11/23 clock issue

Holm Tiffe holm at freibergnet.de
Sun Feb 8 05:20:54 CST 2015

Brent Hilpert wrote:

> On 2015-Feb-07, at 11:20 AM, Holm Tiffe wrote:
> > Pete Turnbull wrote:
> > 
> >> On 06/02/2015 07:44, Holm Tiffe wrote:
> >>> You don't ned no pullup for +5. All open TTL inputs are reading High w/o
> >>> any pullup.
> >> 
> >> Yeah.  So someone at Commodore thought when they designed one version of 
> >> the PET.  We had a few that erratically misbehaved.  It turned out that 
> >> one input on a 74LS00 (I think it was) was floating, and switching noise 
> >> made it erratic.  Floating inputs place the internal circuitry in an 
> >> intermediate state, can cause increased current draw, typically slow the 
> >> device down by increasing switching times, and can cause misbehaviour.
> >> 
> >> TTL is supposed to have a 1K pullup (to limit possible transients); 
> >> LSTTL can be directly connected to Vcc.
> >> 
> >> -- 
> >> Pete
> >> 
> >> Pete Turnbull
> > 
> > Yes Pete, not all People over here are totally braindead.
> > 
> > I've told him that he can leave out the +5V Connection for testing
> > purposes, for nothing other.
> No you didn't. While the context of the discussion is testing and that may have been your intention, your comment specified no such qualification, and as such at best left it ambiguous/unclear.
> Pete's comment was valid clarification and additional information (although I could have minor quibbles with some of the technical phrasing).

You are simply wrong. I'm answered in a thread reagarding a fault in a
clock circuit of an PDP11 Processor Board, not in a discussion regarding
the design of a new board with TTL cicuits.

Besides of that Petes sentence "TTL is supposed to have a 1K pullup (to
limit possible transients); LSTTL can be directly connected to Vcc." above
is plain wrong too.
Direct me to a datasheet containing this please.

> Anyways (for Noel), everyone's right:
> 	- An open TTL input will generally function or act as a logic high.
> 	  It was not unknown for designs to leave inputs open for static-state high,
> 	  but it is considered poor practice and can result in problems in some circumstances.

But not under Noel's circumstances.



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