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DPRG: Logic Family Comparison Chart and example problem between famlies.

Subject: DPRG: Logic Family Comparison Chart and example problem between famlies.
From: Kipton Moravec kmoravec at airmail.net
Date: Tue Aug 24 09:50:53 CDT 1999

Has anyone seen anywhere a table that shows the difference between the
different logic families?

Like what are all the differences between a

74
74AC
74ACT
74AHC
74HCT
74C
74F
74HC
74HCT
74ALS
74LS
74LCX
74LVQ
74S

I just got bit by the differences between a 74LS165 and a 74HC165.  The
74HC165 worked and the 74LS165 did not.




I was interfacing an optoisolator to a digital input. 

            +5
- -------+     |
       |     /
      ---  |/
       V   |
      ---  |\
       |     V
- -------+     +-------+--------74...165 input
                     |
   NEC PS2501        \
                     /
                     \  R1 10K Ohms
                     /
                     |
                     GND

Simple circuit huh?

I guessed that when the PS2501 was turned off, there would be be a very
high impedance, which may be the equivalent of letting the digital input
float. So I put a 10K resistor tied to ground to pull the signal low.

This worked for the 74HC165. When I measured the voltage off was 0V and
on was 5V.

When I put in the 74LS165 off was .3 V, until I told it to READ the
input and then it jumped to 1V, and the 74LS165 read it as a Logic HI!

Running some experiments it looked like the pull down had to be 8300
Ohms or less for this circuit to work.  

I only had 100 boards built. :(  We are now in the process of replacing
the pull down SIPS. (Two 10-pin SIPS are quicker to replace than three
16-pin DIPS).

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