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

Dennis Tomlinson (det@tellabs.com)
Thu, 25 Jun 1998 16:44:13 -0500

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Roland F. Portman wrote:

> 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
>

Hi folks,

I'd like to step in here and ask a few questions if I may:

1. Is pulse monotonicity a requirement?
2. Is first incidence switching necessary, or
3. Is there enough slack in the timing budget so that
second or third incidence is "good enough?"
4. With 10 to 20 Ohms series terminators, is
over/undershoot a problem? Partially loaded? Fully loaded?
5 And finally, is your system malfunctioning because
of SI problems with these signals?

Cheers,

Dennis

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Roland F. Portman wrote:

 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
 

Hi folks,

I'd like to step in here and ask a few questions if I may:

1. Is pulse monotonicity a requirement?
2. Is first incidence switching necessary, or
3. Is there enough slack in the timing budget so that
    second or third incidence is "good enough?"
4. With 10 to 20 Ohms series terminators, is
     over/undershoot a problem? Partially loaded? Fully loaded?
5 And finally, is your system malfunctioning because
    of SI problems with these signals?

Cheers,

Dennis
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