CD2231 Intel Corporation, CD2231 Datasheet - Page 73

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

6.3.1
6.3.2
6.4
Datasheet
Note: SLIP, MNP4, and Automatic In-Band Flow Control modes are only available on Revision B and
Framing
As defined in the original implementation, SLIP frames end with an ‘END’ character and have no
beginning character. However, RFC-1055 suggests that all frames begin and end with ‘END’
characters. The CD2231 uses the ‘END’ character essentially as opening and closing flags. The
defined characters (see table below) are fixed (hardcoded) and cannot be changed by the user.
The CD2231 uses the following conventions when transmitting a SLIP frame:
During receipt of a frame, the CD2231 makes the following substitutions:
The SLIP protocol prohibits in-band flow control. As such, the CD2231 does not respond to XON
and XOFF characters in any special way, they are treated as normal data.
Debugging Aids
For debug purposes, the CD2231 can send the sequence ‘ESC’, ‘END’, by the STCR (Special
Transmit Command register). This is intended as an abort frame function. The STCR also has a
command for sending a bad (0 value) Stop bit, which causes a framing error at the receiving end.
When the CD2231 receives the sequence ‘ESC’, ‘END’, it is reported as ‘receive abort’ in the
RISR register. A bad Stop bit is reported as a FE (framing error) in the RISR.
MNP 4/ARAP Protocol Processing
later devices.
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When an ‘END’ character is to be sent, it is replaced by the character sequence ‘ESC’,
‘ESC_END’.
When an ‘ESC’ character is to be sent, it is replaced by the character sequence ‘ESC’,
‘ESC_ESC’.
When an ‘ESC’ character is found in the data stream, only the ‘ESC_END’ and ‘ESC_ESC’
characters can follow. These two character sequences are replaced with a single character:
Even though the characters ‘ESC_END’ and ‘ESC_ESC’ are the only valid characters
following ‘ESC’, RFC-1055 suggests that when other characters are encountered, the ‘ESC’
should be discarded and the second character should be kept unmodified. The CD2231 follows
this convention.
Defined Character
— The sequence ‘ESC’, ‘ESC_END’ is replaced with ‘END’.
— The sequence ‘ESC’, ‘ESC_ESC’ is replaced with ‘ESC’.
Intelligent Two-Channel LAN and WAN Communications Controller — CD2231
ESC_END
ESC_ESC
END
ESC
Hex Encoding
0xDC
0xDD
0xC0
0xDB
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