Re: Unknown holes in the motherboard of the CBM610

Hi!

On Wed, May 02, 2018 at 10:18:54AM +0200, Francesco Messineo wrote:
> On Tue, May 1, 2018 at 1:33 AM, Segher Boessenkool
> <segher@kernel.crashing.org> wrote:
> > On Mon, Apr 30, 2018 at 08:44:23PM +0200, Mia Magnusson wrote:
> >> Den Mon, 30 Apr 2018 19:59:21 +0200 skrev Gerrit Heitsch
> >> <gerrit@laosinh.s.bawue.de>:
> >> > On 04/30/2018 07:51 PM, Mia Magnusson wrote:
> >> > > But why not just join each of those TTL inputs with +5V directly on
> >> > > the nearby chip?
> >> > >
> >> > > Does some 74xx IC's work better with slightly less drive to +5V on
> >> > > the signal that feeds the inputs?
> >> >
> >> > I don't remember where, but I read somewhere, that you shouldn't
> >> > connect a TTL input directly to +5V.
> >>
> >> Interesting!
> >>
> >> IIRC all data sheets indicate that anything between 2.4V and 5V is
> >> treated like a correct "1".
> >>
> >> Maybe it's different on different TTL families? The B series uses a lot
> >> of S logic.
> >
> > If you have a little bit of overvoltage on a TTL input (say, during
> > power-on) you get a nice big fat short in the TTL device, taking out
> > your power supply (or the whole system) if you have no current-limiting
> > resistor on the input connected to Vcc.
> 
> 
>  Vcc has usually the same speed for both the power pins and the input
> pins connected to them. TTL inputs are usually safe to 7V and by the
> way, there shouldn't be any such overshoot on any proper designed
> digital supply.

74LS inputs have max rating 7V, yes, but 74 only 5.5V (power supply up
to 7V though).

Are we talking about LS devices here?  I totally missed that.

Another explanation for why the designers of this board did this, even
with LS devices, is old habits die hard?  :-)


Segher