参数资料
型号: SCD223110QCD
厂商: INTEL CORP
元件分类: 微控制器/微处理器
英文描述: 2 CHANNEL(S), 256K bps, SERIAL COMM CONTROLLER, PQFP100
封装: METRIC, QFP-100
文件页数: 145/178页
文件大小: 2247K
代理商: SCD223110QCD
Intelligent Two-Channel LAN and WAN Communications Controller — CD2231
Datasheet
69
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 Figure 15.
Using the bit definitions from the standard format (Figure 16), the data bits are identified as D1–
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.
6.2.2
Frame Format
The standard frame format is as follows:
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.
Figure 15. Character Format
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.
START
D1
D2
D3
D4
D5
D6
D7
D8
STOP
The Start bit of the next
character could begin no earlier
than immediately after the
previous Stop bit.
Figure 16. Point-to-Point Protocol Frame
Interframe idle time fill
or next address.
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.
FLAG
A-FIELD C-FIELD FRAME DATA CHARACTERS
FCS-1
FCS-2
FLAG
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