CD2481 Datasheet, PDF (78/222 Pages) Intel Corporation – Programmable Four-Channel Communications Controller

CD2481 — Programmable Four-Channel Communications Controller
7.2.4
7.2.4.1
7.2.4.2
7.2.4.3
Everything between the flags is included in the CRC calculation with two exceptions: control-
escape (7D) characters added for transparency, and mapped characters received without a
preceding control-escape. For mapped characters preceded by a control-escape, the FCS
calculation is made after bit 6 is inverted.
Transparency
Transparency means that there is a protocol method to prevent confusion and ambiguity between
control characters and data characters in the frame.
In PPP mode, there is a control-escape mechanism. Specific characters are identified as ‘control
mapped’ characters. The control map is called the ACCM (async-control-character map).
Whenever there is a mapped character in the data stream, the transmitter precedes that character
with a hex 7D control-escape character. After the control-escape character, the character itself is
transmitted with bit 6 inverted.
For example, if the character 13 is a mapped character, as identified by the corresponding ACCM
bit, then the transmission of 13 is 7D–33.
When the receiver sees the 7D control-escape character, the 7D is removed, and bit 6 of the
following character is inverted. The resultant reconstructed character is passed on to the host as one
received character.
Mapped Characters from 00–1F
When a channel is selected for PPP, two ACCMs are assigned. Each ACCM consists of four
registers (32 bits) to define mapped characters in the range 00–1F. One ACCM is for the
transmitter (TXACCM), and one ACCM is for the receiver (RXACCM). Each bit within the
ACCM points to a particular character within the range. When the bit is set, that character is a
mapped character. Conversely, if the bit is clear, that character is not a mapped character.
As an example, suppose that the TXACCM bit pointing to the character 12 is set, and that the
TXACCM bit pointing to the character 0B is clear. Then whenever a 12 is present for transmission,
the actual transmission is 7D–32. Whenever a 0B is present for transmission, the 0B is transmitted
without modification.
Continuing the example, suppose that the receiver ACCM bits pointing to 12 and 0B are also set
and clear, respectively. Then, a received 12 (without a preceding 7D) is discarded, and a received
0B (without a preceding 7D) is passed through to the host unchanged.
Mapped Characters from 20 and Above
Three characters above 20 may be mapped. These characters are defined in the channel specific
registers TSPMAP[1], TSPMAP[2], and TSPMAP[3].
Characters 7D and 7E as Transmitted Data
Whenever the transmitter sees either 7D or 7E as data for transmission, the transmitter treats these
as mapped characters. Thus, a 7D as data is transmitted as 7D–5D, and a 7E as data is transmitted
as 7D–5E.
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Datasheet