Go Down

Topic: Logic gates without transistors? (Read 2155 times) previous topic - next topic

cjdelphi

Mar 23, 2013, 04:02 am Last 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 am Last Edit: Mar 23, 2013, 05:12 am by Osgeld Reason: 1
I cant see what your talking about but



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
http://arduino.cc/forum/index.php?action=unread;boards=2,3,4,5,67,6,7,8,9,10,11,66,12,13,15,14,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,86,87,89,1;ALL

retrolefty

#2
Mar 23, 2013, 05:16 am Last 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?

http://upload.wikimedia.org/wikipedia/commons/thumb/9/9d/WiredAND.svg/220px-WiredAND.svg.png

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:

http://upload.wikimedia.org/wikipedia/en/8/82/WiredOR.JPG

Lefty

cjdelphi

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


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 am Last 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?


CrossRoads

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

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.

http://www.google.com/search?&site=imghp&tbm=isch&source=hp&biw=1033&bih=858&q=diode+transistor+logic

Osgeld

yes, you can only do rather simple things with diode logic (though its fun to mess with)
http://arduino.cc/forum/index.php?action=unread;boards=2,3,4,5,67,6,7,8,9,10,11,66,12,13,15,14,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,86,87,89,1;ALL

westfw

Which came first, diode logic or relay logic?

Osgeld

I would guess relay as it was introduced into practical application about 60 years before the diode (in practical application)
http://arduino.cc/forum/index.php?action=unread;boards=2,3,4,5,67,6,7,8,9,10,11,66,12,13,15,14,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,86,87,89,1;ALL

Graynomad

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


Which came first, diode logic or relay logic?


http://www.thocp.net/biographies/shannon_claude.htm
"Hello, I must be going..."
"You gotta fight -- for your right -- to party!"
Don't react - Read.
"Who is like unto the beast? who is able to make war with him?"

oric_dan


Which came first, diode logic or relay logic?

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

Krupski


I cant see what your talking about but



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


That diagram reminds me of the logic gates used in the Apollo Guidance Computer.....
Gentlemen may prefer Blondes, but Real Men prefer Redheads!

Go Up