Future Design MCT-MC 4.3, Руководство пользователя

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MCT-MC
4.3
Appendix - 143 - FDC MCT-MC
A.1.2 Serial Communications
The user communications interface for the MC employs serial communication, which is the exchange of data in
a one-bit-at-a-time, sequential manner on a single data line or channel. Serial contrasts with parallel
communication, which sends several bits of information simultaneously over multiple lines or channels. Not
only is serial data communication simpler than parallel, it is also less costly.
Baud Rate
The baud unit is named after Jean Maurice Emile Baudot, who was an officer in the French Telegraph Service.
He is credited with devising the first uniform-length 5-bit code for characters of the alphabet in the late 19th
century. What baud really refers to is modulation rate or the number of times per second that a line changes
state. This is not always the same as bits per second (BPS). However, if you connect two serial devices
together using direct cables then baud and BPS are in fact the same. Thus, if you are running at 9600 BPS,
then the line is also changing states 9600 times per second.
Typical baud rates used for computers are 300, 600, 1200, 2400, 4800, 9600, 14400, 19200, 38400, 57600
and 115200 baud. As the baud rate increases, so does the transmission rate of data. Thus you get more
information in a shorter period of time. However, the faster the transmission rate, the more susceptible it is to
error due to the quality of the cable and sources of electrical “noise” in the environment. To provide the best
balance between speed and noise immunity, the MC uses a baud rate of 9600 baud.
In order for a device to
communicate with the MC, it must have its serial port set for 9600 baud in order for data communications to
work properly.
Start and Stop Bits
The start bit informs the receiving device that a character is coming, and a stop bit tells it that a character is
complete. The start bit is always a 0. The stop bit is always a 1. The human speech equivalent of these bits
could be a clearing of the throat to get someone’s attention (start bit); and a pause at the end of a phrase (stop
bit). Both help the listener understand the message.
A stop bit has a value of 1 - or a mark state - and it can be detected correctly even if the previous data bit also
had a value of 1. This is accomplished by the stop bit's duration. Stop bits can be 1, 1.5, or 2 bit periods in
length. The MC uses the default – and most common – length of 1 period for the stop bit.
A device used to
communicate with the MC must also have its serial port set to use a stop bit of 1 in order for data
communications to work properly.
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. There are several defined parity selections
available for serial communications. They are even parity, odd parity, mark parity, space parity or none at all
can be used.
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. Mark parity means that the parity bit is always set to the mark signal condition and likewise space parity
always sends the parity bit in the space signal condition. Since these two parity options serve no useful purpose
whatsoever, they are almost never used.
The MC offers parity settings of Even, Odd and None.
In order for a device to communicate with the MC, it
must have its serial port set to use the same parity setting in order for data communications to work properly.
The MC is provided with a default parity setting of Even in order to be compatible with Future Design Controls’
Envision software.