CD2231

Manufacturer Part NumberCD2231
DescriptionCD2231 Intelligent Two-channel Lan And Wan Communications Controller
ManufacturerIntel Corporation
CD2231 datasheet
 


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Intelligent Two-Channel LAN and WAN Communications Controller — CD2231
6.2
PPP (Point-to-Point Protocol) Mode
6.2.1
Character Format
The PPP mode uses the async-HDLC character format, which is fixed as one start bit, eight data
bits, and one stop bit. There is no parity bit. The character format is as shown in
Using the bit definitions from the standard format
D8. D1 is the LSB. Characters are identified as either bits (D1–D8) or as hexadecimal values
showing the hex value for bits D5–D8 first, followed by the hex value for D1–D4. Thus, a flag
character is ‘01111110’, and is indicated as a hex 7E. A control-escape character is ‘10111110’ or
7D.
Figure 15. Character Format
START
D1
D2
The line is high prior to the Start
bit. The high value is either due
to a line idle or the Stop bit from
the previous character.
6.2.2
Frame Format
The standard frame format is as follows:
Figure 16. Point-to-Point Protocol Frame
FLAG
A-FIELD C-FIELD FRAME DATA CHARACTERS
The closing flag (7E) of the previous frame can be the same flag
used as the opening of the next frame. This is a shared flag.
A and C fields
The device passes the A and C fields to and from the host. The device does no special processing
on these fields.
6.2.3
Frame Check Sequence
The PPP mode uses the same 16-bit CRC as HDLC mode (V.41).
Everything between the flags is included in the calculation with two exceptions: control-escape
(7D) characters added for transparency, and mapped characters received without a preceding
control-escape. For characters preceded by a control-escape, the FCS calculation is made after bit 6
is inverted.
Datasheet
(Figure
16), the data bits are identified as D1–
D3
D4
D5
D6
D7
FCS-1
FCS-2
Figure
15.
D8
STOP
The Start bit of the next
character could begin no earlier
than immediately after the
previous Stop bit.
Interframe idle time fill
FLAG
or next address.
69