Expander XL state machine question

Fred Dutton fred.dutton at qlogic.com
Thu Feb 24 10:08:31 PST 2005

* From the T10 Reflector (t10 at t10.org), posted by:
* Fred Dutton <fred.dutton at qlogic.com>

While in the XL7:Connected state, an expander PHY is required to send Transmit Dword messages to the XL 
transmitter to transmit all dwords received with Forward Dword indications.

What type of message should be sent to the XL transmitter when there are no Forward Dword indications? 
This can occur because devices may run at slightly different clock rates. So if an expander is running 
faster than an end device or any other device connected to it, it will, every now and then, end up with a 
"dword bubble" in the transfer and, hence, no Forward Dword indication.

If the XL state machine sends Transmit Idle Dword messages you run the risk of inserting an idle dword in 
the middle of a data frame currently being forwarded.

I see two options:

1) always transmit ALIGN primitives when there are no Forward Dword indications, or
2) if a data frame is currently being forwarded, transmit ALIGN primitives when there are no Forward Dword 
indications, otherwise transmit idle dwords.

The second option would obviously require the expander to have the intelligence to know that data frames 
are being forwarded.

Are there any potential drawbacks with either option?


Fred Dutton
Fred Dutton * QLogic * 949.389.6340

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