Hardware Reference
In-Depth Information
Figure 9-3. Serial signal
The standard states that the signal is considered to be in a mark state , when the
voltage is between -3 and -15 V. The signal is considered in a space state if the voltage is
between +3 and +15 V. The RS-232 data line is in the mark state when the line is idle.
Start Bit
When an asynchronous character of data is to be sent, the line first shifts to a space level
for the duration of 1 bit. This is known as the start bit (0). Data bits immediately follow.
Asynchronous lines do not use a clock signal like synchronous links. The
asynchronous receiver must have a clock matching the same baud rate as the transmitter.
The receiver samples the line 16 times in the bit cell time to determine its value. Sampling
helps to avoid a noise pulse from triggering a false data read.
Data Bits
Data bits immediately follow the start bit, least significant bit first. A space is a 0 data
bit, while mark represents a 1 bit. Early teletype equipment used 5 data bits sending
characters in the 5-bit Baudot code. 47 For this reason, serial ports can be configured for 5,
6, 7, or 8 data bits. Before the ASCII character set was extended to 8 bits, it was common
to use 7-bit serial data.
Parity Bit
An optional parity bit can be generated when transmitting or can be detected on the
receiving side. The parity can be odd, even, or stick (mark or space). The most commonly
used setting today is No Parity, which saves 1-bit time for faster communication.
Older equipment often used parity to guard against errors from noisy serial lines. Odd
parity is preferred over even because it forces at least one signal transition in the byte's
transmission. This helps with the data reliability.
Mark or space parity is unusual and has limited usefulness. Mark parity could be
used along with 2 stop bits to effectively provide 3 stop bits for very slow teletypewriter
equipment. Mark or space parity reduces the effective throughput of data without
providing any benefit, except possibly for diagnostic purposes. Table 9-1 summarizes the
various parity configurations.
 
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