Spec issue on MSE leakage current in SPI-3

Walter Bridgewater wally at eng.adaptec.com
Thu Apr 29 17:26:50 PDT 1999


* From the T10 Reflector (t10 at symbios.com), posted by:
* wally at eng.adaptec.com (Walter Bridgewater)
*

The current that might be presented to an output, that it will
have to clamp, may not be from a driver, but from a reflection.

My simulations show that these refelections contain up to 45mA
of current, and maybe more in a open-ended termination case.

So, the drivers clamp circuit might see at least 45mA at any voltage
up to 7 volts.  If the clamp circuit looks like a very low ohmic
connection, then this will be a miss termination issue, and cause
a reflection from the clamping.

In otherwords, if you clamp at the output, the clamp should have
maybe 50 ohms or more of resistance.


Regards,

Wally

> 
> * From the T10 Reflector (t10 at symbios.com), posted by:
> * Daniel_F_Smith at notes.seagate.com
> *
> Correct me if I'm wrong.  A transceiver, sinking maximum current, can only
> pull 2.85V divided by 110 ohm's worth of current from the bus (25.9mA).  If
> an active terminator were to go "open-loop", then the top of a 110 ohm
> termination impedance would see 5.5V maximum (its input supply running 10%
> high).  Calling that worst case, the max current it could sink would be
> 5.5V divided by 110 ohms, or 50mA.  But of course, the input clamp only
> needs to clamp enough current to get the bus voltage down to Vdd of the LVD
> device.  If the input clamp were to draw enough current to protect itself,
> is that enough to interfere with a normal SE bus transfer?  That's the $64
> question.
> 
> 
> 
> 
> 
> "Bill Galloway" <BillG at breatech.com> on 04/29/99 02:44:39 PM
> 
> Please respond to BillG at breatech.com
> 
> To:   "John Lohmeyer" <lohmeyer at ix.netcom.com>, "T10 Reflector"
>       <t10 at Symbios.COM>
> cc:    (bcc: Daniel F Smith)
> 
> Subject:  RE: Spec issue on MSE leakage current in SPI-3
> 
> 
> 
> 
> * From the T10 Reflector (t10 at symbios.com), posted by:
> * "Bill Galloway" <BillG at breatech.com>
> *
> John,
> 
> In general I agree that the receiver will not care if you clamp the line at
> 3 volts. How hard would you clamp it??? I have seen some older designs that
> could put out allot of current above 3 volts.  Would 100ma damage you??
> Would
> 100ma continuos damage the driver??? How would the impedance discontinuity
> of
> your clamping effect signal integrity, reflections, etc.???
> 
> Bill Galloway
> BREA Technologies, Inc.
> P: (281) 530-3063
> F: (281) 988-0358
> BillG at breatech.com
> 
> -----Original Message-----
> From: John Lohmeyer [mailto:lohmeyer at ix.netcom.com]
> Sent: Thursday, April 29, 1999 4:34 PM
> To: BillG at breatech.com; T10 Reflector
> Subject: RE: Spec issue on MSE leakage current in SPI-3
> 
> 
> I do not want to outlaw old devices.  I just want to make it legal for new
> devices to exist on the same bus.  If my new device tends to clamp signals
> at 3 volts (or a little higher) when an old device trys to drive it higher,
> where is the harm?  The receivers will still see a valid high-level
> (assuming SE) and we should not see such high voltages on LVD.
> 
> The 20 uA leakage spec at 4.1V will add a lot of cost and complexity for no
> benefit.
> 
> John
> 
> --
> John Lohmeyer                  Email: lohmeyer at ix.netcom.com
> LSI Logic Corp.                Voice: +1-719-533-7560
> 4420 ArrowsWest Dr.              Fax: +1-719-533-7036
> Colo Spgs, CO 80907              BBS: +1-719-533-7950
> 
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> 
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