On 5/19/2010 6:58 AM, Invisible wrote:
> scott wrote:
>>> "74LS series: Inputs 'float' high to logic 1 if unconnected."
>>>
>>> Oh crap. Well that would explain a thing or two! >_<
>>
>> You should never be leaving any inputs as floating anyway, that's just
>> asking for trouble (for example if you use a different IC series
>> later). Best to always tie your inputs to whatever you want them to be.
>
> Sure. But I'm sitting with the IC between my fingers, poking the pins
> with the end of a wire. Not so easy to tie it all down just for a quick
> test.

So, you put it on a breadboard with each pin tied to a pull-up or 
pull-down resistor, and probe with the opposite sense. If your resistor 
is pull down (i.e. connected to GND) then use VCC to probe (or better 
yet, just get a dip switch ...) If the resistor is connected to VCC, 
then use ground to set the signal to 0

> So it seems that every input pin is supposed to be connected to one rail
> or the other. (Or an output, which I guess is going to be the most
> common case...)

Yes. 5v is high, ground is low, not connected is neither of these, 
though TTL does float high, a non-connected input on a CMOS chip can 
lead to erratic behavior, or even destructive oscillation. It also 
leaves the chip wide open for ESD damage. FETs are interesting in that 
they don't sink or source any current on their inputs. without anywhere 
for the voltage to go, a charge can stay on that input. That charge can 
go dangerously (for the chip) high can cause a dielectric breakdown of 
the gate's insulating layer. Some CMOS gates will have a high-value 
resistor tied to ground on the input to bleed away some of the stray 
voltage as a safety measure for ESD. Don't rely on that.

> The page also claims that you can configure an output pin to drive an
> LED so that it lights when the pin goes *low*... Man, electronics is weird!

LED anode connected to VCC, cathode connected to out. ;) Not too strange 
when you think about it. A logic output is either connected to VCC or GND.

One other thing to be cautious of: If a TTL chip has an open collector 
output, make sure to use a pull-up resistor before connecting it to a 
CMOS chip. High is floating on an open collector output.

Oh! and by the way: with TTL chips VCC is *critical* due to the nature 
of the bipolar transistors used in the gates, they must have a regulated 
voltage. Look at Falstad's circuit simulator to figure out why. They 
have a resistor set to a particular value to bias the transistor. If the 
supply voltage goes out of range, the transistor is no longer properly 
biased, which will cause misinterpretation of signals.

CMOS doesn't require a bias, so can run on any voltage (within limits)
-- 
~Mike

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