Diode-Resistor AND gate question

[Christopher Collins]

An AND gate can be made of two diodes and a resistor:

+5V
|
R
|
A -|<|--+---- OUT
|
B -|<|--+

Here, R is a resistor and the |<| thingies are small diodes.

As long as either of the inputs A and B is connected to ground
(logical 0),
the output will be a 0 as well (not exactly zero volts, since there's
a
voltage drop over the diode). If both inputs are open, or connected to
logical 1, the output will be a 1 as well. Hence, the output is the
logical
and of the inputs.

My question: What's the point of the resistor here? It seems to me
this could work without it.

Christopher

 

[Brian]

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).

"Christopher Collins" <christopher.m.collins@gmail.com> wrote in message
news:f4f3a83b.0409201410.370e25c@posting.google.com...

An AND gate can be made of two diodes and a resistor:

+5V
|
R
|
A -|<|--+---- OUT
|
B -|<|--+

Here, R is a resistor and the |<| thingies are small diodes.

As long as either of the inputs A and B is connected to ground
(logical 0),
the output will be a 0 as well (not exactly zero volts, since there's
a
voltage drop over the diode). If both inputs are open, or connected to
logical 1, the output will be a 1 as well. Hence, the output is the
logical
and of the inputs.

My question: What's the point of the resistor here? It seems to me
this could work without it.

Christopher

 

[Hal Murray]

An AND gate can be made of two diodes and a resistor:

+5V
|
R
|
A -|<|--+---- OUT
|
B -|<|--+

Here, R is a resistor and the |<| thingies are small diodes.

[snip]

My question: What's the point of the resistor here? It seems to me
this could work without it.

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

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[Rich Grise]

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

An AND gate can be made of two diodes and a resistor:

+5V
|
R
|
A -|<|--+---- OUT
|
B -|<|--+

Here, R is a resistor and the |<| thingies are small diodes.

[snip]

My question: What's the point of the resistor here? It seems to me
this could work without it.

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

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

 

[John Popelish]

Christopher Collins wrote:

An AND gate can be made of two diodes and a resistor:

+5V
|
R
|
A -|<|--+---- OUT
|
B -|<|--+

Here, R is a resistor and the |<| thingies are small diodes.

As long as either of the inputs A and B is connected to ground
(logical 0),
the output will be a 0 as well (not exactly zero volts, since there's
a
voltage drop over the diode). If both inputs are open, or connected to
logical 1, the output will be a 1 as well. Hence, the output is the
logical
and of the inputs.

My question: What's the point of the resistor here? It seems to me
this could work without it.

Christopher

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...
--
John Popelish

 

[Glenn Gundlach]

"Brian" <bellis350@comcast.net> wrote in message news:<Xc2dnd6w57-JytLcRVn-jA@comcast.com>...

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).

"Christopher Collins" <christopher.m.collins@gmail.com> wrote in message
news:f4f3a83b.0409201410.370e25c@posting.google.com...
An AND gate can be made of two diodes and a resistor:

+5V
|
R
|
A -|<|--+---- OUT
|
B -|<|--+

Here, R is a resistor and the |<| thingies are small diodes.

As long as either of the inputs A and B is connected to ground
(logical 0),
the output will be a 0 as well (not exactly zero volts, since there's
a
voltage drop over the diode). If both inputs are open, or connected to
logical 1, the output will be a 1 as well. Hence, the output is the
logical
and of the inputs.

My question: What's the point of the resistor here? It seems to me
this could work without it.

Christopher

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

 

[Watson A.Name - \"Watt Su]

"Glenn Gundlach" <stratus46@yahoo.com> wrote in message
news:acb22b57.0409202047.405aba8f@posting.google.com...

"Brian" <bellis350@comcast.net> wrote in message
news:<Xc2dnd6w57-JytLcRVn-jA@comcast.com>...
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).

"Christopher Collins" <christopher.m.collins@gmail.com> wrote in
message
news:f4f3a83b.0409201410.370e25c@posting.google.com...
An AND gate can be made of two diodes and a resistor:

+5V
|
R
|
A -|<|--+---- OUT
|
B -|<|--+

Here, R is a resistor and the |<| thingies are small diodes.

As long as either of the inputs A and B is connected to ground
(logical 0),
the output will be a 0 as well (not exactly zero volts, since
there's
a
voltage drop over the diode). If both inputs are open, or
connected to
logical 1, the output will be a 1 as well. Hence, the output is
the
logical
and of the inputs.

My question: What's the point of the resistor here? It seems
to me
this could work without it.

Christopher

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

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

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

 

[Rich Grise]

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

"Glenn Gundlach" <stratus46@yahoo.com> wrote in message

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

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

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

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

Cheers!
Rich