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Chapter 13 Serial Communication
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Odd parity: Among all bits to be transferred (including bits of characters and the parity bit),
the number of all “1” is an odd number, for example:
In the 8-bit data 01100101, the number of “1” is an even number; we append a 1 to change it to an
odd number, so the parity bit is 1.
In the 8-bit data 01100001, the number of “1” is an odd number; we append a 0 and it remains to
be an odd number, so the parity bit is 0.
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Even parity: Among all bits to be transferred (including bits of characters and the parity bit),
the number of all “1” is an even number, for example:
In the 8-bit data 01100101, the number of “1” is an even number; we append a 0 and it remains to
be an even number, so the parity bit is 0.
In the 8-bit data 01100001, the number of “1” is an odd number; we append a 1 to change it to an
even number, so the parity bit is 1.
When the parity check is adopted, 1-bit error can be detected, but 2-bit or high errors cannot be
detected. Furthermore, detected errors cannot be corrected. Upon detection of an error, it will
request for retransmission. However, since it is easy to implement, it is widely applied.
To implement communication between two parties, the settings of data bit, start bit, stop bit, bits
per second and parity check for serial ports of both parties must be set to be the same.
The Kaspro CASTOR/Gemini/Galaxy and most PLCs support only asynchronous transfer mode.
Our description is also based on asynchronous transfer mode. (If you are interested in
synchronous transfer mode, please refer to related documents). Certainly, entity interfaces are
required as transmission media to send or receive data in asynchronous transfer mode. This is the
difference between RS-232, RS-422 and RS-485, as detailed respectively below.
13.2 RS-232
Presently, RS-232 is the most widely applied serial interface in the PC and communication industry.
RS-232 is defined as a single-ended standard to extend communication distance in low-rate serial
communication. RS-232 adopts an Unbalanced Data Transmission (UDT) interface, under which
the voltage of a single cable relative to a grounding signal cable is used to indicate the logic status
Mark or Space. A typical connection method is shown as follows:
TX
RX
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