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DPRG: Logic Family Comparison Chart and example problem between fa

Subject: DPRG: Logic Family Comparison Chart and example problem between fa
From: john.r.strohm at bix.com john.r.strohm at bix.com
Date: Wed Aug 25 00:18:15 CDT 1999

>I was interfacing an optoisolator to a digital input. 
>
>            +5
>-------+     |
>       |     /
>      ---  |/
>       V   |
>      ---  |\
>       |     V
>-------+     +-------+--------74...165 input
>                     |
>   NEC PS2501        \
>                     /
>                     \  R1 10K Ohms
>                     /
>                     |
>                     GND

The above is not a good approach for driving bipolar TTL.

74xx bipolar inputs are transistor emitters.  In order to activate an
emitter input, you need to provide a VERY low-resistance ground, such as
(for example) the collector of an NPN transistor that is being driven ON. 
For an inactive (high) input, it suffices to tie it high through a
relatively-large resistor.

The Texas Instruments "TTL Data Book", if you can find it, has a wealth of
information on this kind of thing.

You'll probably get better results with this geometry.  Note that the
signal polarity presented to the 74xx165 is reversed: light ON = LOW input.

             +5
              |
              \
              /
              \  R1
              /
              |
- -------+      +--------------- 74...165 input
       |     /
      ---  |/
       V   |   NEC PS2501
      ---  |\
       |     V
- -------+     |
             |
             |
            GND

Mike has posted something about the various 74xxNNN families.  I'm not
offhand sure what value would be good for the pullup; I'd probably start
experimenting with something around 10K and see how it worked.

I used to have a good book about TTL design, but it is buried very deep at
the moment.

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