Datasheet

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Debugging Serial Buses in Embedded System Designs
Figure 26 shows one byte of RS-232 data. The byte is
composed of these bits.
Start - The byte begins with a start bit.
Data - Several bits of data follow.
Eight bits of data is the most common; some applications
use seven bits of data. Even when only seven bits are
transmitted, the data is often informally referred to as a
byte. In UART to UART communication, 9 bit data words
are sometimes used.
Parity - An optional parity bit.
Stop - 1, 1.5, or 2 stop bits.
An RS-232 bus does not have a clock line. Each device uses
its own clock to determine when to sample the data lines. In
many designs, a UART uses the rising edges of the Tx and Rx
signals to synchronize its clock with the other device’s clock.
When probing RS-232 signals, it is often helpful to use
a breakout box. This device allows you to easily probe
the signals inside an RS-232 cable. Breakout boxes are
inexpensive and readily available from electronics dealers.
The RS-232 standard does not specify the content
transmitted across the bus. ASCII text is most common, but
binary data is also used. The data is often broken up into
packets. With ASCII text, packets are commonly terminated
by a new line or carriage return character. With binary data
other values, such as 00 or FF hex are commonly used.
Devices often implement RS-232 using a universal
asynchronous receiver/transmitter (UART). UARTs are widely
available in off-the-shelf parts. The UART uses a shift register
to convert a byte of data into a serial stream, and vice versa.
In embedded designs, UARTs can also communicate directly
without the use of RS-232 transceivers.
Figure 26. RS-232 byte structure.
Start
1 bit 1 bit
StopData0
1 bit
Data1
1 bit
Data2
1 bit
Data3
1 bit
Data4
1 bit
Data5
1 bit
Data6
1 bit
Data7
(opt.)
1 bit 1-2 bits
Parity
(opt.)