On 5/19/2010 7:31 AM, scott wrote:

> 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.


There's even a name for that trick: Charlieplexing.

The 3 state had to be simulated by switched, but here's a circuit for 
the Falstad simulator (http://www.falstad.com/circuit/):

$ 1 5.0E-6 10.20027730826997 50 5.0 50
L 224 176 176 176 0 1 false 5.0 0.0
L 224 224 176 224 0 1 false 5.0 0.0
162 352 176 352 224 1 2.1024259 1.0 0.0 0.0
162 384 224 384 176 1 2.1024259 1.0 0.0 0.0
w 352 176 336 176 0
w 384 176 368 176 0
w 352 224 336 224 0
w 384 224 352 224 0
w 368 176 352 176 0
L 224 272 176 272 0 1 false 5.0 0.0
w 384 272 352 272 0
162 416 176 416 272 1 2.1024259 1.0 0.0 0.0
w 416 272 448 272 0
w 416 176 448 176 0
162 448 272 448 176 1 2.1024259 1.0 0.0 0.0
w 416 176 384 176 0
w 352 272 336 272 0
162 384 272 384 224 1 2.1024259 1.0 0.0 0.0
s 272 176 336 176 0 0 false
s 272 224 336 224 0 0 false
s 272 272 336 272 0 0 false
w 384 272 416 272 0
162 352 224 352 272 1 2.1024259 1.0 0.0 0.0
r 272 176 224 176 0 100.0
r 272 224 224 224 0 100.0
r 272 272 224 272 0 100.0


-- 
~Mike

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On 5/19/2010 8:13 AM, 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>

Ah, sweet! Thanks for that link! <Bookmarked>


-- 
~Mike

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scott wrote:
> 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.

Yep. Indeed, there are a lot of busses that are organized such that you have 
an output to it and an input, and you start putting your address on the bus 
bit by bit until you see a bit different than your address. That means 
someone else put on a 0 while you put on a 1 (assuming floating high). Every 
packet gets delivered with no contention, with lower addresses getting 
priority.  It's a kind of cool way to work it.

-- 
Darren New, San Diego CA, USA (PST)
    Ada - the programming language trying to avoid
    you literally shooting yourself in the foot.

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Invisible wrote:
> that particular LED doesn't do anything now.

It's a DED component. It emits dark.

-- 
Darren New, San Diego CA, USA (PST)
    Ada - the programming language trying to avoid
    you literally shooting yourself in the foot.

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Mike Raiford wrote:
> There's even a name for that trick: Charlieplexing.

Cute. Is there a name for when you have a red LED in one direction and a 
green LED in the other and you use AC to make it yellow? I want to use that 
around my colleges and sound like I know what I'm talking about. ;-)

-- 
Darren New, San Diego CA, USA (PST)
    Ada - the programming language trying to avoid
    you literally shooting yourself in the foot.

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>> that particular LED doesn't do anything now.
> 
> It's a DED component. It emits dark.

Hey, so long as it doesn't start emitting negative light...

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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Darren New wrote:

> Cute. Is there a name for when you have a red LED in one direction and a 
> green LED in the other and you use AC to make it yellow? I want to use 
> that around my colleges and sound like I know what I'm talking about. ;-)

Most people call that a tri-colour LED - but sometimes that term is used 
to refer to an LED with three different-coloured dies inside it (and 
hence 3 pins). Which is obviously a different component.

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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scott wrote:
>> 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.

OK, so if I'm understanding this right... The 74126 (go look it up) 
contains 4 "buffers" - gates who's output is logically equal to their 
input. But each gate also has an "enable" pin. When the enable pin is 
high, the gate works like normal. When the enable pin goes low, it's 
like the output pin isn't connected to anything any more.

And it seems that this allows you to connect several outputs together, 
forming a kind of wired-OR configuration, provided that at all times 
only one gate is "enabled".

Is that right?

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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Am 19.05.2010 16:16, schrieb Invisible:
> While we're here, does anybody comprehend what VCC, VSS, VDD and related
> symbols mean?

According to JEDEC,

VBB = substrate power voltage (bias voltage), a voltage < VSS/GND.

VCC = logic power voltage, serving as a reference for high signal levels 
(often sharing a common rail with VDD, in which case they may be used 
interchangeably).

VCCQ = output stage logic power voltage, separate (positive) power rail 
to drive output loads.

VDD = drain power voltage. separate (positive) power rail to drive the 
chip's internals.

VDDQ = output stage drain power voltage. Effectively the same as VCCQ.

VEE = emitter power voltage. Only used for ECL devices.

VHH = special function enable voltage. An extra-high voltage to enable 
certain special functions of a device.

VPP = programming voltage. An extra-high voltage to supply the power for 
the programming function of a device (e.g. EPROM)

VREF = reference power supply, serving as a special reference voltage, 
e.g. for ADC inputs.

VSS = GND = ground reference / source power voltage, serving as both 
(negative) power rail and low signal level reference.

VSSQ = GNDQ = output stage source power voltage / ground reference, 
serving as both a (negative) power rail and low signal level reference 
specifically for output signals (should always be at the same level as 
VSS=GND).

(This is just a subset of all the symbols used; for instance, 
digital/analog devices such as programmable amplifiers or ADCs/DACs 
often use separate voltage rails for the analog signals.)

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clipka wrote:

> According to JEDEC,
> 
> VCC = logic power voltage, serving as a reference for high signal levels 
> (often sharing a common rail with VDD, in which case they may be used 
> interchangeably).
> 
> VDD = drain power voltage. separate (positive) power rail to drive the 
> chip's internals.
> 
> VSS = GND = ground reference / source power voltage, serving as both 
> (negative) power rail and low signal level reference.
> 
> VSSQ = GNDQ = output stage source power voltage / ground reference, 
> serving as both a (negative) power rail and low signal level reference 
> specifically for output signals (should always be at the same level as 
> VSS=GND).

Suddenly the picture becomes even more complex than you feared...

> (This is just a subset of all the symbols used; for instance, 
> digital/analog devices such as programmable amplifiers or ADCs/DACs 
> often use separate voltage rails for the analog signals.)

Thank god I don't have to deal with anything analogue yet! ;-)

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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