Subject: OPEN BIRD7 Comments

From: [email-masked] ( Bob Ross)
Date: Thu, 20 Jan 94 16:55:27 PST

Kellee, In you comments on the "open" bird7, you have raised some 
interesting points.

The real issue is to specify whether the pullup is open or the pulldown
is open - i.e., "the missing V/I data for the output."  By knowning this,
the appropriate ramp can be interpreted for the turn-off transition.

"Open_drain" is already defined in IBIS Version 1.1.  Furthermore, in 
NOTES ON DERIVATION DATE #7, "The rise time of an open-drain device
must be measured into a 50 Ohm pullup resistor tied to the power pin."
Based on this, there is a strong implication that "Open_drain" means
open pullup.  However, I think you are technically correct in pointing
out that in CMOS logic, Open_drain is ambigous as to which side is
open - it could either the p-channel or the n-channel device.

The IEC logic symbol definitions for open outputs unfortunately give
little guidence in this area.  One is defined for "N-P-N open-collector or
similar output ...", and the other is for "N-P-N open-emitter or similar 
output ...".  Based on this, it would be ideal to use open_collector
and open_emitter generically to cover both bipolar and CMOS technologies
and to drop "Open_drain" as a Model_type.

On the other hand, I know of one example where "open source" and "open
drain" are used.  The Toshiba TC140G/14L/150G Series Macrocell data
book specifies open drain, open source, bidirectional open drain
and bidirectional open source output buffers.  (Actually these are 3-state
open drain and open source output buffers!)  I am faxing to you, Kellee,
some of the pages.  For terminology purposes the four Model types are
given for open pullup and open pulldown and the I/O versions of the same.
>From the circuits shown, I think you could argue that the names are
technically incorrect.

One part of the issue then is what terms should we use?  Are there
any common usage guidelines we could follow?  What I would like to see
is the best way to specify "open-pullup", "open-pulldown", "I/O-open-pullup"
(such in the ABT Futurebus line), and "I/O-open-pulldown" with, hopefully,
only four Model_types.  We should also specify "3-state-open-pullup" and
"3-state-open-pulldown" for completeness.

Regarding another point, I agree that the dV/dt of the off side has to be
specified.  The text of Note 7 above should be expanded.  Would this 
address your concern?

Bob Ross
Interconnectix, Inc.