Dynapro Technologies Dynapro ET TCS, Руководство разработчика приложений

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2-10
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Because communication with the host computer occurs over a serial hardware link, noisy or
faulty connections can possibly corrupt the data stream and introduce errors. The TCS uses
techniques to minimize the possible effects of several types of communication errors. In
particular, to maintain communication integrity, the TCS monitors and reports the following
types of errors:
•
Framing Error
A Framing Error occurs when a code is received without a following stop bit. When a
framing error occurs, the single mnemonic character
FE
is displayed in place of the character
in error.
•
Buffer Overflow
A Buffer Overflow Error occurs when one or more codes have overflowed the TCS input
buffer and been lost. When a buffer overflow error occurs, the single mnemonic character
BO
is displayed on the screen instead of the codes that were lost.
•
Overrun Error
An Overrun Error occurs when a received code is overwritten by the next code before it can
be read from the Universal Asynchronous Receiver/Transmitter (UART) in the TCS. When
an overrun error occurs, the code that was overrun is lost, the single mnemonic character
OE
is displayed, and the code that overran the previous code is interpreted normally.
•
Parity Error
A Parity Error occurs when a code is received with an incorrect parity. When a parity error
occurs, the single mnemonic character
PE
is displayed in place of the code in error. (The TCS
checks for parity errors only when the user has enabled parity checking; see Parity Bit earlier
in this chapter.)
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To prevent the TCS and host from overflowing each other’s input buffers, the TCS supports two
methods of communication control: exercising the SRTS and CTS control lines and sending
XON (17 decimal) and XOFF (19 decimal) codes. The method used depends on whether the
XON/XOFF mode is set. For a complete description of XON/XOFF, refer to Chapter 4.
NOTE
Throughout this manual, the terms stall and unstall refer to the stopping and starting of data flow between
the TCS and host, using either the control lines or the XON and XOFF codes.
To prevent its own input buffer from overflowing, the TCS stalls the host. When the TCS is
ready to accept more data, it unstalls the host. Similarly, to prevent the host’s input buffer from
overflowing, the host stalls the TCS, and when ready to receive more data, the host unstalls the
TCS. The input and output buffers each hold a maximum of 636 codes.