AM79C30AVC datasheet(37/101 Pages) AMD

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Am79C30A/32A Data Sheet

that a flag sequence is not repeated within the flag

boundaries of the frame, a logical 0 bit is automatically

inserted after each sequence of five contiguous logical

1s. When receiving, the DLC examines the frame con-

tent between the opening and closing flags and auto-

matically discards the first logical 0 which directly

follows five contiguous logical 1s.

D-Channel Address Recognition

The address field, shown in Figure 4, allows for three

types of addresses:

1. 1-byte address signified by the LSB of the first ad-

dress byte being set to a logical 1

2. 2-byte address signified by the LSB of the first ad-

dress byte being set to a logical 0, and the LSB of

the second address byte being set to a logical 1

3. More than 2-byte address signified by the LSB of

both the first and second address bytes being set to

a logical 0

In the case of the LAPD operating environments, the

address is a 2-byte address where the first byte is anal-

ogous to the Service Access Point Identifier (SAPI) and

the second byte is analogous to the Terminal Endpoint

Identifier (TEI) as defined by the CCITT recommenda-

tions.

The DLC is able to recognize D-channel addresses of

all of the three types outlined above. Note that only the

first two bytes of a more than 2-byte address can be

checked by the DLC. There are four First Received

Byte Address Registers (FRARs) which hold the values

used to match against the first byte of the incoming ad-

dress. Similarly, there are four Second Received Byte

Address Registers (SRARs) which hold the values

used to match against the second byte of the incoming

address.

FRAR4 defaults to FE hex; SRAR4 defaults to FF hex.

This default is analogous to the broadcast address de-

fined by the CCITT recommendations. The type of ad-

dress recognition which is enabled is shown in Table 38

Figure 4.

Level-2 Frame Structure Formats

OCTET 2

OCTET 3

SAPI

TEI

EA =1

EA =0 C/R

FLAG

ADDRESS

CONTROL

FCS

FLAG

01111110

16 bits

8 bits

16 bits

01111110

12,3

5,6

OCTET

FLAG

01111110

ADDRESS

16 bits

2,3

CONTROL

8 bits

INFORMATION

N bits

5 …

FCS

16 bits

N – 1

FLAG

01111110

OCTET

Minimum Packet

General

Notes:

EA = Address Field Extension bit

C/R = Command/Response Field bit

SAPI = Service Access Point Identifier

TEI = Terminal Endpoint Identifier

FCS = Frame Check Sequence

09893H-4

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