Dynapro Technologies Dynapro ET TCS, Руководство разработчика приложений

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NOTE
In Figure 2-1, indicates a space (logical zero) and indicates a mark (logical one).
However, on the RS-232 line itself these are inverted, so a mark is a low voltage and a space is a high
voltage. On the RS-485 line, the Data + signal appears as shown in the above figure.
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The start bit tells the receiving station that the next seven bits (or eight, depending on how the
Data Bits parameter is set) represent an ASCII code, transmitted with the least significant bit
first. The start bit is always a single zero bit (a space).
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The stop bit tells the receiving station that it has received the end of the code. The stop bit is
always a one bit (a mark), which is the idle state of the line. A stop bit can be immediately
followed by the start bit preceding the next code, or it can just return the line to its idle state. The
latter case, for example, occurs when the stop bit follows the last code to be transmitted.
To meet the requirements of various host computers and interfaces, the TCS can use either one or
two stop bits. The number of stop bits is selected by the user through the Setup Screen.
Instructions for selecting the number of stop bits are provided in the Installation Guide, under the
heading, Using the Setup Screen.
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The parity bit follows the seven or eight data bits. The parity bit is used to check for transmission
errors. Parity checking can use either even or odd parity.
Even parity means that the parity bit and the data bits together contain an even number of binary
ones. For example, for the data bits 1010100, the parity bit is 1, making the total number of ones
even (in this example there are a total of four ones).
Odd parity means that the parity and data bits together contain an odd number of ones. For
example, for the data bits 1010100, the parity bit is 0, making the total number of ones odd (in
this example there is a total of three ones).
By setting two TCS parameters (Parity Enable and Parity Sense), the parity bit can be used in
five ways, as shown in Table 2-2. Instructions for setting the Parity Enable and Parity Sense
parameters are provided in the Installation Guide, under Using the Setup Screen.