Seagate Fibre Channel Interface - Manual (Page 47)

Fibre Channel Interface Manual, Rev. D 33
8.0 FC Arbitrated Loop concepts
This section describes some basic Fibre Channel Arbitrated Loop (FC-AL) concepts. Seagate disc drives sup-
port FC-AL as the topology for connectivity in Fibre Channel environments.
Fibre Channel is a serial data channel that provides logical point-to-point service to two communicating
devices. With FC-AL, you can have a maximum of one point-to-point circuit at any one time. When this circuit
is active, only two L_Ports are communicating, but up to 127 devices may be attached to and participating on
the same loop. All of the L_Ports that are on the loop but that are not one of the two communicating L_Ports
“see” all data transferred across the loop and retransmit this information so that the data reaches its intended
destination.
8.1 Arbitrated Loop physical address (AL_PA)
Each device communicating on an arbitrated loop must have an Arbitrated Loop Physical Address (AL_PA).
The AL_PA is an 8-bit (1-byte) 8B/10B encoded value that is a valid data character. This 8-bit character, when
encoded to 10 bits, must have an equal number of 1’s and 0’s in the address to maintain neutral running dis-
parity. Neutral running disparity is required so that the AL_PA data character does not change the current run-
ning disparity of the current transmission word. There are 134 characters that result in neutral disparity. See
Table 12. Seven of the 134 neutral disparity characters are reserved (see Table 13 on page 35). This leaves
127 valid addresses.
Table 12: 8B/10B characters with neutral disparity
D xx.y y
Hex value xx 0 1 2 3 4 5 6 7
00
* * * 00, 80, E0
01
* * * 01, 81, E1
02
* * * 02, 82, E2
03
* * * * * 23, 43, 63, A3, C3
04
* * * 04, 84, E4
05
* * * * * 25, 45, 65, A5, C5
06
* * * * * 26, 46, 66, A6, C6
07
* * * * * 27, 47, 67, A7, C7
08
* * * 08, 88, E8
09
* * * * * 29, 49, 69, A9, C9