Talk:Diode logic

message: How about some citations? Cbdorsett 14:25, 14 November 2006 (UTC)
thread_title:  Citations 

message: thumb|right|300px|Revealing the basic idea behind the diode AND logic gate by applying De Morgan's laws on the whiteboard
Diode logic gates were introduced in 50's; I met them for the first time in the late 60's when I went to technical school. I remember I understood the explicit diode OR gate but I was in a maze by this so odd, exotic and absurd diode AND gate... I asked myself many questions as: "Why the diodes were back to front? Why the resistor was connected to +V instead to ground? Why there was no input current when the input voltage was high? And why there was input current when the input voltage was low? But why the current went out of the diodes and went in the input source? What was this absurd? Why it was impossible to make inverting diode gate? Why AND gate was supplied by an additional voltage source while OR gate had not such a source?...and so on...and so on...
It is interesting fact that many years later I continued to not understand what the basic idea behind diode AND was. I perfectly knew the circuit but I didn't understand it... I "saw the trees but I didn't see the forest for them"... And what is more interesting I didn't see any reasonable explanations of this humble circuit somewhere to answer my childhood questions... instead I was seeing thousands of "not seeing the forest" explanations...three months ago), during the laboratory exercises with my students on Digital circuits, I began realizing the great idea behind these legendary diode circuits. I was amazed how simple it was. I shared and considered my insights with these young people and they approved of them; they admired this "elegant simplicity". We decided to share the truth about diode AND gates with wikipedians and Wikipedia readers. That is why, I have written these explanations in the article. In addition, I have listed (in bold) and explained (in italic) below the key points of understanding diode AND gates extracted from the main article. 
Is it an original research? I have already explained it in the article about Miller theorem. Yes, it is... but in the common sense of this term, not in the specific Wikipedia OR sense. Really, I have exposed my insights about this circuit solution but I have managed to reduce them to extremely clear, evident and simple explanations  that should not be treated as original research in the Wikipedia OR sense; they do not need to be referenced. If you do not agree with me, please insert your comments below the items. Circuit dreamer (talk, contribs, email) 18:10, 15 January 2011 (UTC)
thread_title:  The mystery of diode AND gate 

message: * In logic gates, logical functions are performed by parallel or series connected switches controlled by input logical variables. It is well-known truth, see also , , , , etc.
* In diode logic, the switches are implemented by diodes: when forward biased, a diode switch is closed; when backward biased, the switch is open. It is well-known truth, see also , , , etc.
* OR logic gates are implemented by parallel connected switches. It is well-known truth, see the sources above.
* In a diode OR logic gate, diode switches are connected so that they act as normally open switches: if the input voltage is high (input logical 1), the according diode switch is closed; if the input voltage is low (input logical 0), the diode switch is open. It is obvious, there is no problem to understand it in the case of a diode OR gate.
* Diode OR logic gates consume current from input sources with high voltages and they do not inject current to input sources with low voltages; so, they belong to so-called current-sourcing logic. We will discuss this phenomenon thoroughly below; for now see . See also the text I have extracted from this forum (we will use it when explaining diode AND gate):
:"...If you take one of the simple power source, a battery. If the battery is 9V, and you put a load across the two poles, then the current flows through the load. The current will go in one direction. One pole could be referenced as GND, the other as Vcc. Now, take two batteries - one is the above 9V battery, the other is a 12V battery. You connect the negative poles together, let it be the GND reference. Then, if you connect a load between the two positive poles (one pin on 9V battery + pole, the other pin on the 12V battery + pole), then, current will flow, but will go in the *other* direction, in the 9V battery, than in the first case. When the 9V battery provides current, which goes out of 9V (using conventional current), the battery is said to *source* current. When the battery is absorbing current, it is said to *sink* current. Source and Sink really only does tell in which direction the conventional current flow..." (I have corrected only some spelling mistakes)
* AND logic gates should be implemented by series connected switching elements. It is a well-known truth, see the sources above.
* In contrast to transistors, diodes are two-terminal switching elements, in which the input and output are not separated; they are the same. As a result, series connected diode switches cannot be driven by grounded input voltage sources. The 4-terminal relays are the most suitable electrically-controlled switches since their input and output circuits are completely separated. The 3-terminal transistors can still be used as 4-terminal switches by grounding the emitter (source); thus, the input and output circuits have a common point (the ground) but they do not interfere in each other's operation. The situation with 2-terminal diodes is the worst - here the input source and the load are connected in the same circuit and there is no way to connect the diodes in series and to drive them by grounded voltage sources.
*  To solve this problem, diode AND logic gates are constructed in the same manner as OR diode gates - by parallel connected diode switches (the same idea is used in the input stage of TTL logic gates where base-emitter junctions are connected in parallel). If you find it difficult to see the parallel connection, imagine that all the input voltages are low (i.e., all the inputs are connected to ground).
* In a diode AND logic gate, diode switches are connected so that they act as normally closed switches in respect to the input voltage: if the input voltage is high, the switch is open; if the input voltage is low, the switch is closed... To realize this idea,  the diodes are reverse connected and forward biased by an additional voltage source +V (a power supply) through the pull-up resistor R1. This is a key concept in understanding the diode AND logic gate; so, I will explain it thoroughly.
:The name of this circuit technique is biasing (it is not only electrical; it is a general idea that can be seen around us in this world). To be more precise, this is "biasing with the same but opposite to the input quantity" with the purpose to reverse the behavior of some element. In this arrangement, we apply not the genuine input voltage across the element; we apply the difference between the input and biasing (offset) voltage, i.e., we apply the inverted input voltage. As a result, the current flowing through the element is inverted as well.
:First, see the extracted forum text above that explains the same concept. Or imagine another problem: we have a relay with normally open contact but we need a relay with normally closed contact. What do we do? We can invert the relay behavior by connecting the one coil terminal to +V instead to ground. This means that we have inserted an additional voltage that is equal but opposite to the input voltage (traveling along the loop +V -> VIN -> coil). As a result of this biasing, when applying +V to the other coil terminal, the resulting voltage drop across the coil will be zero and no current will flow trough it; and v.v., when applying 0 to the coil terminal, the resulting voltage drop across the coil will be -V and current will flow trough it in opposite direction (+V -> coil -> input source -> ground). The relay with normally open contact has become a relay with normally closed contact. 
:We may see the same trick in a CMOS logic gate, in a PNP transistor stage or in any complementary transistor stage that are driven and supplied by positive voltages. In these circuits, the upper PMOS or PNP transistor is actually driven by negative input voltage (with respect to +V), which is the inverted positive input voltage. From this viewpoint, a diode AND gate with positive input voltages is actually a diode OR gate with negative input voltages.
* To obtain AND instead OR function according to De Morgan's laws, the input and output logical variables are inverted:
:Y = NOT ((NOT (X1) OR NOT (X2)) = NOT (NOT (X1 AND X2)) = X1 AND X2.
:Finally, we have only to describe the diode AND structure by logical expressions and to simplify them by applying De Morgan's laws. There are a lot of sources about such transformations (e.g., ).
* Therefore, the diode AND logic gate is a modified diode OR logic gate: the diode AND gate is actually a diode OR gate with inverted inputs and output. This final conclusion evolves from the considerations above.
* As diode AND logic gates do not consume current from input sources with high voltages and they inject current to input sources with low voltages, they belong to so-called current-sinking logic. We have just explained this phenomenon above. There are also a lot of sources about current-sinking logic.
Circuit dreamer (talk, contribs, email) 18:10, 15 January 2011 (UTC)
thread_title:  Key points of explaining diode AND gate 

message: I have a question and a few comments.
1. When was DL invented? Is there a specific date?
2. "output logical 1" and similar expressions in the descriptions of OR and AND operations are repetitive. Stating what 0 and 1 mean once is sufficient.
3. "Powerful diode OR circuits are used in the simple UPS to switch between the main and the standby supply sources." I believe it should sound more like "Powerful diode OR circuits are used in simple UPSs to switch between the mains and the standby supply sources."
ICE77 (talk) 22:01, 16 February 2011 (UTC)
thread_title:  Question and comments 

message: With respect to the name "Mickey Mouse logic", whose addition  reverted, the person from whom I first heard the term told me it came from some CMOS cookbook or TTL cookbook by Don Lancaster. --Damian Yerrick (talk | stalk) 16:53, 26 September 2011 (UTC)
replies: text: The problem is, that's about the only place you find that term used (or, people quoting Lancaster). It's not a widespread phrase. --Wtshymanski (talk) 17:49, 26 September 2011 (UTC)
thread_title:  Mickey Mouse logic 

message: It should be noted that this article has been significantly changed as of August 20, 2014.  The purpose is to correct the theory that a diode is a switch to the p-n diode theory that a diode is a passive nonlinear impedance with a low impedance and low voltage in the forward direction and a very high impedance in the reverse.Thingmaker (talk) 14:10, 20 August 2014 (UTC)
replies: text: It's OK but not mandatory  to describe changes you've done; the edit history is there for all to check. I think the reason that textbooks use "switch" is as a shorthand for "nonlinear passive...etc." - which is mouthful to say and is just one way of thinking about a switch anyway. --Wtshymanski (talk) 16:16, 20 August 2014 (UTC)
thread_title: Note

message: "But why do we need to pull down to -6 volts?" says our hypothetical bright 12-year-old reader, "Doesn't that make the output of the gate -6 V for logic 0, not 0 V ? "  And if we have ideal diodes anyway, we don't need the pull-down to a negative voltage. Once we explain the topology of the ideal gates, then we can get to the ugly reality of non-ideal devices with finite switching speed and leakage current, capacitance, minority carrier storage, resistance (of the lossy variety, forsooth) and so forth. A description of how these non-idealities militate against use of diode logic in complex systems would be quite encyclopedia worthy. --Wtshymanski (talk) 02:11, 24 August 2014 (UTC)
replies: text: Are you asking a hypothetical question (on behalf of your 12-year-old) or do you really not understand?  I suspect that you do understand, but anyway, the logic 0 output is not -6 V, it is 0 V (less one diode forward drop), assuming that the gate input is a true 0 V.  Even with ideal diodes, the pull down is still doing something.  Without it, both sides of the diode are connected to 0 V, the diode is sitting right on the origin of its I-V curve, and it is thus undefined whether it is turned on or not.  It is thus undefined whether the output is connected to the low impedance source (via the diode) or through the load resistor to the 0 V rail.  The pull down guarnatees low impedance.  I think it is good to have real circuits in the article.  Having said all that, I agree with you that basic circuits should be presented first, free of all extraneous items like output transistors.  If for no other reason than book sources invariably show the simple circuits in dealing with this subject.  Otherwise, this page is in danger of covering the same ground as diode-transistor logic. SpinningSpark 08:10, 24 August 2014 (UTC)
replies: text: I questioned the need for DL myself.  I see no problem with it existing but it makes more sense when presented with a practical application such as DTL.  By itself it has very limited application.  Thanks Sparks for providing your response.  Adding more to the answer for the 12 year old, if RD returns to ground it has no output drive at the 0 level which would make it even more useless.  The 0 input allows the input diodes to limit the output to 0.  If the resistors return to 0 volts for the OR and +6 volts for the AND then neither could drive the other since they would have no output current drive at those levels.Thingmaker (talk) 11:12, 24 August 2014 (UTC)
replies: text: If it's not in the article, it ought to be. Why -6 volts? This is the problem we get into when we start mixing our "ideal" diodes with real world circuits. Since we're assuming ideal diodes with zero resistance and no voltage drop, there seems to be no motivation for the pull-down; we don't need "output drive" since that's only necessary for non-ideal diodes. But as the reader can see from the 608 gate schematics, real diodes need a pull down. --Wtshymanski (talk) 13:56, 24 August 2014 (UTC)
replies: text: As I said above, the pull down still makes sense with ideal diodes. SpinningSpark 14:50, 24 August 2014 (UTC)
replies: text: Wtshymanski are you suggesting we should have a section that discusses the non ideal diode and its implications to diode logic design.  I am working on a section I hope to add to the p-n diode article that addresses the p-n diode from a circuit design standpoint (connecting the equations to real life).  I could probably write a condensed version for the diode logic article or wait and simple provide a link to the final version, it you allow it to stand.Thingmaker (talk) 14:44, 25 August 2014 (UTC)
replies: text: Ideal diodes still must work in a real world of electronics. They have a zero forward voltage and zero reverse current.  They still need a resistor to keep the output from drifting away and it still needs output drive if it has any value except in a lab experiment which would still need an output resistor.  I believe even with an ideal diode the designer would choose voltages and resistors.  I have corrected some of the changes to reflect this fact.Thingmaker (talk) 21:59, 25 August 2014 (UTC)
thread_title: Ideal diodes? And logic levels?

message: A negative logic AND is not a NOT AND - it's an OR (with negative logic levels). The logic function produced by a set of diodes with common anodes depends on how you assign logic 1 and 0 to voltage levels.  Getting a NOT AND without changing the representations of 1 and 0 requires an inverter, which we don't have in pure diode logic. I suppose there could well be circuits where we use positive logic definitions for the first column of diodes, switch to negative logic definitions for the second column, and so on...but I don't think I'd care to present an example. --Wtshymanski (talk) 20:50, 25 August 2014 (UTC)
replies: text: NAND is not a negative AND it is a Negated AND or an AND Invert. I probably misused the NOT.  I think I was trying to preserve the "NOT NOT" logic from the previous article.  I believe NOT is reserved for the naming of inverted signal names such as A and B going into a  NAND gate results in an output of NOT (A AND B) or NOT A OR NOT B.  An AND is a negative OR and an OR is a negative AND.  A logic designer does not need an inverter to choose to use negative logic.  He can name and change the logic levels to suite his needs.  Today with logic families providing every logic function anyone would need it is not so important.  There was a whole generation of logic designers that did their job using only AND Inverts or only OR Inverts. It was important that the AND Invert was also a negative OR Invert or they would not have had an OR function.  They just called it what they needed.Thingmaker (talk) 23:27, 25 August 2014 (UTC)
replies: text: This has to be clear in the article; changing the representation of logic 1 and 0, for the exact same circuit, changes it from AND to OR. Got to be careful to explain that the difference is only a change in the point of view of the designer for that stage. --Wtshymanski (talk) 00:55, 26 August 2014 (UTC)
thread_title: NOT AND is not NAND

message: Does the 550 switching diodes for the clock in the Diode–transistor logic article prove that the PN diode is a switch?  I have the explanation of how a diode switch works.  Click on this User talk:Thingmaker#Reference point.  The simple PN diode chip puts integrated circuits to shame.  Possibly instead of looking for published documentation of “someone” calling the diode a switch one might just read p-n junction or p-n diode and see if they can find how the pn diode is a switch.Thingmaker (talk) 20:40, 27 August 2014 (UTC)
thread_title: A diode switch

message: Why did you remove the note about handmade transistors as unsuppoted?  This book says that making handmade transistors ended in 1952, which means they were likely not used in the IBM 608 and the note was probably correct that they were used in the IBM 360 and 370 (and since that is the subject of the book, makes it even more likely). That has now left the article saying something almost certainly not true. SpinningSpark 19:03, 17 August 2019 (UTC)
replies: text: I have trimmed this down further to hopefully eliminate the possibility of publishing something wrong. I removed the note about IBM 360 and 370 because there wasn't enough context to understand what that was trying to say and no reference cited to help me. I tried to clarify what was left and it is possible I changed meaning and made things worse. I have now removed all the details about components because of WP:V issues and because this is not important information in this context (would be good the articles about 608, 360 or 370). ~Kvng (talk) 16:18, 18 August 2019 (UTC)
replies: text: Not so sure this doesn't belong.  IBM handmaking transistors by modifying germanium diodes seems to still be within the context of "diode logic".  It should certainly be somewhere on Wikipedia and an overview article seems better to me than spreading the information over several individual products.  Besides, the source I found was not specific on what products it was used in.  We can infer that from the dates given, but that would be WP:SYNTH. SpinningSpark 12:12, 19 August 2019 (UTC)
replies: text: I don't think we should restore what was there but I am not opposed to putting in something about this that's verifiable and that readers can understand. ~Kvng (talk) 14:16, 21 August 2019 (UTC)
replies: text:  Did you read my addition this morning? Are you ok with that? SpinningSpark 16:03, 21 August 2019 (UTC)
replies: text: That is clear and verifiable. It doesn't really make a connection to the section topic (Early diode logic with transistor inverter) but this section is already disjointed. Hopefully someone will be able to tie all this information into a coherent narrative at some point. ~Kvng (talk) 16:45, 24 August 2019 (UTC)
thread_title:  Handmade transistors