11/23 clock issue

Brent Hilpert hilpert at cs.ubc.ca
Sat Feb 7 22:08:21 CST 2015

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).

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.

	- Some sources specify using a pull-up R instead of direct connection to +5, but in practice many considered the R's as unnecessary/overkill.
	  Some designs used pull-up R's, some designs directly connect to +5.
	  I've even seen both in the same piece of equipment (probably multiple engineers involved in different parts of the design).

This is for TTL (and DTL) only, MOS/CMOS is another matter.

> That has nothing todo with your Commodore problem.
> Regards,
> Holm

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