VITESSE
SEMICONDUCTOR CORPORATION
Data Sheet
High Performance Serial
Backplane Transceiver
VSC870
Figure 14: Back Plane Interconnect Using VSC870 Transceivers
Port Card
Memory
System
Trans
Trans
VSC870
Parallel
VSC870
Switch
Fabric
Port Card
Memory
System
Trans
Trans
VSC870
VSC870
4.2 Data Encoding Format
When the BYPASS signal is HIGH, the RXTYP[1:0] and TXTYP[1:0] signals are the direct representation of the
overhead bits (B[1:0]) in the serial channels. For data words, the user can use these bits for signaling to the receiving
port card. Information such as start of frame and end of frame can be passed with the data in this manner. Scrambling
must be used in all cases. At the serial interface, the data words are scrambled to increase the signal edge transition
density by setting the SCRAM input HIGH. When scrambling is enabled, a command word must be used to initialize
the scrambling sequence. The transceiver checks for bit patterns on IDLE words to detect a link error condition. The
encoding for the data at the parallel interface and the two overhead bits at the serial interface are described in the
following table. The format for data words and command words is described in section 1.2.
TXTYP[1:0]RX
TYP[1:0]
Direct Mode
CELLSYN = 0
B[1:0]
SCRAM = 1
SCRAM = 0
Error Check
0 0
0 1
1 0
1 1
0 0
0 1
1 0
1 1
Command Word
Data Word
No Scramble
Scramble
Scramble
Scramble
No Scramble
No Scramble
No Scramble
No Scramble
IDLE words
No
No
No
Data Word
Data Word
4.3 Loop Timing
Loop timing depends on the source of the transmitters bit clock. For the case of using the transceiver in a simple
port to port switching system, one transceiver can act as the master and one as the slave. The master will have LTIME
set LOW so that its CMU will generate the bit timing. The slave will have LTIME set HIGH so that its transmitter
uses the bit clock recovered from its clock recovery unit, therefore the whole system is in a single clock domain.
For the case of using the transceivers as a backplane interconnect link, all transceivers on the switch card have
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Page 30
G52190-0, Rev 4.1
01/05/01
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