Clamp diodes inside TTL devices are useless for termination on the high side
because they are so far above a TTL high signal (diode doesn't turn on until
~5.6V, while the signal is ringing around 3V).
Honeywell, Space Systems Division, M/S 934-5
13350 US 19 N., Clearwater, FL, 34624
From: Andrew Ingraham [SMTP:Andrew.Ingraham@digital.com]
Sent: Tuesday, October 27, 1998 6:39 PM
Subject: RE: [SI-LIST] : Schottky diode termination
I first saw Schottky diode termination described in a rather old
Fairchild ECL book. Diode termination was not often used for ECL,
perhaps because the normal ECL parallel termination usually worked
well if you could afford the power dissipation.
The TTL logic families had clamp diodes in their input structures,
one of their intended functions actually was to provide some amount
diode termination. This is one of the reasons why TTL was easy to
with, sometimes even when the wires got long compared to the
Many people who have used TTL do not realize that they have been
use of diode termination all along.
Most CMOS families also have input clamp diodes, and again they
generally work as both input protection and partial signal
Those CMOS devices without input diodes tend to be SI nightmares.
In a multi-drop bus configuration, diode terminators may be needed
several places, perhaps as many as one per input. For a daisy-chain
route, they may only be needed at the two ends. The best way to
what works is to try, preferably in simulation with GOOD models.
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