[sdiy] Re: Bottom Ten ICs

[Scott Gravenhorst]

"Cornutt, David K" <david.k.cornutt at boeing.com> wrote:
>From: Harry Bissell Jr [mailto:harrybissell at prodigy.net]
>
>> 74xx
>> 74xxx
>
>What does everyone have against 7400-series logic?
>Admittedly, it's awfully 1980s now, and there are
>better technologies available.  But it was a great
>leap forward at the time.  It it wasn't for the
>minicomputers built out of 7400 logic, we wouldn't
>have the circuits we have now.  

No disagreement with this, but there is high supply current
requirements and the noise spike offered to the rails during
state switch.  One nasty thing about totempole outputs that
TTL uses is that for an instant, both bipolar output
transistors are on simultaneously.  This has a speed
advantage at the expense of impulse current.  This current
can cause problems with other devices powered by the same
supply, especially if the supply is not up to the task and if
bypassing is omitted.  This is quite apparent in the FatMan
design where unmodified, the synth's VCOs will soft sync
easily.  It is cured one of two ways, a better bypassing
arrangement or simply replace the 555 ICs with 7555 CMOS ICs.
 The bipolar 555 uses a standard totempole output for pin 3.


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