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16 Fibre Channel Interface Manual, Rev. D
Idle (IDLE)
An Idle is transmitted on the loop to indicate the node is operational and ready for frame transmission and
reception. Idles are transmitted when frames, R_RDY, or primitive sequences are not being transmitted. This
maintains word synchronization and minimum spacing between frames.
Receiver ready (R_RDY)
R_RDY indicates that a frame was received and that the interface buffer that received the frame is ready for
another frame. R_RDY is preceded and followed by a minimum of two Idles. R_RDY establishes buffer-to-
buffer credit during data transmissions between an initiator and target.
5.4.1.1 Primitive signals used as frame delimiters
Primitive signals can also be frame delimiters. A frame is an indivisible information unit that may contain data to
record on disc or control information such as a SCSI command.
Note. All ordered sets (except for the End-of-frame delimiter, EOF) require the running disparity from the
previous word to be negative. The second character of an ordered set (except EOF) will be positive
and the third and fourth characters are neutral.
Frame delimiters mark the beginning and end of frames. There are several frame delimiters available in Fibre
Channel. There are nine Start-of-frame (SOF) delimiters and six End-of-frame (EOF) delimiters as listed in
Table 4.
Start-of-frame (SOF) delimiters:
• mark the beginning of a frame,
• indicate whether this is the first frame of the sequence, and
• indicate the class of service for the frame.
End-of-frame (EOF) delimiters:
• mark the end of a frame, and
• indicate whether this frame is the last frame of the sequence.
The second character of EOF delimiters differentiates between normal and invalid frames. The EOF delimiter
also ensures that negative running disparity results after processing the set by assigning the appropriate sec-
ond character.
The third and fourth characters of the delimiter functions (SOF and EOF) are repeated to ensure that an error
affecting a single character will not result in the recognition of an ordered set other than the one transmitted.
See Table 4 below.
Table 3: Primitive signals
Primitive signal Signal
Beginning
running
disparity Ordered set (FC-1) Ordered set (hex)
Idle IDLE Negative K28.5 D21.4 D21.5 D21.5 BC 95 B5 B5
Receiver_Ready R_RDY Negative K28.5 D21.4 D10.2 D10.2 BC 95 4A 4A