Re: Unknown holes in the motherboard of the CBM610

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.
Quoting from the TTL handbook that I linked before:

UNUSED INPUTS
For best noise immunity and switching speed, unused TTL inputs should
not be left floating, but should be held between
2.4 V and the absolute maximum input voltage.
Two possible ways of handling unused inputs are:
1. Connect unused input to VCC, LS TTL inputs have a breakdown voltage
> 7.0 V and require, therefore no series resistor.
2. Connect the unused input to the output of an unused gate that is forced HIGH.

I think the series resistor is used to allow the lines to be held down
in case of testing and troubleshooting maybe. Or because the designer
was thinking about old original TTL gates.
Frank