Abdalla,
It is a common assumption that a Tx and Rx are connected by daisy chained
transmission lines (and tees to stubs or other I/O buffers. For proper
signal integrity, the thru daisy chain path often has series resistors and
blocking caps. What I am trying to say is that there is no clear definition
of what a "good" connection is and the electrical elements comprised
thereof. A "good" connection is one that works, we use simulation models to
predict what works and what does not work.
Walter
Walter Katz
wkatz@xxxxxxxxxx
Office 978.461-0449 x 133
Mobile 720.417-3762
-----Original Message-----
From: ibis-macro-bounce@xxxxxxxxxxxxx <ibis-macro-bounce@xxxxxxxxxxxxx> On
Behalf Of Abdelgader Mahmoud Abdalla Mahmoud
Sent: Friday, December 13, 2019 6:27 PM
To: wkatz@xxxxxxxxxxxxx; Arpad_Muranyi@xxxxxxxxxx; IBIS-ATM
<ibis-macro@xxxxxxxxxxxxx>
Subject: [ibis-macro] Re: What is a connection?
Walter
Sorry, I don't get it
you will interconnected TX and Rx by series resistor as if you are
represented transmission line or what As far as I know, the transmission
line should be represented by characteristic impedance, am I right?
Abdalla
________________________________________
From: ibis-macro-bounce@xxxxxxxxxxxxx [ibis-macro-bounce@xxxxxxxxxxxxx] on
behalf of Walter Katz [wkatz@xxxxxxxxxxxxx]
Sent: 13 December 2019 18:51
To: Arpad_Muranyi@xxxxxxxxxx; IBIS-ATM
Subject: [ibis-macro] Re: What is a connection?
Arpad,
"connected legally" means the circuit works. It is a function of the total
DC load and AC load. A board drawing 100 amps of current from a 2V VDD
supply is not going work legally (i.e. pass the Rx and Tx buffer rail
voltage requirements if there is a 100 Ohm series resistor between the VDD
connector pins and the VDD designator pins. A 100 Ohm series resistor might
be OK if the board was drawing 1 mA.
Bottom line is IBIS assume a connection between Tx and Rx models, and this
defines "connected"
In the layout world this could be giving the Tx and Rx pins the same CAD
Net. Or there could be a series resistor between two different CAD nets. Now
it is the choice of how the model maker constructs his interconnect
circuits. The series resistor can be an element in the ISS interconnect
model between the Tx and Rx which are "connected". Or he can have the series
resistor be an IBIS component, and then there would be two CAD nets, one
connection from the Tx to the series resistor and another from the series
resistor to the Rx. All EDA tools have the ability to sniff this out and
generate Extended Nets. So the "connection" between the Tx and the Rx can be
implement as a daisy chain of connected sections.
Walter
From: ibis-macro-bounce@xxxxxxxxxxxxx <ibis-macro-bounce@xxxxxxxxxxxxx> On
Behalf Of Muranyi, Arpad
Sent: Friday, December 13, 2019 1:37 PM
To: IBIS-ATM <ibis-macro@xxxxxxxxxxxxx>
Subject: [ibis-macro] Re: What is a connection?
Walter,
Thanks for the thoughts. I have a little hard time with the "DC impedance"
terminology here.
If you look at the Wiki article:
https://en.wikipedia.org/wiki/Electrical_impedance
you will see that they say:
"In addition to resistance as seen in DC circuits, impedance in AC circuits
includes the."
So I feel that at DC we should be talking about resistance, not impedance.
The other thing that still bothers me is your statement about Rail
connectivity. What does "connected legally" mean?
If I were to summarize what you are saying, than the only difference between
PDN and signal connectivity is that in PDN we require a low DC resistance,
but in signal connections we can also consider a low AC impedance (at
Nyquist) in addition to a low DC resistance as a "good connection".
Thanks,
Arpad
=========================================================================
From:
ibis-macro-bounce@xxxxxxxxxxxxx<mailto:ibis-macro-bounce@xxxxxxxxxxxxx>
[mailto:ibis-macro-bounce@xxxxxxxxxxxxx] On Behalf Of Walter Katz
Sent: Friday, December 13, 2019 9:08 AM
To: IBIS-ATM <ibis-macro@xxxxxxxxxxxxx<mailto:ibis-macro@xxxxxxxxxxxxx>>
Subject: [ibis-macro] What is a connection?
All,
I/O Connectivity
The IBIS specification defines the electrical IV and VT characteristics of
an I/O buffer. It is fundamentally based on the fact that there is an
electrical path that can be defined between a Driving Output buffer and one
or more Input (and other Output) buffers. It is expected that the
transitions of the Driving Output Buffer are reflecting in transitions at
the Receiving Input buffers between Vinl and Vinh. One way of describing
these electrical connections between the I/O buffers are with IBIS ISS (and
Touchstone File) models. A connections is a "good" connection if the
transitions at the Driver generate transitions at the Receiver that satisfy
the receiver "rules" in the IBIS file. A good connection could have a low DC
impedance, or in some cases a low impedance at a Nyquist frequency.
Rail Connectivity
If two nodes in a rail distribution system are "connected" legally then
there is a "low" DC impedance between the two nodes. Again, "low" is
dependent on the current t flow and proper operation of the I/O buffers.
Walter
Walter Katz
wkatz@xxxxxxxxxx<mailto:wkatz@xxxxxxxxxx>
Office 978.461-0449 x 133
Mobile 720.417-3762
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