Re: TDI and TDO signal

[Timothy Hellman/PicTel <Timothy_Hellman@smtpnotes.pictel.com>]

I'm convinced that the JTAG signals on the PCI connector are useless unless
you're designing an embedded system and know exactly what cards are
going into it.  As pointed out below, you can't use the scan chain on a 
motherboard unless all the slots are populated, and even then you'll have 
to count on all your add-in boards implementing the JTAG pins properly.

On the other hand, add-in board manufacturers are strongly discouraged 
from actually connecting the JTAG pins to any JTAG devices on their 
boards, because not all motherboards implement these lines properly
(as was discussed recently in this forum), thereby allowing accidental 
triggering due to noise coupling.

It was a nice idea, though.

Tim

=========================
Hi All,

This is one area of PCI that has confused me, perhaps someone can clarify.  Is
there some type of specification, that mechanically, a PCI socket will keep
TDI and TDO connected when a PCI card is not present?  If not, then
I'm not convinced that connecting TDI and TDO together is the proper
solution, either.  It would appear to me that PCI slots that are not populated 
would make TDI and TDO appear open, so even if you connect TDI and TDO together 
in your card, there's still an incomplete scan path when the system is not 
fully 
populated with PCI cards.  

However, by connecting TDI and TDO together on your card, you have made your 
card
incompatible in systems supporting a 1149.1 multidrop architecture.   In a 
multidrop 
architecture, the TDI, TDO, TMS and TCK are common to all daughter boards in 
the system.  
Each daughter board requires a multi-drop interface chip (see TI '8996 
addressable 
shadow port or similar) in order to address the daughter boards in the system 
and select
which board will drive on TDO during the SHIFT-DR or SHIFT-IR states of 1149.1. 
 
One advantage would be that not all slots need to be populated to complete the 
scan 
chain.

Regards,
CJ 
IEEE 1149.1 working group member (however, these are my personal opinions and
not the opinion of the working group)



>>
>>In <199608210937.CAA12145@scruz.net> "John R Pierce"
><uunet!scruznet.com!pierce> writes:
>>>Read the PCI 2.1 SPEC.  Those books are not the SPECIFICATION.
>>
>>>But, yes, you hook them up.  otherwise other boundary scan boards would
>fail if
>>>the system supported boundary scans...
>>
>>>-jrp
>>>(btw, your email address is broken, '@@' don't work!)
>>
>>>----------
>>>> From: Carsten Bode <cb@@Diehl.DE>
>>>> To: Mailing List Recipients <pci-sig-request@znyx.com>
>>>> Subject: TDI and TDO signal
>>>> Date: Wednesday, August 21, 1996 4:56 AM
>>>> 
>>>> Hello
>>>> I have a simple question regarding the design of a PCI card ( standard PC
>>>format ) that does not
>>>> support boundary scan.
>>>> Should I connect TDI and TDO or leave them open or pull them up or down 
with
>>>a resistor?
>>>> 
>>>> I found different statements in the Mindshare PCI book ( connect TDI and 
TDO
>>>) and
>>>> the PCI book from Ed Solari ( recommends 5K pullups/pulldowns on various
>T...
>>>signals ).
>>>> 
>>>> Regards,
>>>>  Carsten Bode
>>>> 
>>
>>
>>
>>-- 
>>My email: cjclark@intellitech.com
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>>

>--------------------------------
>  Alan Deikman, ZNYX Corporation
>  alan@znyx.com


>End of forwarded message.

>-- 
>My email: cjclark@intellitech.com
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>+ Intellitech Corporation                     Scan Tools               +
>+ 66 Route 25, Meredith, NH 03253        for Design and Test.          +
>+ PH:603-279-6308 FX:603-279-5135     For info: info@intellitech.com   +
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-- 
My email: cjclark@intellitech.com
------------------------------------------------------------------------
+ Intellitech Corporation                     Scan Tools               +
+ 66 Route 25, Meredith, NH 03253        for Design and Test.          +
+ PH:603-279-6308 FX:603-279-5135     For info: info@intellitech.com   +
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