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### Topic: Logic gates without transistors? (Read 8982 times)previous topic - next topic

#### cjdelphi

##### Mar 23, 2013, 04:02 amLast Edit: Mar 23, 2013, 06:44 am by cjdelphi Reason: 1

i don't see how both diodes either and being triggered would not pull high to low? both diodes would do the same? i don't get how AND is made?

#### Osgeld

#1
##### Mar 23, 2013, 05:10 amLast Edit: Mar 23, 2013, 05:12 am by Osgeld Reason: 1

if 4 or 5 is low, then current is sinked thus logic = low

if both are high then current sinks though 1, thus logic = high

#### retrolefty

#2
##### Mar 23, 2013, 05:16 amLast Edit: Mar 23, 2013, 05:19 am by retrolefty Reason: 1
Are you referring to the classic wired-AND gate using two diodes and a resistor pull-up?

Two high inputs equal a high output, either (or both) inputs low equals a low output, that statisfies the classic AND truth table.

Then you can contrast that with the classic wired-OR gate:

Lefty

#### cjdelphi

#3
##### Mar 23, 2013, 06:47 am
Sorry, the image was not showing....  what's going on with those 2 diodes and pull up, so there's always 9v high ...

I would presume either not both but either of the diodes would pull it ground? not both like an AND logic calculation

#### retrolefty

#4
##### Mar 23, 2013, 06:51 am

Sorry, the image was not showing....  what's going on with those 2 diodes and pull up, so there's always 9v high ...

I would presume either not both but either of the diodes would pull it ground? not both like an AND logic calculation

You don't seem to understand how a truth table works. Just map out the four possible input voltage combinations (low-low, low-high, high-low, high-high) that are possible and the resulting voltage output for each of the conditions and see if you can't spot the AND function after all.

Lefty

#### cjdelphi

#5
##### Mar 23, 2013, 07:00 amLast Edit: Mar 23, 2013, 07:09 am by CrossRoads Reason: 1
[path deleted - seems to have been hijacked by a semi-porn site - Moderator]

if pin 11 and pin 14 are low (B and C of the binary output)

Reset triggers, but i don't understand why this circuit ONLY triggers off when both B and C are LOW, i don't understand
why would it not tigger off if B was low and C was high? B would provide a low signal to trigger reset?  how's the AND part
to this circuit working via the 2 diodes and a pull up?

#6
##### Mar 23, 2013, 07:08 am
But it is an AND.  Look at the truth table:

In1  In2   Out
0     0       0
0     1       0
1     0       0
1     1       1

Thus Wired AND - both inputs must be High for output to be High.

If Both needed to be High to make the output Low, that would be a NAND -  if you were to take that output and invert it using a NPN collector follower,

In1  In2   Out
0     0       1
0     1       1
1     0       1
1     1       0

And there's Wired OR, with diodes flipped around - either input High makes outpt High:
In1  In2   Out
0     0       0
0     1       1
1     0       1
1     1       1

If you were that output and invert it using a NPN collector follower, then you have NOR:

In1  In2   Out
0     0       1
0     1       0
1     0       0
1     1       0

In all cases, the diode serve to isolate the inputs from each other.

If either but not both were to affect the output, that would be Exclsive OR, XOR:

In1  In2   Out
0     0       0
0     1       1
1     0       1
1     1       0

XOR/XNOR more difficult to pulloff with just diodes & resistors tho.

and inverted, XNOR

In1  In2   Out
0     0       1
0     1       0
1     0       0
1     1       1

Generally, Wired OR and  Wired AND are all that's needed - if any of several inputs go low, the output goes low - such as with a keypad, pressing any key to Gnd can be used to create an interrupt and tell the code to go scan the keypad.

If you want action only with 1 or the other, that's easier to mimic in software - similar example, with multiple keys pressed, scan the inputs, if more than 1 was pressed, ignore the result.
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

#### oric_dan

#7
##### Mar 23, 2013, 08:19 am
For historical reference, the diagram in reply #2 is probably about the very first AND
gate ever invented. Since you can't make a very good computer that way [ie, it loses
noise margin after about 3 gates in series], they added an NPN inverter to it's output,
and invented the modern computer world, starting from there.

#### Osgeld

#8
##### Mar 23, 2013, 08:25 am
yes, you can only do rather simple things with diode logic (though its fun to mess with)

#### westfw

#9
##### Mar 23, 2013, 08:35 am
Which came first, diode logic or relay logic?

#### Osgeld

#10
##### Mar 23, 2013, 08:42 am
I would guess relay as it was introduced into practical application about 60 years before the diode (in practical application)

#11
##### Mar 23, 2013, 08:54 am
I had a calculator once that was entirely diode logic, there must have been 1000s of them plus some magnetic core memory as well and to top it off a nixie display.

I hope I've still got it somewhere.

As for relay logic, we built an entire runway lighting system using relay logic, flip flops, NAND, AND, OR gates etc were each a large PCB. All at 48V IIRC.

______
Rob
Rob Gray aka the GRAYnomad www.robgray.com

#### runaway_pancake

#12
##### Mar 23, 2013, 12:22 pm

Which came first, diode logic or relay logic?

http://www.thocp.net/biographies/shannon_claude.htm
"Who is like unto the beast? who is able to make war with him?"
When all else fails, check your wiring!

#### oric_dan

#13
##### Mar 23, 2013, 08:26 pm

Which came first, diode logic or relay logic?

Yeah, I knew someone would bring this up, ;-).

#14