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>> 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!
>
> Why is that weird? The LED doesn't care which leg is connected to the
> logic output pin, it's just a voltage. You could even connect the LED
> between two output pins*, then it would only turn on when one was high
> and the other was low.
I always thought of it as logic high meaning "connected to the +V rail"
and logic low meaning "not connected to anything". From the looks of
things, it seems that actually logic low means "connected to the -V
rail". So it's like a bipolar setup...
> * Using a series resistor if required...
Yes, that's the other fun thing. Depending on which type of gates you're
using, they supply about 20 mA. But some of the LEDs I'm looking it draw
40 mA...
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Mmm, 74HC00, 79p. That's quite cheap...
sound quite so cheap any more. o_O
Remind me, *why* am I embarking on this crazy project? ;-)
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Am 19.05.2010 13:07, schrieb Invisible:
> Ah yes. That's the fun thing about doing digital electronics. You can't
> just *buy* a 7400. No, you have to decide whether you want a 74LS00 or a
> 74HC00 or a 74HCT00 or...
>
> Reading the datasheets is like walking into another world. (Especially
> since the datasheets are usually poorly-scanned grainy PDFs that don't
> match the part number you actually asked about!)
Texas Instruments has a nice collection of 74xx data sheets in a single
proper PDF file at <http://focus.ti.com/lit/ug/scyd013b/scyd013b.pdf>
("Digital Logic Pocket Data Book"), apparently covering all their
current products (as of 2007) in the 74xx line, from single-gate 5-pin
thingies like the 74x1G00 (single NAND gate) to 100-pin monsters like
the 74x32543 (36-bit registered bus transceivers with 3-state outputs).
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Am 19.05.2010 14:51, schrieb Invisible:
> Yes, that's the other fun thing. Depending on which type of gates you're
> using, they supply about 20 mA. But some of the LEDs I'm looking it draw
> 40 mA...
That's why normally you'd use an additional transistor to drive the LED
(if you have multiple LEDs to drive you may want to use a transistor
array in an IC package).
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> I always thought of it as logic high meaning "connected to the +V rail"
> and logic low meaning "not connected to anything".
Logic "high" and "low" are just names given to a certain agreed range of
voltages. The difference between "not connected to anything" (floating) and
"logic low" is very important when working with data busses. If you have
several devices all with their outputs connected to a data bus, then only
one device at a time must be driving the data lines to logic low/high, the
rest must set their outputs to floating.
> From the looks of things, it seems that actually logic low means
> "connected to the -V rail". So it's like a bipolar setup...
But don't assume a "logic low" output will be exactly -V, it is only
guaranteed to be within the range of voltages given for "logic low" in the
data sheet.
> Yes, that's the other fun thing. Depending on which type of gates you're
> using, they supply about 20 mA. But some of the LEDs I'm looking it draw
> 40 mA...
What you mean is, they draw *up to* 40 mA? How much they actually draw
depends on what value series resistor you put in there. Using Ohm's law and
the V-I curve of the LED from the datasheet you can work out exactly what
resistor size you need for a given current.
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Am 19.05.2010 15:29, schrieb scott:
>> I always thought of it as logic high meaning "connected to the +V
>> rail" and logic low meaning "not connected to anything".
>
> Logic "high" and "low" are just names given to a certain agreed range of
> voltages. The difference between "not connected to anything" (floating)
> and "logic low" is very important when working with data busses. If you
> have several devices all with their outputs connected to a data bus,
> then only one device at a time must be driving the data lines to logic
> low/high, the rest must set their outputs to floating.
... except of course in data buses that operate with one dominant and
one recessive symbol (e.g. CAN), in which sending the recessive symbol
is perfectly equivalent to not sending anything - which allows for some
neat tricks to deal with collisions (i.e. the situation where two
devices start sending simultaneously).
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>> I always thought of it as logic high meaning "connected to the +V
>> rail" and logic low meaning "not connected to anything".
>
> Logic "high" and "low" are just names given to a certain agreed range of
> voltages.
Indeed. You can use any two states you want, so long as it's easy to
distinguish them, and all the components are using the same rules.
> The difference between "not connected to anything" (floating)
> and "logic low" is very important when working with data busses.
There is that...
>> From the looks of things, it seems that actually logic low means
>> "connected to the -V rail". So it's like a bipolar setup...
>
> But don't assume a "logic low" output will be exactly -V, it is only
> guaranteed to be within the range of voltages given for "logic low" in
> the data sheet.
Heh. It gets weirder and weirder...
>> Yes, that's the other fun thing. Depending on which type of gates
>> you're using, they supply about 20 mA. But some of the LEDs I'm
>> looking it draw 40 mA...
>
> What you mean is, they draw *up to* 40 mA?
No idea. The spec sheet just says "forward current".
> How much they actually draw
> depends on what value series resistor you put in there. Using Ohm's law
> and the V-I curve of the LED from the datasheet you can work out exactly
> what resistor size you need for a given current.
Heh. And here I was thinking it's just a case of "if the voltage is less
than X, you don't need a resistor"...
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Hey, check this out:
http://www.maplin.co.uk/Media/PDFs/N24AZ.pdf
content of this datasheet means! :-D
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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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clipka wrote:
> Texas Instruments has a nice collection of 74xx data sheets in a single
> proper PDF file at <http://focus.ti.com/lit/ug/scyd013b/scyd013b.pdf>
> ("Digital Logic Pocket Data Book"), apparently covering all their
> current products (as of 2007) in the 74xx line, from single-gate 5-pin
> thingies like the 74x1G00 (single NAND gate) to 100-pin monsters like
> the 74x32543 (36-bit registered bus transceivers with 3-state outputs).
Ah, neat!
Most of the books I've seen only cover the 7400, 7401, 7402, etc., and
don't mention things like the 741G00. Could be useful to know. (Except
that Maplin don't seem to sell this stuff... *sigh*)
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