LVD SCSI Question

Walter Bridgewater x2371 wally at eng.adaptec.com
Tue Oct 15 13:56:39 PDT 1996


* From the SCSI Reflector (scsi at symbios.com), posted by:
* wally at eng.adaptec.com (Walter Bridgewater x2371)
*
Gentlemen,

In reference to hot plugging a single-ended SCSI device into a
SCSI bus operating in LVD mode:

Hasn't this issue already been resolved?

You can only use hot plugging case 1, when it will change the SCSI bus
|from LVS mode to SE mode.

In other words: If you want to plug a single ended device into a SCSI
bus already running in LVD mode, you have to power down the system
and re-cable it, because; 1) How to switch modes is not defined & 
2) You may not have the right cable length & loading to run in single-
ended mode.

This is what I remember from from the SPI-2 work group meetings.

Regards,

Wally


Enclosures of original questions follow.
> 
> * From the SCSI Reflector (scsi at symbios.com), posted by:
> * "Tony Priborsky" <Tony_Priborsky at maxtor.com>
> *
> George and all:
> 
> Your note about hot-plugging and managing the states of the dual-mode LVD 
> transceivers was timely.  I was going to post a similar question myself.
> 
> I'd had a discussion with Bill Ham about this, and I too was wondering how 
> to handle the hot-plugging of a legacy HVD or SE device.  This causes the 
> DIFFSENS line to transition in a manner that affects the entire bus.  The 
> scenario you mentioned about a device device having negotiated for FAST-40 
> transfers suddenly being pulled to single-ended mode was the question I 
> posed to Bill.   After thinking about it a bit, it became clear to me that 
> device (and probably host adaptor) firmware must actively monitor the 
> state of the DIFFSENS signal and react to changes in it.   SPI-2 does 
> specify some debounce intervals, but doesn't say what to do if it changes 
> after initial power up.  I was going to suggest that one way to handle it 
> would be to treat the event as a kind of SCSI bus reset, and have the 
> devices affected process it as such (i.e., reverting to 8-bit, asynch 
> mode).  A new Unit Attention ASC/ASCQ pair would indicate that the bus 
> configuration had changed.
> 
> It may be possible to silently change modes under some circumstances, but 
> the debounce intervals make it difficult to think about doing it with 
> concurrent I/O processes.
> 
> I was also wondering how terminators would deal with this event.  If there 
> are going to be dual-mode terminators on the drives, firmware may need to 
> change their mode or protect themselves too (unless they are crafted to do 
> so automatically), and standalone terminators need to change or protect 
> themselves automatically.
> 
> Can SPI-2 be considered complete until this issue is addressed, especially 
> with respect to this hot-plugging scenario?
> 
> Cheers,
> Tony Priborsky
> Sr. Staff Engineer
> Maxtor Corp
> 2121 Miller Dr.
> Longmont, CO  80501
> 303/682-4880
> 
> ----------------------[Reply - Original Message]----------------------
> 
> Sent by:"George Penokie" <GOP at rchvmp3.vnet.ibm.COM>
>  * From the SCSI Reflector (scsi at symbios.com), posted by:
> * "George Penokie" <GOP at RCHVMP3.VNET.IBM.COM>
> *
> In SCSI-2 and SPI the differential dirver protection is a simple circuit
> that outputs directly into the differential drivers enable pin.  The
> wording and the circuit (SPI only) are very clear as to how the protection
> works.  It is basicly an on/off switch and when off the device will just
> not be able to access the bus.
> 
> Now enter LVD where the on/off switch become an on/on/off switch and there
> is no longer a clear indication as to what to do with the three resulting
> states.
> 
> One can only assume the hardware would have logic that would continuously
> monitor the three signals as follows.
> -The HVD signal would cause all the drivers to be disabled in the
> same fashion as in SCSI-2 and SPI.
> -The LVD signal would cause the LVD drivers to be enabled and the single
> ended driver to be disabled.
> -The single ended signal would cause a the single ended drivers to be 
> enabled
> and the LVD drivers to be disabled.
> 
> So far so good. One could argure that, based on the descriptions in SPI,
> that is the only logical conclusion.
> 
> But there is more: To support LVD and single ended operations the software
> must be involved.  In SCSI-2 and SPI how the software handled the mixing
> was not importance because the drivers where turned off so the device
> could not receive or send anything on the bus.  This not the case for
> LVD, however, where the device must operated properly in both a
> single ended mode and a LVD mode.  In other words the devices software
> must immediatly know when there is a mode change so it can transfer
> information correcly on the bus.
> 
> The problem is the current description in SPI-2 could lead you to think
> these mode changes will only occur after a reset or power on.  This is
> NOT the case.  A device could be hot-plugged without a corresponded
> reset or POR occurring. If this occured and the new device was single 
> ended
> the drivers would switch but the software would not and you could end up
> transfering fast-40 data on a single ended bus (not good).
> 
> I belived that SPI-2 should make it clear that the DIFFSENS line
> can change at any time and the devices hardware and software must 
> immediately
> respond to any of those changes.
> 
> Any comments???
> 
> Bye for now,
> George Penokie
> *
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