Some sequencer fustration and CD40xxB replacement question.

[Hairy Harry]

Absolutely !

I got really screwed by this once... Two counters (4040 I think) both
with "schmitt trigger" inputs. Only ONE manufacturer had the trigger
as the input stage (the way you would expect) and the other had a
DeMorgan equivalent of that stage, with the schmitt trigger as the
second stage. (too bad for me they were in production units!)

Funny how unit one acted like it had a schmitt trigger input, and unit
two did not....  Same as your problem.

Try some different manufacturers. Sometimes a few picofarads of capacitance 
added to the clock input can make the triggering reliable
also... maybe 10-20pF.

Beware the LM555 and NE555 they are not exactly the same either...

H^) harry


>From: "Theo" <t.hogers at home.nl>
>To: <synth-diy at node12b53.a2000.nl>
>Subject: Some sequencer fustration and CD40xxB replacement question.
>Date: Wed, 15 Nov 2000 16:38:15 +0100
>
>I had some problems with a new version of a 4017 based sequencer I did 
>years
>ago.
>Problem is the clock pulse.
>Turns out not all 4017 are created equal.
>The CD4017B I used in the original has a smith trigger on the clock input
>and allows for unlimited clock rise and fall times.
>The CD4017BC I used in the new unit has no smith trigger and 5u sec max 
>rise
>and fall times. :((
>Does anybody know a modern 40xx series that DOES do direct replacement for
>CD40xxB?
>Also, are this kind of hidden differences common between logic manufactures
>?
>It is the first time I encountered such a small but essential difference
>among supposedly 'identical' ICs
>
>Reg. Theo
>

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