11/23 clock issue

Holm Tiffe holm at freibergnet.de
Sun Feb 8 14:58:21 CST 2015

Pete Turnbull wrote:

> On 08/02/2015 11:20, Holm Tiffe wrote:
> >Brent Hilpert wrote:
> >>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.
> Ooh, somebody's still grumpy today.  Not me though, I'm writing this 
> with a smile on my face :-)
> >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.
> Yet it will sit in the archive for years and since the message itself 
> says nothing about that, it could easily be taken out of context by 
> someone who knows less than you, I, or Brent.  Moreover, it doesn't 
> always work as a reliable test, simply because there /are/ circumstances 
> where letting an input float will sufficiently change the operating 
> characteristics.
> >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.
> No, you're wrong.  Try looking in the Texas Instruments TTL Data Book; 
> in the Fifth European Edition, it's clearly described on pages 5-4 to 
> 5-5.  It's also mentioned in chapter 3 of my 1971 edition of the Texas 
> Instruments book "Designing with TTL Integrated Circuits", published by 
> McGraw-Hill, which also has a lot to say about switching speeds, DC and 
> AC noise margins, and noise rejection.  You might also care to read 
> Chapter 9 especially section 9.06 "Some comments about logic inputs" in 
> "Horowitz & Hill "The Art of Electronics" (one of the standard 
> textbooks) culminating in their analysis of unused TTL inputs left open, 
> and hence having /zero/ noise margin.
> -- 
> Pete
> Pete Turnbull

Pete I'm not a natural english speaker as you know, and it really isn't
easy for me the express what I think "between the lines".
If you think it should be easy, we could try it the other way around, in

It is clear that TTL inputs should'nt be left open floating in a proper
designed circuit, but all considerations of that fact are related to
noise pulled up on these open inputs, a fact that is in the context above
simply irrelevant.
An 74S240 has to read an open Input as H in the same way it has to read it
trough a 1 K Resistor or directly connected to VCC.
It has to read it as high as long as the input voltage is above 0.8V.
(the switch point has to be somewhere between 0.8 and 2.4 Volts to be


That is the only thing that is relevant for Noels testing purposes and that
is how I meant the hint. You simply pulled what I wrote out of the context.

If you really wanted to say something then don't piss on my feet, simply
explain that in a real cicuit the inputs should be pulled high with an
pullup resistor to prevent noise pickup. This where te right way, not hat
what you've done.

Since the Inputs are emitters (as you for sure know) there is no real
difference between LS TTL directly connected to VCC and Standarad, H or S
TTL connected trough a resistor. It is electrically the same since there is
no current flowing out of the emitter.

Nevertheless I'll try to read what you've mentioned above.


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