From: petrus.bitbyter@hotmail.com
"Computer Nerd Kev" schreef in bericht
news:XnsA21EE8E85ADEBCSKJHNMW985780A@94.75.214.90...
> Hello,
>
> I am working on a circuit in which I want to use 74164 SIPO
> Shift Registers as something like basic DACs (Shift Registers
> outputting a stepped current).
>
> Whatever you want to call it, it involves connecting the
> outputs of the 74164s together through individual high value
> resistors so that current increases with each output going
> high, the parallel output driving the base of a transistor. I
> couldn't see a problem with the idea until I read this line in
> "Newnes Digital Logic IC Pocket Book" (P. 44):
>
> "Note that normal totem-pole outputs should not (except in a
> few special cases) be connected in parallel."
>
> Nothing is said regarding what these "special cases" might be,
> although if viewed from the perspecive of logic interfacing, I
> can see how mine may be one of them. Indeed it seems likely
> that the statement refers to cases such as in a wired-OR
> circuit where outputs are connected directly, without series
> resistors.
>
> Nevertheless, I saw need for clarification so I searched books
> and the internet for reference to TTL totem-pole outputs in
> parallel, but found nothing to directly address my concern.
> Hence I ask here for clarification on the matter.
>
> --
> __ __
> #_ < |\| |< _#
Ever had a look at the schematic of a totempole? Then you'd know that
putting them in parallel will give a short from Vcc to Gnd when one is high
and the other low. There is off course a collector resistor but way too low
to stand the short circuit current for longer then some ns. Connecting
resistors to the output will lower the current but you'll have to calculate
carefully to stay within de margins of the chips fanout. Be aware that
resistors tend to conduct current in both directions.
petrus bitbyter
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