FW: FW: Question about the Ignore Wide Residue message

[Skip Jones]

This is our take.

Regards,

Skip Jones
_______________________________________________________________________________
From: Gary on Mon, Jun 6, 1994 2:50 PM
Subject: Re: FW: Question about the Ignore Wide Residue message
To: sk_jones at qlc.com


Skip:

This is inreply to the message that you forwarded me from
shiang at corp.cirrus.com 
in regards to the Ignore Wide Residue message.  I have not forwarded this to
Shinga.

The questions were:
     1. Why does this message only apply to the target side of a SCSI bus. 
     The target sends this message to the initiator, but the initiator does 
     not send this message to the target. 
     Also, is it necessary for a target to send this message when the DATA 
     IN is a read command, since the initiator already knows that the block 
     length is not a multiple of the scsi width? 
     
     2. In what commands does the target send this message to the 
     initiator?  

The answers are:     
     1. The message applies only to the target side of the SCSI bus, because
        the target is the side of the bus that determines the amount of data
        actually moved on the SCSI bus.
        
        It is necessary for the target to send this message whenever the number
        of "valid" data bytes does not match the current (negotiated) bus
width.
        If the block length is 511 and 3 blocks are sent accross a 16 bit 
        data path, then the target must send the Ignore Wide Residue message 
        because 8 bits of the last trasfer is invalid. The block size is 
        determined by the target.  I don`t know of any target devices that
        support Wide SCSI that also support odd block sizes (511, 1025, ...),
        but SCSI defines a protocol to handle this.

     2. The type of commands that the target may need to send this message will
        typically be Inquiry, Request Sense, and Mode Sense.  The amount of
        data returned by these commands is typically not known by the initiator
        and is not required (by SCSI) to be divisible by the current negotiated
        bus width.  The CDB setup by the initiator only specifies a maximum
        exceptable transfer length (allocation length).  Many initiators will
        set the allocation length to the maximum value of 255, the actual data
        length is deteremined by the target.
 
        As an example: a 33 byte mode sense page is 100% legal SCSI and if
        the current negotiated bus width is other than 8 bits, an Ignore Wide
        Residue message is in order.

Gary