits. The start bit
indicates when the data byte is about to begin and the stop bit(s) indicates
when the data byte has been transferred. The process of identifying bytes with
the serial data format follows these steps:
1 When a serial port pin is idle (not transmitting data), then it is in an “on”
state.
2 When data is about to be transmitted, the serial port pin switches to an “off”
state due to the start bit.
3 The serial port pin switches back to an “on” state due to the stop bit(s). This
indicates the end of the byte.
Data Bits
The data bits transferred through a serial port may represent device
commands, sensor readings, error messages, and so on. The data can be
transferred as either binary data or ASCII data.
Most serial ports use between five and eight data bits. Binary data is typically
transmitted as eight bits. Text-based data is transmitted as either seven bits
or eight bits. If the data is based on the ASCII character set, then a minimum
of seven bits is required since there are 2
7
or 128 distinct characters. If an