i have a project that i am tieing the outputs of a cd4081 AND gate together,
right now i have diodes at the outputs to be safe before i tie them together,
is this necessary??? also, the same question with the 4017 counter???
please let me know what would be best thing to do.
thanks carl

 

Using diodes is the correct way to tie gate outputs together. You don't want to tie the outputs directly unless you want to let the smoke out... and we all know smoke is what makes this stuff work! Ha!

Note that putting two diodes in series with the output of gates is typically referred to as a 'diode-or' configuration because because either of the gates can drive the output high.

An OR gate could also be used.

 

Hello,

You can only tie outputs together when these are open collector outputs.
For all other outputs you must use diodes or OR gates as tindel said.

Bertus

 

I disagree... excepting open collector outputs you can NEVER tie two outputs together, you need to combine them together in some sort of logical fashion following the logical dictates of your circuit.

If either output being high means that a high should be registered by the next gate, then yes, you can use diodes (plus a resistor) to create an OR gate. Or just use an OR gate.

If both outputs need to be high for the next gate to register that high... you need to use an AND gate. It may be possible to create an AND gate with the same diodes and resistor.

(Open collector outputs are an exception as they are by design a form of a partial gate.)

 

I've seen application notes from IC manufacturers who actually put logic gate outputs in parallel to increase the available current output. Thought it was strange but they did it. If the gates inside one package have matching switching characteristics there shouldn't be a problem.

 

As long as there will never be contention, except during short switching transients, you can tied gate outputs together to increase drive strength. Within an IC, this is the most common way of getting large increases in drive strength. For driving our long-lines, we had a standard cell that was 200 inverters in parallel.

But if the outputs might contend (some be driving HI while others are driving LO), then you have to do something to make this contention benign. One way to do that is to use open collector/drain outputs and another is to use diodes. Putting a diode in series with a logic output effectively turns it into a poor man's open collector/drain output because, depending on polarity, one of the output drivers can no longer influence the output on the other side of the diode. If the cathode is facing the common node, then then any of the gates can pull the common node HI but none of them can pull it LO, hence you need a pulldown resistor (or other load) and have created a wired-OR connection. If the anode is facing the common node, then the opposite is true and any output can pull the common node LO but none of them can pull it HI, hence you need a pullup resistor and have created a wired-AND connection.

 

ok,thanks all, i'll try to explain more. i'm using all 4 of the 2 input cd4081 AND gates, only
one gate will be on "1" at a time will the other three gates being off "0" pull my on "1" gate
low or cancel it out. i would try to breadboard this right now,i have the circuit working with
diodes, but i am worried about damaging the 4081, i'm very limited to electronic supplyers,
i have to go thruogh mail order.... minimum orders,shipping is a humbug. i would like to use a 4 input OR gate or make one with other OR gates but i don't have one at hand. so i used diodes to make one. the bottom line is i would like to eliminate the diodes to save space and diodes i allready have six on 4017 outputs that would like eliminate as well,i have the same question? i'm worried about damage to the chips. i hope this explain my situation.thanks all i'll keep checking in to see whats going on,thanks again...carl

 

If you tie two active gate outputs together and one is trying to pull HI and the other is trying to pull LO, then you don't know what the result will be. If you have enough information about the specs of the gates, you can make a pretty good guess, but even then you don't know how much better than the spec'ed performance each gate will be. About all you can be sure of is that each gate is going to be supplying/sinking more current than it should and you risk damaging or destroying one or more of them.

Even if, as in your case, you know that (at most) one will be HI and three will be LO, you don't know that the one pulling HI might not be strong enough to pull all three of the others far enough up in voltage to make the node too high to be firmly recognized as a LO.

Bottom line, do not ever allow active outputs to contend with each other (except for very short switching transients and even those should be avoided when possible) unless you are designing the output drivers specifically to permit that, which means designing and analyzing them as analog circuits, among other things.

 

thanks WBahn, i think i'll leave the diodes there ,they work fine. until i can order a "4072" four input OR gate its a dual gate i can use the other half in this circuit.this will take 8 diodes off and look i little better. thanks.....carl

 

Yeah, use the diodes as a temporary approach to let you test the circuit and verify that the logic is correct, then replace it with a true gate as soon as practicable. Using a real logic gate will not ony make things cleaner, but will almost certainly give you better and more reliable performance out of the box.