am79c30a Advanced Micro Devices, am79c30a Datasheet - Page 37

• Manufacturer Part Number: am79c30a
• Description: Digital Subscriber Controller ?dsc ?circuit
• Manufacturer: Advanced Micro Devices

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-
2. 2-byte address signified by the LSB of the first ad-
3. More than 2-byte address signified by the LSB of
OCTET
OCTET
Notes:
EA = Address Field Extension bit
SAPI = Service Access Point Identifier
FCS = Frame Check Sequence
dress byte being set to a logical 1
dress byte being set to a logical 0, and the LSB of
the second address byte being set to a logical 1
both the first and second address bytes being set to
a logical 0
01111110
FLAG
1
01111110
FLAG
1
EA =0 C/R
EA =1
ADDRESS
1
16 bits
2,3
2
ADDRESS
16 bits
Figure 4. Level-2 Frame Structure Formats
2,3
3
CONTROL
4
Am79C30A/32A Data Sheet
8 bits
4
TEI
SAPI
CONTROL
5
8 bits
4
6
INFORMATION
7
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
N bits
C/R = Command/Response Field bit
TEI = Terminal Endpoint Identifier
5 …
8
16 bits
FCS
5,6
OCTET 2
OCTET 3
16 bits
N – 1
FCS
01111110
FLAG
7
01111110
FLAG
N
Minimum Packet
09893H-4
General
37