Seagate Fibre Channel Interface - Manual (Page 28)

14 Fibre Channel Interface Manual, Rev. D
• If the number of ones and zeros in a sub-block are equal, running disparity is neutral and the value of run-
ning disparity at the end of the sub-block remains the same as the preceding character even if it is separated
by neutral characters.
Note. The rules of running disparity prohibit consecutive positive or consecutive negative characters even
if they are separated by neutral disparity characters. In other words, the negative and positive dis-
parity characters must alternate, even if separated by a neutral disparity character. See Table 2.
5.2 Buffer-to-buffer data transfers
Fibre Channel devices transfer information from an output buffer in the transmitting node to an input buffer of
the receiving node. This is called a buffer-to-buffer transfer. Each node may have from 1 to n buffers. The num-
ber of buffers in each node does not have to be equal. Each buffer is the size a frame may transfer in its pay-
load. The Fibre Channel standard does not define the actual length of the buffer or the method used to store
the bytes in the buffer. Figure 5 shows how data is sent from the transmit buffer and received by the receive
buffer.
Figure 5. Serialization process
The bytes being transmitted are sent in increasing displacement. 1
The basic unit of transfer for the contents of a buffer-to-buffer data transfer is the frame.
Table 2: Running disparity examples
Sub-blocks
Valid? 1 2 3 4 5
+ N – + – Yes
+ N + No
+ + No
– – No
1 This means that if byte 0 is sent first, bytes 1, 2, 3, and 4 are then transmitted in that order. If byte 100 is sent first, bytes
101, 102, 103, and 104 are then transmitted in that order.
Transmit
Buffer
8-bit
byte 8-bit
byte 8-bit
byte
8-bit character 10-bit character
Serial transfer
8-bit
byte
8B/10B
Encoder
Serializer
Receive
Buffer
8-bit
byte 8-bit
byte 8-bit
byte
8-bit character
8-bit
byte
10-bit character
8B/10B
Decoder
Deserializer