Subject: Re: open source and open drain

From: [email-masked] (C. Kumar)
Date: Tue, 17 Jan 95 09:37:31 -0500

This is reply to a question I asked of Stephen Peters of Intel. Like
Stephen suggested it may
be of general interest.

Hello Kumar:

     Good question.  Open drain/collector buss do have some advantages:

     1.  In the old days before 1ns edge rates they allowed one to
         do logic functions (wire-or or wire-and) without the 
         delay of an actual logic gate.

     2.  They allow multi-party operation and bus contention on signals.

     2.  In a dense ASIC they save I/O buffer size because now there
         is no pullup device.  This also saves on suppling VCC power to
         the I/0 ring.

     3.  If the pullup resister is selected to match the network impedence
         you can terminate the net quite nicly (This is why designing with
         ECL is so nice -- you have to work NOT to have a terminated net).
         Adjusting VTT allows for 
         reduced signal swing (with a pullup device you go from gnd to VCC).
         Of course, the shape of the rising edge is now controlled by
         by the network characteristics and not an active pullup xsistor
         on the output.

There are probably other advantages as well.  This might be a good question
for the IBIS reflector (or the SI reflector -- I assume you are on that?)

                Stephen

Stephen:

I have a general question for you regarding open drain or any open devices.

Under what circumstances you use such devices and what are their advantages?

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