SFF-8636 -- Common Management Interface

Olawsky, Barry Barry.Olawsky at hp.com
Tue May 25 08:45:49 PDT 2010

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The burden on the system integrators such as HP was considered. Low port
count designs (for example, low profile adapter cards with two 4x HD ports)
pay a small penalty in having to share or switch a two-wire interface between
two cables. High port count devices such as switches have a higher burden and
may require more additional external logic to share a single two-wire
interface. However, attempting to troubleshoot a locked two-wire BUS in the
field may require disconnecting every cable connected to the ports sharing
the common two-wire bus. This solution is unacceptable for many customers. In
HP's opinion, the added system cost and design complexity is worth the
improved serviceability.
With regards to your second question, there is nothing preventing system
integrators from connecting the interrupt pins from all managed cable ports
together. However, a fair amount of effort is required to identify the
source(s) of interrupt for a single managed port. Busing them together forces
system side to service all ports at once. Given the relatively slow two-wire
bus data rate, a common interrupt signal appears to be an excessive burden on
the firmware.
Barry Olawsky
Hewlett Packard
From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of Ballard,
Curtis C (StorageWorks)
Sent: Friday, May 21, 2010 9:57 AM
To: Shah, Amit M; t10 at t10.org
Subject: RE: SFF-8636 -- Common Management Interface
I had the same question on point 1 and talked to one of the members working
on that specification.	The response I received was that with the multi-drop
interface it was possible for one device to completely lock up the bus and
prevent access by any other devices if that device didn't play nicely or had
a fault.  Having worked with I2C in the past and seen that behavior I can
appreciate that concern but it would be nice to have multi-drop capabilities
so devices with a lot of external ports don't have to have multiple
management interfaces or a MUX.
Curtis Ballard
Hewlett Packard
StorageWorks Platforms Tape
Fort Collins, CO
(970) 898-3013
From: owner-t10 at t10.org [mailto:owner-t10 at t10.org] On Behalf Of Shah, Amit M
Sent: Friday, May 21, 2010 12:29 AM
To: t10 at t10.org
Cc: Shah, Amit M
Subject: SFF-8636 -- Common Management Interface
I had couple of question about the proposal "SFF-8636" related to Common
Management Interface.
1.	  This proposal talks about 2 wire management protocol which is sort
of similar to I2C. The Slave Addr is fixed at 50h. This means that the
interface cannot be used as a multi-drop interface and each cable will need
its own individual interface. So  the question is that why a multi-drop
interface was not considered for common management interface?
2.	 The Spec also talks Interrupt. Also SFF-8449 talks about IntL. Can
this Interrupt be shared across multiple interfaces?
Will appreciate your response.
Amit Shah

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