Re: [SI-LIST] : Terminations scheme for bi-directional bus

Roland F. Portman (portman@bitmicro.com)
Thu, 25 Jun 1998 11:08:50 -0700

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Susan,

Try adding schottky diode terminations at 3 places along your backplane,
(far left, middle, far right).
Keep the series terminators at each transceiver. The 74actq245 have
balanced 24mA drivers. The
series terminators at 10-20 ohms are really acting like series loads and
limit the dynamic current rush
of the driver (which is far more than 24mA).

Take a look at the app notes at California Micro Devices They have
some good write ups.
CMD Technotes Unfortunatly, CMD only has arrays to terminate busses
of 8 or 16 signals.

In the recent version of EDN magazine (online at www.ednmag.com) there
is an article
on termination techniques which includes an on-line si tool. EDN
termination and Signal Integrity Tool

I also came across an HP article describing the use or their dual series
schottky diodes for signal clamping.
Non-RF Applications for the Surface Mount Schottky Diode Pairs
HSMS-2802 and HSMS-2822
It specifically mentions the HSMS-2822 data sheet There are two
diodes in series in a SOT-23 package.

Good Luck,

Roland

M. Susan Tweeton wrote:

> There was a question about bi-directional buses a few days ago, but
> this
> one poses some interesting problems.
>
> I have 8 boards connected to a backplane. At least four, but as many
> as
> all 8 boards CAN be inserted at one time. Each board has a
> transceiver
> connected to one bi-directional bus. Any of the transceivers can be
> the
> driver. The transceiver nets to the connectors are about 3 inches
> long
> and the backplane traces between connectors are about 1.25 inches.
> The
> transceiver is a 74ACTQ245. All boards have a Z0 of about 60 ohms.
>
> What termination techique should I try? I've tried using 10 or 20 ohm
>
> series resistors near each transceiver, but the driver signal has a
> ring
> back that crosses the 0.8 volt threshold on the falling edge. Because
>
> any number of receivers can be on the bus at one time, I can't adjust
> the termination resistance to accommodate all loading options.
>
> ANY IDEAS?
>
> Thank you in advance for your comments.
>
> Susan Tweeton
> SI/Design Engineer
> Rockwell-Collins
> Cedar Rapids, IA
> **** To unsubscribe from si-list: send e-mail to
> majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE
> si-list, for more more help, put HELP. si-list archives are
> accessible at

--
Roland F. Portman
BiT Microsystems, Inc.
48499 Milmont Drive
Fremont, CA  94538

(510) 623-2341 Ext 129 (510) 623-2342 Fax

portman@bitmicro.com http://www.bitmicro.com

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Susan,

Try adding schottky diode terminations at 3 places along your backplane, (far left, middle, far right).
Keep the series terminators at each transceiver.   The 74actq245 have balanced 24mA drivers.  The
series terminators at 10-20 ohms are really acting like series loads and limit the dynamic current rush
of the driver (which is far more than 24mA).

Take a look at the app notes at California Micro Devices    They have some good write ups.
 CMD Technotes    Unfortunatly, CMD only has arrays to terminate busses of  8 or 16 signals.

In the recent version of EDN magazine  (online at www.ednmag.com)  there is an article
on termination techniques which includes an on-line si tool.    EDN termination and Signal Integrity Tool

I also came across an HP article describing the use or their dual series schottky diodes for signal clamping.
 Non-RF Applications for the Surface Mount Schottky Diode Pairs HSMS-2802 and HSMS-2822
It specifically mentions the HSMS-2822    data sheet   There are two diodes in series in a SOT-23 package.
 

Good Luck,

Roland
 

M. Susan Tweeton wrote:

There was a question about bi-directional buses a few days ago, but this
one poses some interesting problems.

I have 8 boards connected to a backplane.  At least four, but as many as
all 8 boards CAN be inserted at one time.  Each board has a transceiver
connected to one bi-directional bus.  Any of the transceivers can be the
driver.  The transceiver nets to the connectors are about 3 inches long
and the backplane traces between connectors are about 1.25 inches.  The
transceiver is a 74ACTQ245.  All boards have a Z0 of about 60 ohms.

What termination techique should I try?  I've tried using 10 or 20 ohm
series resistors near each transceiver, but the driver signal has a ring
back that crosses the 0.8 volt threshold on the falling edge.  Because
any number of receivers can be on the bus at one time, I can't adjust
the termination resistance to accommodate all loading options.

ANY IDEAS?

Thank you in advance for your comments.

Susan Tweeton
SI/Design Engineer
Rockwell-Collins
Cedar Rapids, IA
**** To unsubscribe from si-list: send e-mail to majordomo@silab.eng.sun.com. In the BODY of message put: UNSUBSCRIBE si-list, for more more help, put HELP.  si-list archives are accessible at

 

--
Roland F. Portman
BiT Microsystems, Inc.
48499 Milmont Drive
Fremont, CA  94538

(510) 623-2341 Ext 129
(510) 623-2342 Fax

portman@bitmicro.com
http://www.bitmicro.com
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