Figure 2.1 Transmission Of "C - Omega i-SERIES User Manual

• Parity Bit
Besides the synchronization provided by the use of start and stop bits, an additional bit
called a parity bit may optionally be transmitted along with the data. A parity bit affords a
small amount of error checking, to help detect data corruption that might occur during
transmission. You can choose either even parity, odd parity or no parity at all. When
even or odd parity is being used, the number of marks (logical 1 bits) in each data byte
are counted, and a single bit is transmitted following the data bits to indicate whether the
number of 1 bits just sent is even or odd.
For example, when even parity is chosen, the parity bit is transmitted with a value of 0 if
the number of preceding marks (1's) is an even number. For the binary value of 0110 0011
the even parity bit would be 0. If even parity were in effect when the binary number
1101 0110 is sent, then the parity bit would be 1. Odd parity is just the opposite, and the
parity bit is 0 when the number of mark bits (1's) in the preceding word is an odd number.
Parity error checking is very rudimentary. While it will tell you if there is a single bit error
in the character, it doesn't show which bit was received in error. Also, if an even number
of bits are in error then the parity bit would not reflect any error at all. No parity ignores
the parity bit. When transmitted, each character is preceded by a start bit and followed
by a stop bit plus an optional parity bit, making train of 10 or 11 bits for each transmitted
character. The Figure 2.1 below shows transmission of the 7 bits of the ASCII lower case
"c" with start, stop and even parity bits.
• Baud Rate
The baud rate refers to the data transmission. It specifies the communication rate over
the bus. When a change in signal represents one data bit, baud rate is equal to bits per
second (bps). Standard baud rates for computers are 300, 600, 1200,2400, 4800, 9600
and 19200 baud.
7 - BIT CHARACTER
STOP BIT
1 2 3 4 5 6 7 8
1
0
START BIT EVEN PARITY BIT
Figure 2.1 Transmission of "c" with start, stop, and even parity bits.
• Communication Protocol
A data communication protocol defines the rules and structure of messages used by all
devices on a network for data exchange. This protocol also defines the orderly exchange
of messages, and the detection of errors. i-Series controllers use i-SERIES and
MODBUS communication protocols.
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