Product Manual

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Section 7. Data Communications
Introduction to data communications:
In the PC-400, data is sent to a printer or computer by using “asynchronous
serial data communications.” Data is broken up and sent one piece at a time to
the printer or computer. In spite of this apparent simplicity, a basic
understanding of serial data communications is needed when setting up the PC-
400.
The PC-400 transmits letters and numbers 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 PC-400 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
• knowing when data starts and stops
• the ability of the receiving equipment to digest the data sent
The transmission rate determines how fast the data is sent from the scale and is
measured in Baud or bits per second. (For applications such as the PC-400,
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 term “Asynchronous Serial Data 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 setting to determine how fast data should be
sent. 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. When this happens, 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.
The PC-400 and the equipment connected to it resolve this dilemma by sending
a “start bit” at the beginning of each data byte. This bit tells the printer or
computer that a new data byte is on the way. When the start bit is received, the
bit timer starts running and runs until it has received the correct number of bits.