CD2231 Intel Corporation, CD2231 Datasheet - Page 70

• Manufacturer Part Number: CD2231
• Description: CD2231 Intelligent Two-channel Lan And Wan Communications Controller
• Manufacturer: Intel Corporation

CD2231 — Intelligent Two-Channel LAN and WAN Communications Controller
6.2.4
6.2.4.1
6.2.4.2
6.2.4.3
6.2.4.4
70
Transparency
Transparency means that there is a protocol method to prevent confusion and ambiguity between
control characters and data characters in the frame.
For 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 control-escape character of 7D. After the control-escape, the character itself is transmitted
with bit 6 inverted. For example, if the character 13 is a mapped character, 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 mode, 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 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.
When the bit is clear, that character is not a mapped character.
For example, suppose 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, if 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 can 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.
Mapped Characters in the FCS Field
Whenever the transmitter sees that the FCS result to be transmitted contains a mapped character, it
handles that character as any other mapped character. See
For example, if an FCS field is A7–7E, the transmitter would send three characters in the FCS
field, A7–7D–5E. The receiver would convert the received FCS back to A7–7E before completing
the FCS computation.
Section
5.2.4.
Datasheet