Terminators

Gerry_Houlder at notes.seagate.com Gerry_Houlder at notes.seagate.com
Mon Jan 10 13:58:41 PST 2000


* From the T10 Reflector (t10 at t10.org), posted by:
* Gerry_Houlder at notes.seagate.com
*
Your approach seems risky.

(1) You would need a switch for each individual resistor (so when the terminator
is disabled you don't end up with one end of all the resistors connected to each
other but not connected to 3.3 V.). Doing all of these switches discretely will
use a lot more board space than buying a comercially available switchable
terminator.

(2) Also the passive terminator you are defining might work OK for single ended
interface at slower transfer speeds but clearly won't work for LVD drivers. This
may not matter to you is you only intend to use single-ended products.





Dimiter Popoff <tgi_earth at yahoo.com> on 01/09/2000 09:11:52 AM

To:   t10 at t10.org
cc:    (bcc: Gerry Houlder)

Subject:  Terminators



* From the T10 Reflector (t10 at t10.org), posted by:
* Dimiter Popoff <tgi_earth at yahoo.com>
*
Hi all,
I am replacing the SCSI bus terminators in a new revision of a product
so they will be software selectable. Because of existing board-space
constraints I cannot use one of the off-the-shelf active terminators;
I have placed a single resistor (75 Ohm OK?) with a Shottky diode
in series to it to each line. All Shottky diodes have their anodes shorted
and connected to a 3.3V source, which can be switched on/off (even
when the device is not powered and termpwr is present, it will work
according to the settings in the device).
If someone thinks this is the wrong way to do this, or if someone
knows this is the right way to do it, please let me know - I'll give
the boards to the PCB manufacturer within a few days and it is not
too late to change something, and I can certainly use an assurance
I have understood what is meant by the "active termination".

Thanks in advance,
Dimiter

Dimiter Popoff
Transgalactic Instruments, Gourko Str. 25 b, 1000 Sofia, Bulgaria
http://www.freeyellow.com/members2/transgalactic/
Email: tgi at bulnet.bg, tgi_earth at yahoo.com
Phone: 00359/2992/3340, 00359/2/9805997, Fax: 00359/2/9540384

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