Diode-Resistor AND gate question

Without the resistor, when both diodes go to a logical "1", there would be nothing to pull the output to a logical "1" (or +5 volts in this case).

Without the resistor, how much current would you get through one of the diodes when the inputis at 0V?

We used to call this MML or Mickey Mouse Logic. Handy for CMOS (no loading) and slow signals like door open switches. gg

with

Yeah. Besides, how didja come up with this "AND Gate" stuff? ;-)

The best way for you to understand the function of the resistor is to add a model of the load of the logical inputs that this and gate drives.

If they are cmos logic, they look something like capacitance ot the two supply rails. In that case, what is the source of charging current to pump these capacitors positive when the diodes are reverse biased?

If the load is TTL, each load gate contains some internal pull up resistance since the inputs are current operated (the input is a logic zero when you pull current out pf the input till the input voltage falls below about 1.2 volts). In that case, the diode pull up resistance may be redundant.

So the answer is, it depends...

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Didja hear that Michael Eisner is going to be leaving Disney?

Because it's a Mickey Mouse Operation. ;-)

On Monday 20 September 2004 06:24 pm, Hal Murray did deign to grace us with the following:

That's simple. All of it. ;-)

Why did you give us this url ?

To show a totally faulty way of doing it?

Look at the first circuit on that web page. If both inputs are high there is no problem, the output is high, it is directly connected to the plus 6 Volt power supply.

If input A goes low, what happens?

Unless it can take out the power supply and kill it, nothing happens to the output. It is still directly connected to the plus 6 Volt power supply.

If the input to A is made up of a switch to ground it will burn the diode, or if a very sturdy diode is used, the power supply will burn.

The link shows an "OR" gate. There seems to be a disconnect in the definition.

It is possible to have an "AND" gate or an "OR" gate using the same components.

There is also the possibility that there may be some confusion in the definition of a 1 and 0. Let us assume a 1 = positive voltage above 1V. 0 = less than 1V.

An "AND" for positive logic has a pull up to VCC and the cathodes of the diodes connected so that pulling the anode of either diode to a "LOW" (GND by definition) will give one diode drop (nominal 0.7V) at the output. GND + (nominal 0.7V)

AN "OR" for positive logic (as shown in the link,) has the resistor pulling down, and the diodes connected with the cathodes common to the resistor. Pulling the Anode of either diode to VCC results in an output of VCC - (Nominal

Is that any clearer?

No, it doesn't. If anything it is an always gate, the output is always high because it is directly connected to the plus 6 Volt supply.

How could it ever be anything but high, unless the power supply is turned off, or breaks down?

No. And I can't believe that you are trying to defend that circuit as anything but a big mistake.

Don't you see that the output is directly connected to plus 6 Volt?

Compare how the connection to plus 6 Volt is drawn in the the following circuits. The connection is drawn exactly alike in the first circuit, so there can be no doubt that the output is connected to plus 6 Volt.

Ah-ha! I see what you mean. I thought it referred to the design maximum voltage. But IF it is actually a power supply (battery?) it is really wired wrong. But since it was an educational web site, perhaps the questions should be "WHAT is WRONG with this picture?"

This web page is obviously not intended to show examples of faulty circuits, so it is simply a mistake.

So I was too subtle?

Maybe just a bit too fast, from being wrong to being subtle.

Remember that we are writing in an international newsgroup, where you cannot be too subtle without a big risk of being misunderstood. So we need to establish what is right and wrong clearly first, and then you can be subtle as much as you like.

Dark

Owww, you get a howler for that one!!

I was not wrong, the web site was! Remember the diagram was NOT mine. I did not refer to it other than to say it was connected as an "OR" which is is. It has as you pointed out, a fatal flaw. What I wrote was entirely correct.

Don't blame me if you didn't understand. English IS the common language here.

On Monday 20 September 2004 10:37 pm, Watson A.Name - "Watt Sun, the Dark Remover" did deign to grace us with the following:

I heard that Mickey wants to divorce Minnie on the grounds of insanity because she's f****ng Goofy.

Cheers! Rich