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DACS: Framing
 
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Framing

Before a raw data stream may be sent, it must be framed. A stream is of variable length, and may contain full 8-bit data. No error detection or correction is employed at this level of the protocol, however simple CRC (Cyclic Redundancy Check) calculation could be added in at a later date. Three bytes are defined to facilitate framing the data. Most importantly, the data link escape (DLE) character, defined as 0x10, tells the serial receiver that the next character received has special meaning and is not simply data. The two framing characters STX and ETX (Start Transmission and End Transmission, respectively) are used to signify the beginning and end of a frame when preceded by a DLE. STX and ETX are defined as 0x02 and 0x03 respectively. Figure 102 depicts a raw data stream properly framed by DLE, STX and ETX.

Figure 102: Serial protocol, 'clean' data framed for transmission.
\begin{figure}
\psfig{file=fw/serproto-1.eps,width=6in}
\end{figure}

Figure 102 shows a raw data stream with no embedded DLE bytes. However, it is certainly possible that the raw data stream may contain one or many DLE bytes. In this case, the sender must character stuff, or escape the embedded DLE characters. It simply doubles them up as it comes across them. The receiver merely strips the extra DLE off upon receipt of the frame. Figure 103 shows an example of a character stuffed frame.

Figure 103: Serial protocol, 'unclean' data character stuffed and framed for transmission.
\begin{figure}
\psfig{file=fw/serproto-2.eps,width=6in}
\end{figure}


next up previous contents
Next: Firmware Modules Up: Serial Communications Protocol Previous: Physical Layer   Contents
Steve Richardson 2000-07-06
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