Number of pad bytes with DT transfers

Richard Moore r_moore at qlc.com
Wed Mar 8 14:44:12 PST 2000


* From the T10 Reflector (t10 at t10.org), posted by:
* Richard Moore <r_moore at qlc.com>
*
Wolfgang,

DT transfers include a pad mechanism which is controlled through the P_CRCA
signal. P_CRCA is switched low for two REQ edges (an assertion and a
negation
of REQ) to indicated a CRC transfer with no pad bytes. P_CRCA is switched
low
for three REQ edges (a negation, an assertion, and another negation) to
indicate
two pad bytes followed by CRC. This is sufficient for transfers of length
mod 4
equal to 0 or 2.

For the 1 and 3 cases, a single residue byte, or a single residue byte plus
two
pad bytes using P_CRCA, is used. The residue byte is indicated by IGNORE
WIDE
RESIDUE, and can only be placed at the end of the transfer, not on
intermediate
CRCs. On the other hand pad bytes, which always come two bytes at a time,
can
happen immediately prior to any CRC.

The above description is for the non-packetized case. Packetized will have
the
transfer length explicitly declared in the L_Q packet that precedes the data
packet.

 -- Richard Moore
    QLogic Corp.


>-----Original Message-----
>From: Wolfgang Michel [mailto:wolfgang_michel at vortex.de]
>Sent: Tuesday, March 07, 2000 2:26 AM
>To: t10 at t10.org
>Subject: Number of pad bytes with DT transfers
>
>
>* From the T10 Reflector (t10 at t10.org), posted by:
>* "Wolfgang Michel" <wolfgang_michel at vortex.de>
>*
>
>Given a sequential device (like a DAT, etc.) is transfering data to 
>the initiator with DT data group transfers. And further given the 
>device has to disconnect from the initiator due to an internal 
>resource condition on a non module 4 boundary. The device than has to 
>fill up the last data field up to the next 4 byte boundary with 
>padding bytes before sending the CRC. How is the initiator able to 
>identify the number of pad bytes (1,2 or 3)? Up to my understanding 
>the IGNORE WIDE RESIDUE message "invalidates" only 1 byte.
>
>Your assistance/help is really appreciated.
>
>Thanks
>Wolfgang
>
>
>
>***************************************************
>Wolfgang Michel, Director R&D
>ICP vortex Computersysteme GmbH
>Konrad Zuse Str. 9
>74172 Neckarsulm, Germany
>Tel.: +49-7132-9620-535
>Fax : +49-7132-9620-200
>email: w_michel at vortex.de
>
>http://www.icp-vortex.com
>
>http://www.vortex.de
>*************************************************** 
>
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