RTL Logic

From: der Mouse <mouse_at_Rodents.Montreal.QC.CA>
Date: Sun Jan 9 04:15:40 2005

> Wired-OR will work fine for open-collector output TTL.

Well, except you generally get wired-AND, because the open collectors
are generally NPN collectors with emitters to ground, not PNP
collectors with emitters to Vcc.

> It will fail miserably with totem-pole output parts. A 'battle of
> the transistors' will ensue and one output or another will win.

Except that totem-pole outputs usually have just a transistor to
ground, but a transistor and resistor to Vcc. This means that the one
driving the signal low will win, and, provided not too many outputs are
wired together, it will sink the current without damage. (I've
actually built circuits that depended on this, though I've never liked
it and never considered it suitable for more than preliminary hackery.
I've also always never done it for anything where the conflict will
last more than nanoseconds, as when building an ~R/~S flip-flop out of
two cross-coupled inverters rather than the more usual NANDs or NORs.)

Of course, this depends on the details of the output pin circuitry and
hence does not really apply to things like CMOS TTL-alikes (unless they
go to the trouble to also simulate this aspect of TTL - I don't know
whether they do).

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Received on Sun Jan 09 2005 - 04:15:40 GMT

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