Spec issue on MSE leakage current in SPI-3

Daniel_F_Smith at notes.seagate.com Daniel_F_Smith at notes.seagate.com
Thu Apr 29 17:02:19 PDT 1999


* 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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