High(low) impedances by TTl logic

The discussion revolves around the concept of high and low impedances in TTL (Transistor-Transistor Logic) logic circuits, exploring their implications for inputs and outputs, as well as the potential consequences of connecting various components to these ports. The scope includes technical explanations and conceptual clarifications regarding TTL logic and its comparison to CMOS technology.

Discussion Character

Technical explanation
Conceptual clarification
Debate/contested

Main Points Raised

Some participants explain that TTL outputs are low impedance, allowing one unit to drive multiple inputs without overloading, while inputs are high impedance, drawing little current.
Others describe the operation of TTL outputs in high and low states, detailing how current flows and the implications for connected devices.
One participant cautions against connecting multiple TTL outputs together due to the risk of creating a short circuit, while suggesting alternatives like open-collector outputs for specific applications.
Another participant expresses skepticism about the relevance of TTL in modern applications, arguing that CMOS technology offers better performance, lower power consumption, and more stable threshold voltages.
Some participants note the complexity of understanding TTL impedances, emphasizing that they are non-linear and vary with voltage levels, which complicates the discussion.
One participant questions the clarity of the original question posed about TTL impedances, suggesting that a deeper understanding of transistor behavior is necessary for meaningful answers.

Areas of Agreement / Disagreement

Participants express differing views on the relevance and complexity of TTL technology compared to CMOS. There is no consensus on the necessity of understanding TTL impedances, with some arguing for its complexity and others questioning its practicality in current contexts.

Contextual Notes

Participants highlight limitations in understanding TTL impedances due to their non-linear nature and dependence on various conditions, suggesting that a thorough study of transistor behavior is essential for clarity.

Messages 11 Reaction score 0

Can someone tell me what does high or low impedances mean for the inputs and outputs for TTL logic,and how you can see that a port is high(low) impedant. And what if you connect other things to such ports?

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Kenneth Mann Messages 424 Reaction score 3

Impedance consequences

The typical TTL device output consists of two (NPN) transistors, connected from power to ground, with the emitter of the "upper" transistor going (through a diode) to the collector of the "lower" transistor, and its emitter to ground. The collector of the "upper" transistor is connected through a very small resistor, to the power (Vcc). The "output" is generally between the diode and the "lower" transistor. This output is "low impedance", which allows one unit to drive the inputs of several devices (fan-out). Those inputs are of relatively "high impedance", allowing several of them to be driven by a single output without overloading it.

When the output is in a "high" state ("1", "ON"), the upper transistor is turned ON and the lower transistor turned OFF. This means that current flow is from Vcc, through the output, to the input(s) of the following device(s), to ground. These inputs are of moderately high impedance, so that they don't draw an especially large current. In this condition (ON), the inputs serve as current sinks, and since they draw little current, several can be driven by a single output. The result is a "1" voltage from the output to the connected inputs.

When the output goes to a "low" state ("0", "OFF"), the upper transistor is turned OFF and the lower transistor turned ON. This means that current flow is now inward via the output, and goes to ground through the lower transistor. That current now must come from the inputs that follow (via their Vcc's). Again, since the inputs are of relatively high impedance, they will not overload the output that is connected to them, even though that output is almost a dead short to ground. (And, it doesn't take much current to control the inputs.) The result in this case, is a "0" from the output onto the connected inputs.

This type of output is referred to as a "Totem Pole" output. There is one caution in this design configuration; the low impedance of the output means that two such outputs must never be connected together. To understand this, consider the case in which two are connected and one goes to "ON" while the other goes to "OFF". The output that is ON is connected directly to Vcc, while the output that is OFF is connected directly to ground, creating a near dead short, and resulting (usually within minutes) in barbecued IC. (Besides, the logic may also be wrong, if not planned for.)

There are, however, cases in which we want to be able to connect outputs together. One such condition occurrs when it is desired to create a "wire-OR" case (which I suspect is rarely used these days). This is simply the case in which, rather than using an OR gate, outputs are tied directly together. I don't recommend it; it sometimes confuses the logic - mainly because it is rarely used. It can definitely create a circuit tracing nightmare (An invisible OR-gate) for a technician who must do maintenance. To do this, ICs with "Open-Collector" outputs are used (along with pull-up resistors).

A more common case comes up when it is desired to create a "Bus". This entity, which is very common in computer design and with similar units, simply allows an output to be "disabled" by putting that output into a "high-impedance" state. Several outputs may be connected each line in a bus, however at any time, only one can be enabled, thus keeping our rule intact. To accomplish this, we use what are called "Tri-State" gates, etc. These are simply IC devices, each of which has an input that allows its output to be disabled. All this input does is turn off Both transistors in the output of that device (no matter what the other inputs to that device are doing). As a result, if we follow our rule, and disable the outputs of all ICs that go to the bus line except the one chosen to be active, it will be the only one seen by the bus line, and there will be no conflicts. The high impedance output allows an IC to be 'hidden'. Then when we want it, we simply enable its output, and disable all the rest. I hope that this is understandable.

Messages 283 Reaction score 0

Cant really understand why anybody would wish to look at TTL these days when Cmos does a far better job and is far more understandable. Threshold voltages in TTL are poorly defined , temperature dependent , not at 1/2 supply , the technology takes gobs of current most wasted , it's only claim is speed and yet at high speed it is sometimes hard to apply due to the levels of current switched. High speed Cmos wastes very lttle power , has thresholds which even if not specified are within a few % of 1/2 supply giving the greatest noise margin , supplies do not have to be accurate -- in undefined circumstances you can often operate at 1 volt at reduced speed , and today the speed is such that most casual users would never use it. There are many adherents to TTL -- but it was a tecnology which grew out of the Bipolar transistor era when the King was the Opamp , also the initial experiments with MOS tecnology ( nmos ,pmos, resulted in VERY hard to understand and design circuits) But that ceased with CMOS --- today no-one would consider making a microprocessor in any thing but Cmos -- except under extrordinary circumstances such as the extreme of speed and power be damned. Since processors reach speeds of ~ 4 Ghz I would suggest that most mere mortals are not even equipped with the know how as to use it . You cannot build Bipolar devices with the dimensions of a simple Cmos device they are just too complex for that ( with hidden problems ) . Just a little while ago I had made some comment on a Forum about feature sizes in IC Chips , and he came back with a comment that I was full of SHT and that feature sizes were only 3 microns , in fact feature sizes are at 20 nMeters in some special applications --- He is also an advocate of TTL . I do not know what problem you are dealing with Gotilio -- because you did not say if it's homework -- then I guess you have to give an answer and the other contributor gave a good answer -- if it's by choice , then I would say ditch TTL it is an extremely limited and old technology . Ray.

Messages 11 Reaction score 0

It's a chapter in a book that I have to study,but I dindn't understand the part about the impedances. thanks for the explanation!

Messages 283 Reaction score 0

Exactly -- and I wonder why -- the real truth about impeances in TTL is extremely complex -- they change value for any level of voltage applied -- they are totally non-linear -- so the answers can only be given in the simplest of cases -- so I think you can see why I question why anybody would ask you this --- to really understand this topic you would have to study the precise action of transistors ( both npn and pnp ) under a variety if bias conditions -- and not just at DC but at speed . To me the way you pose the question it is unanswerable -- and I question whether the question poser knows what the heck he is talking about . I think that you need clarification about the question , impedance as such is usually considered as some simple linear proerty of a linear circuit -- including inductance capacitance and resistance , TTL hardly qualifies . Ray.

Kenneth Mann Messages 424 Reaction score 3 rayjohn01 said:

There is and has for years, been a generally accepted practice of using the term "impedance" with respect to digital circuitry in its "English" context as that which 'impedes' the flow of current, and thus defines the level of an output signal. Granted, it does not fall within the general linear design context of resistance, capacitance or inductance, but then it is not needed in that context. If you have a better term, I'm sure the world will welcome it.

In the mean time, I propose a compromise: I won't become a TTL advocate if you'll not put forth that which simply confuses people.

With great respect, KM

Messages 283 Reaction score 0

Then with great respect you will realize that the sink impedance of TTL is dependent on the current sunk as it is a saturation resistance unless arranged to be caught by some diode arrangement. And the source impedance ( at a transistor emmitter is dependent on that transistor feed arrangements and its beta. Generally then you will not talk of impedances but sink or source voltages under load. Ray.

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