CD2481 Datasheet, PDF (81/222 Pages) Intel Corporation – Programmable Four-Channel Communications Controller

Programmable Four-Channel Communications Controller — CD2481
7.3
7.3.1
Option
RxChk
{COR3}
RTPR
Description
If RxChk is set, the receiver tests the FCS at the end of each frame, and reports the
result.
If RxChk is clear, the receiver makes no FCS computation.
The RTPR timer is disabled when all bits are 0. RTPR is enabled with a non-zero value.
See Section 4-5.
SLIP Processing
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 CD2481 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.
Defined Character
END
ESC
ESC_END
ESC_ESC
Hex Encoding
0xC0
0xDB
0xDC
0xDD
The CD2481 uses the following conventions when transmitting a SLIP frame:
• 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’.
During receipt of a frame, the CD2481 makes the following substitutions:
• 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:
— The sequence ‘ESC’, ‘ESC_END’ is replaced with ‘END’.
— The sequence ‘ESC’, ‘ESC_ESC’ is replaced with ‘ESC’.
• 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.
The SLIP protocol prohibits in-band flow control. As such, the CD2481 does not respond to XON
and XOFF characters in any special way, they are treated as normal data.
Datasheet
81