Allied Data Technologies TRON SECURE 33.6, Руководство пользователя

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1 DATA COMMUNICATION
Computers can exchange (communicate) data (information) through a disk (floppy disk), through
the connection of two serial ports, or through a network.
With the use of a modem, information is exchanged through the serial (or RS232) port.
1.1 Data bits, parity bits and speed
The RS232 standard describes how serial information is processed (passed on). In the computer,
bits are processed simultaneously (parallel). Through the serial port, these bits are passed on one
by one. A couple of extra bits are sent along to assure that the receiving side can interpret them
correctly. These bits are: a starting bit, the so-called parity bit (for error detection), and one or two
stop bits. Together this forms a frame or character.
The two frames used most frequently for standard asynchronous communi-cation are built up as
follows:
1 start bit + 7 data bits + 1 parity bit + 1 stop bit or
1 start bit + 8 data bits + 1 stop bit
Beside the frame (character), the speed (baudrate) has to be set as well. The speed is expressed
in Bits per Second (bps) or baud. Although there is a differen-ce, these concepts are often used
alternately. However, the indication bps is always cor rect. Therefore, at V21 300 bps is the same
as 300 baud. However, at V22bis a 2400 bps connection is 600 baud. This is the case since per
modulation (per baud), 4 bits are transferred simultaneously.
Possible speeds are: 75, 300, 600, 1200, 2400, 4800, 7200, 9600, 12000, 14400, 16800, 21600, 24000,
26400, 28800, 31200 and 33600 bps.
When the data bits, parity bits, and speed have been set on both sides (sender and receiver), a
connection can be established. Information can then be exchanged.
1.2 ASCII and the meaning of data
For the interpretation of received data, with most programs the ASCII table is used as a basis, in
which each character has a unique bit combination. Sending text is relatively simple if both sides
stick to this ASCII table.
Control characters are also included in the ASCII table. These are characters that do not
represent characters or punctuation, but comman-ds that may be followed by a certain action.
An example of such a command is the ASCII character with the number 12 (Form Feed).
This command assures that the printer moves to the next page.
This way, we also have characters which are used in data communication to indicate the
beginning of a text block that we would like to send (char.2 STX: Start of text) or its end (char.3
ETX: End of Text). There are also characters, that inform the sender whether a text block was
received correctly or not. This is done by character 6 (ACK: acknowledge) and 21 (NAK: negati-
ve(ly) acknowledge).
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