Transparentization (Transparent XON/XOFF)

Transparent XON/XOFF Flow Control scans the data stream for XON ($11), XOFF ($13), and DLE ($10) characters, with either 0 or 1 in the high-order bit position. When one is found, a DLE with same parity character is inserted ahead of it. The original character is then made transparent by exclusive-ORing the character with the value $21 (hexadecimal 21, decimal 33, binary 00100001), which preserves the parity bit value. XON and XOFF characters are thus hidden in the data stream.
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