Section 6. Data Communications
Introduction to data communications:
In both the Model 7000XL and the Model 7000, data is sent to a printer or
computer using "asynchronous serial communications." While the name is
quite impressive, the actual concept is fairly simple. Data is broken up
and sent one piece at a time to a printer or computer. In spite of this
simplicity, a basic understanding of serial data communications is helpful
when setting up these devices.
The 7000XL and 7000 transmit letters and numbers to a printer or
computer by replacing the letter (or number) with an eight bit ASCII code.
This code is then transmitted, one bit at a time. A bit is the smallest unit
of data and can have a value of "1" or "0." By combining eight bits into a
byte, it is possible to get 256 unique bit patterns. These patterns are used
to create the ASCII codes used by the 7000XL and the 7000 to represent
letters and numbers.
When setting up a serial communications system, there are several
concerns which affect the configuration of that system. These are:
transmission rate (Baud Rate);
knowing when data starts and stops;
detecting noise on the data.;
knowing if the receiving equipment is ready for more data.
The transmission rate determines how fast data is sent from the scale to
the printer (or computer) and is measured in Baud or bits per second.
(For applications such as the 7000XL or 7000, Baud and bits per second
are interchangeable.) The transmission rate controls how many bits can
be sent in a given time. It is important that the sending and receiving
units are set to the same Baud settings. Typical values are 1200, 2400,
4800 and 9600 baud. The higher the Baud rate the faster the data is
sent.
The term "asynchronous serial communications" implies that the sending
unit has no way of telling the receiving unit when a data bit has been sent
or when to expect the next bit. To correct this problem, both the sending
and receiving units use the baud rate settings to determine how fast to
send data. If the baud rates at the sending and receiving units differ, the
receiving unit will expect data to arrive at a different time than when the
transmitting unit sent it. As a result, the data will be lost. When the baud
rates match, the receiving unit has no problem with the data arriving early
or late. The only problem is knowing when the data transmission started.
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