AM79C864AKCW AMD [Advanced Micro Devices], AM79C864AKCW Datasheet - Page 40

• Manufacturer Part Number: AM79C864AKCW
• Description: Physical Layer Controller With Scrambler (PLC-S)
• Manufacturer: AMD [Advanced Micro Devices]

Using the Stream Cipher Scrambler
The system has access to the scrambler and
descrambler through a pin and a register. Pin 41
(SCRM)
PLC_CNTRL_C enable the scrambler. SCRM and
Repeat Filter
The main function of the Repeat Filter is to prevent the
propagation of code violations and invalid line states
from the inbound link to the outbound link. This function
is not required if a MAC layer is present in the station
configuration (since MAC will not propagate invalid line
states and code violations). But certain station configu-
rations consist of only PHY layer entities (such as the
path of the secondary ring inside a station with just one
MAC in the primary ring). In such cases, a PHY layer
implementation is expected to provide the Repeat Filter
function.
3-42
AMD
SCRM
HIGH
HIGH
LOW
LOW
Table 18. Stream Cipher Enable
and
S_Data
bit
SCRM_RSYNC
Cipher Enable
5
0
HIGH
HIGH
LOW
LOW
5
(CIPHER_ENABLE)
The SUPERNET 2 Family for FDDI 1994 Data Book
Deduced
Line State
Figure 5. Stream Cipher Descrambler
5
Scrambler
Disabled
Enabled
Enabled
Enabled
P R E L I M I N A R Y
+
–
+
–
Synchronizer
of
State
5
5
CIPHER_ENABLE are ORed together so either one
HIGH overrides the other (Table 18). Pin 41 is a ground
pin on previous versions of the PLC; therefore if the
PLC-S is to be used in the fiber mode, it can be inserted
in a previously manufactured board without any
changes.
The Repeat Filter in the PLC-S filters the symbol stream
at the output of the Remote Loopback MUX. Invalid line
states are not allowed to propagate through a station;
they will be turned into an Idle symbol stream. Also, if the
repeat filter detects a corrupted frame, it truncates the
frame by transmitting four Halt symbols and then Idle
symbols until a JK symbol pair is seen. The Halt symbols
will cause the next MAC entity in the logical ring to count
the frame as a lost frame.
Another function of the Repeat Filter is called the
GOBBLE_BYTE function.
11
5
Generator
Random
10
5
Capture
Figure 4. Stream Cipher Scrambler
Data
Data/D_Data
9
8
5
15535B-17
7
6
5
When the Repeat Filter
4
3
2
15535B-16
1
S_Data