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Serial String of Variable Speed Devices Disclosure Number: IPCOM000118007D
Original Publication Date: 1996-Aug-01
Included in the Prior Art Database: 2005-Mar-31
Document File: 4 page(s) / 111K

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Truestedt, HL: AUTHOR


Disclosed is a method which allows variable speed devices to exist on a serial string interface.

This text was extracted from an ASCII text file.
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This is the abbreviated version, containing approximately 52% of the total text.

Serial String of Variable Speed Devices

      Disclosed is a method which allows variable speed devices to
exist on a serial string interface.

      When using a serial string interface (e.g., Fibre Channel
Arbitrated Loop, FC-AL), it may be desirable to mix high-speed and
low-speed devices on the same string.  The current architecture of
FC-AL requires that only homogeneous speed devices be attached to the



      For simplicity, the language of the FC-AL is used in the
following discussion.  Although the description covers a serial
copper Loop, (i.e., the readily available bypass circuits are copper
only at this time), it is extendable to optical Loops as well.

      It is assumed that during Loop Initialization (discovery of
other L_Ports' operational characteristics), each L_Port can describe
the frequencies at which it can operate.  If all L_Ports are able to
run at the lowest speed, the Loop manager (i.e., Small Computer
System Interface (SCSI) controller) may keep the Loop at its slowest
rate except for data transfers to a high-speed L_Port.

      Bypass circuits are not shown in the following figures, but
are assumed to be present to allow the transmitter of an L_Port to be
bypassed.  When an L_Port is bypassed, the transmitter output is not
place on the Loop, however, the receiver is still able to "see"
everything on the Loop.

      It is expected that all frequency shifts occur during Loop idle
periods (i.e., control or data frames are not being transmitted) to
avoid possible frame corruptions.  It is also assumed that a receiver
can tell the difference between the six Fibre Channel frequencies
(i.e., 133, 266, 531, 1062.5, 2125, and 4250 Megabits/sec.).  If this
is not the case, a new primitive signal could be used to identify the
speed of the Loop.  The signal would always be propagated before a
frequency shift such that an L_Port would know to bypass itself since
it cannot operate at that frequency.

      Model 1 - all L_Ports can operate at lower speed

      Fig. 1 shows four L_Ports (FC-AL ports) attached in a Loop
where some of the L_Ports are able to run at a slow data rate only
(e.g., L_Ports B and C) and others are able to run at both a slow
data rate and a high data rate (e.g., L_Ports A and D).

To allow the above four L_Ports to operate on the Loop as shown in
Fig. 1, the following conditions need to be met:
  1.  The transmitters and receivers of the Loop are always
       operating at the lowest data rate when data frames are
       not being transferred.  This allows all L_Ports to arbitrate
       for access to the Loop.  Only when data frames are being
       transferred between L_Ports that support the higher
       frequency, is...