Motorola MPC823e Reference Manual page 725

16.9.19.4 TRANSMITTER TRANSPARENCY ENCODING. The SCCx ASYNC HDLC
controller maps characters according to the RFC 1549 standard. It examines the outgoing
data bytes and performs a transparency algorithm on a given byte if one of the following
conditions apply:
• The byte is a flag (0x7E-PPP, 0xC0/0xC1-IrLAP)
• The byte is a control-escape character (0x7D)
• The byte has a value between 0x00 and 0x1F and the corresponding bit in the
TX control character table is set
When a condition applies, a two-byte sequence is transmitted in place of the byte. The
sequence consists of the control-escape character (0x7D) followed by the original byte
exclusive-OR'ed with 0x20.
16.9.19.5 RECEIVER TRANSPARENCY DECODING. The SCCx ASYNC HDLC
controller maps characters according to the RFC 1549. To recover the original data, it
examines the incoming data bytes and performs the transparency algorithm in the following
ways:
• Discards any character that has its corresponding bit set in the RX control character
map. This character is assumed to have been inserted in the character stream by an
intermediate device and is not part of the originally transmitted frame.
• Reverses the transmission transparency sequence by discarding a received
control-escape character (0x7D) and exclusive-OR'ing the following byte with 0x20
before performing the CRC calculation and writing the byte into memory.
MOTOROLA MPC823e REFERENCE MANUAL
Communication Processor Module
16-271