4 ms·

Note that for hobbyists RTL is still interesting when building a logic circuit from discrete components.

by irdc 3y ago

adrian_b 3y ago
While RTL is a possible choice, DTL circuits (diode-transistor logic) are both easier to design and more efficient from the POV of power consumption. DTL circuits can also be faster, especially when implemented with Schottky diodes, which should be the normal choice for them. DTL circuits are composed of transistors used as inverters and of diode circuits that are either minimum circuits a.k.a. AND or maximum circuits a.k.a. OR. When considered as min/max circuits instead of and/or, they can work even with a multi-level logic, not only with binary logic.
- amelius 3y ago
  Ok, but diodes are more expensive than resistors, and from what I can gather you also need more components in total.
  - mrob 3y ago
    You can get small signal Schottky diodes for less than 0.01USD each, even in small quantities. The bulk of the cost is going to be the boards and assembly.
    - amelius 3y ago
      Yeah, ok, but I suppose you need more components, so more space, so a bigger board and higher assembly cost.
      - adrian_b 3y ago
        No, DTL circuits do not have more components than RTL, they normally have much less components. A maximum or minimum circuit with diodes, (i.e. an OR or AND gate) has just one diode for each input and one resistor for the output. In most cases each AND or OR gate must be followed by one inverter, to provide amplification, i.e. to restore the logic levels to values compatible with an input. The complexity of the inverter depends on what kind of transistors are available and on what switching speed is desired, but sometimes the inverter can be made just with a single transistor without any other biasing components. The economy of components vs. RTL appears in any complex circuits, because it is very easy to make DTL gates with a very large number of inputs, while RTL gates are restricted to a much smaller number of inputs, due to insufficient amplification from the transistors, so you need many RTL gates to replace one DTL gate with many inputs. Using a diode matrix (and a decoder circuit, which can be made with another diode matrix), you can make a PLA or a ROM memory (the difference between PLA and ROM is that in the former both diode matrices have arbitrary diode patterns, while in the latter the diode matrix of the decoder has a fixed pattern, for selecting 1 of 2^N outputs with an N-bit input), and such PLAs or ROMs made with discrete diodes were still used in computers (e.g. for storing microprograms) many years after the integrated circuits had replaced simpler gates and flip-flops, because the ICs still had cost and size disadvantages. The diode matrices have disappeared completely only after the introduction of integrated PROMs and PLAs (during the early seventies) that could be programmed by the end-users, by burning internal NiCr fuses.
        amelius 3y ago
        It sounds interesting. I wonder though if diodes cause more current spikes in a system, so that perhaps currents exceed maximum ratings of the components for very brief moments.
        adrian_b 3y ago
        Because a diode that becomes on has a very low resistance, there will indeed be great spikes of current, which is a desirable thing, because this causes faster switching times between the logic levels. This is a reason why DTL circuits can be much faster than the RTL circuits, because in the latter the resistances slow down the charging and discharging of the various parasitic capacitances of the circuit. The current spikes are not a problem (because they are very short in time), they just require the presence of good decoupling capacitors close to the gates, to keep the current spikes localized, and various other shielding or decoupling measures may be needed, to avoid creating radio interference above what is accepted by standards. This is unavoidable for fast logic circuits, regardless of their schematic. There are only two ways to make a logic circuit faster, either making it smaller so that its capacitances become smaller too, or increasing the current spikes that can be delivered by the transistors. The problem in logic circuits is that the transistors always limit the current spikes to lower values than desired, which limits the attainable clock frequency, and never that the current spikes are too high.
  - deleted 3y ago
    [deleted]
- tgflynn 3y ago
  Out of curiosity, what would be the advantage of DTL over TTL, which I would think would be the default choice for building discrete logic circuits ?
  - kragen 3y ago
    diodes are cheaper, smaller, and easier to solder than transistors; ttl took off with integrated circuits ttl was the default choice for building discrete logic circuits until about 01980, after which point it was obsolete because cmos (74hcxxx, not cd4xxx) was better in every way except esd if you're running an educational computer lab on a tight budget, esd is still the dominant consideration, because students will burn out all your cmos chips with static after only a few dozen uses, while ttl chips will survive most of their mistakes but outside a circuit lab or possibly repair of 40-year-old devices there's no reason to use ttl
  - projektfu 3y ago
    It depends how "discrete" you mean. If you're using gate ICs, TTL has most of the advantages. If, for some reason, you're limiting yourself to discrete transistors, TTL is going to be hard to implement. These transistor-based projects are mostly done for the bragging rights.
  - adrian_b 3y ago
    TTL is just an implementation variant of DTL, which had some advantages for low supply voltages. When making DTL circuits with bipolar transistors, especially at low supply voltages, in order to make the logic levels at the output equal to the logic levels at the input, a simple solution was to add a diode at the output of an AND gate made with diodes, to shift down the logic levels by the voltage drop over that diode. When the diodes used for gates were bipolar diodes, not Schottky diodes, they stored a big electric charge when on and they could be turned off quickly only if there was a path to evacuate the stored charge. Also the bipolar transistor of the inverter stored a big charge in the base, which had to be evacuated quickly for turning it off. The current needed to evacuate the stored charge was required to pass in reverse direction through the level-shifting diode, which is not possible, so the DTL gates with level-shifting diodes were slow. In bipolar IC technology, all diodes are made from transistor junctions, in order to not have separate process steps for making diodes. The junctions suitable for fast diodes are the emitter-base junctions. So all the diodes of a DTL gate were made inside an IC as multiple emitters of a transistor, with a short over the base-collector junction, to disable the transistor effect and make it work as a bunch of diodes. At this point in the history of integrated DTL circuits, someone made the observation that seems trivial in hindsight, that removing the short over the base-collector junction allows to use it as the level-shifting diode, saving a diode. Moreover, this not only saved a diode, but the bipolar transistor effect results in passing current through the reverse-biased level-shifting diode, allowing the fast turn off of the inverting transistor. So in those early times, when DTL circuits could be made only with bipolar diodes and with bipolar transistors, TTL was the best variant. Later, when the bipolar diodes were replaced with Schottky diodes, which store negligible charge when on, and when diode clamps were used over the inverting transistor, so that it no longer reached deep saturation and it no longer stored a big charge, TTL was no longer the optimal implementation and some of the so-called Schottky TTL families were actually DTL circuits, not TTL, but they had retained TTL as a marketing term, as this had become almost synonymous with bipolar logic integrated circuit. TTL could never be used in discrete circuits, because it is based on bipolar transistors with multiple emitters and/or multiple bases, which have never been available as discrete parts.

Note that for hobbyists RTL is still interesting when building a logic circuit from discrete components.

by irdc 3y ago

Note that for hobbyists RTL is still interesting when building a logic circuit from discrete components.

While RTL is a possible choice, DTL circuits (diode-transistor logic) are both easier to design and more efficient from the POV of power consumption. DTL circuits can also be faster, especially when implemented with Schottky diodes, which should be the normal choice for them. DTL circuits are composed of transistors used as inverters and of diode circuits that are either minimum circuits a.k.a. AND or maximum circuits a.k.a. OR. When considered as min/max circuits instead of and/or, they can work even with a multi-level logic, not only with binary logic.

amelius 3y ago
Ok, but diodes are more expensive than resistors, and from what I can gather you also need more components in total.
- mrob 3y ago
  You can get small signal Schottky diodes for less than 0.01USD each, even in small quantities. The bulk of the cost is going to be the boards and assembly.
  - amelius 3y ago
    Yeah, ok, but I suppose you need more components, so more space, so a bigger board and higher assembly cost.
    - adrian_b 3y ago
      No, DTL circuits do not have more components than RTL, they normally have much less components. A maximum or minimum circuit with diodes, (i.e. an OR or AND gate) has just one diode for each input and one resistor for the output. In most cases each AND or OR gate must be followed by one inverter, to provide amplification, i.e. to restore the logic levels to values compatible with an input. The complexity of the inverter depends on what kind of transistors are available and on what switching speed is desired, but sometimes the inverter can be made just with a single transistor without any other biasing components. The economy of components vs. RTL appears in any complex circuits, because it is very easy to make DTL gates with a very large number of inputs, while RTL gates are restricted to a much smaller number of inputs, due to insufficient amplification from the transistors, so you need many RTL gates to replace one DTL gate with many inputs. Using a diode matrix (and a decoder circuit, which can be made with another diode matrix), you can make a PLA or a ROM memory (the difference between PLA and ROM is that in the former both diode matrices have arbitrary diode patterns, while in the latter the diode matrix of the decoder has a fixed pattern, for selecting 1 of 2^N outputs with an N-bit input), and such PLAs or ROMs made with discrete diodes were still used in computers (e.g. for storing microprograms) many years after the integrated circuits had replaced simpler gates and flip-flops, because the ICs still had cost and size disadvantages. The diode matrices have disappeared completely only after the introduction of integrated PROMs and PLAs (during the early seventies) that could be programmed by the end-users, by burning internal NiCr fuses.
      - amelius 3y ago
        It sounds interesting. I wonder though if diodes cause more current spikes in a system, so that perhaps currents exceed maximum ratings of the components for very brief moments.
        adrian_b 3y ago
        Because a diode that becomes on has a very low resistance, there will indeed be great spikes of current, which is a desirable thing, because this causes faster switching times between the logic levels. This is a reason why DTL circuits can be much faster than the RTL circuits, because in the latter the resistances slow down the charging and discharging of the various parasitic capacitances of the circuit. The current spikes are not a problem (because they are very short in time), they just require the presence of good decoupling capacitors close to the gates, to keep the current spikes localized, and various other shielding or decoupling measures may be needed, to avoid creating radio interference above what is accepted by standards. This is unavoidable for fast logic circuits, regardless of their schematic. There are only two ways to make a logic circuit faster, either making it smaller so that its capacitances become smaller too, or increasing the current spikes that can be delivered by the transistors. The problem in logic circuits is that the transistors always limit the current spikes to lower values than desired, which limits the attainable clock frequency, and never that the current spikes are too high.
- deleted 3y ago
  [deleted]
tgflynn 3y ago
Out of curiosity, what would be the advantage of DTL over TTL, which I would think would be the default choice for building discrete logic circuits ?
- kragen 3y ago
  diodes are cheaper, smaller, and easier to solder than transistors; ttl took off with integrated circuits ttl was the default choice for building discrete logic circuits until about 01980, after which point it was obsolete because cmos (74hcxxx, not cd4xxx) was better in every way except esd if you're running an educational computer lab on a tight budget, esd is still the dominant consideration, because students will burn out all your cmos chips with static after only a few dozen uses, while ttl chips will survive most of their mistakes but outside a circuit lab or possibly repair of 40-year-old devices there's no reason to use ttl
- projektfu 3y ago
  It depends how "discrete" you mean. If you're using gate ICs, TTL has most of the advantages. If, for some reason, you're limiting yourself to discrete transistors, TTL is going to be hard to implement. These transistor-based projects are mostly done for the bragging rights.
- adrian_b 3y ago
  TTL is just an implementation variant of DTL, which had some advantages for low supply voltages. When making DTL circuits with bipolar transistors, especially at low supply voltages, in order to make the logic levels at the output equal to the logic levels at the input, a simple solution was to add a diode at the output of an AND gate made with diodes, to shift down the logic levels by the voltage drop over that diode. When the diodes used for gates were bipolar diodes, not Schottky diodes, they stored a big electric charge when on and they could be turned off quickly only if there was a path to evacuate the stored charge. Also the bipolar transistor of the inverter stored a big charge in the base, which had to be evacuated quickly for turning it off. The current needed to evacuate the stored charge was required to pass in reverse direction through the level-shifting diode, which is not possible, so the DTL gates with level-shifting diodes were slow. In bipolar IC technology, all diodes are made from transistor junctions, in order to not have separate process steps for making diodes. The junctions suitable for fast diodes are the emitter-base junctions. So all the diodes of a DTL gate were made inside an IC as multiple emitters of a transistor, with a short over the base-collector junction, to disable the transistor effect and make it work as a bunch of diodes. At this point in the history of integrated DTL circuits, someone made the observation that seems trivial in hindsight, that removing the short over the base-collector junction allows to use it as the level-shifting diode, saving a diode. Moreover, this not only saved a diode, but the bipolar transistor effect results in passing current through the reverse-biased level-shifting diode, allowing the fast turn off of the inverting transistor. So in those early times, when DTL circuits could be made only with bipolar diodes and with bipolar transistors, TTL was the best variant. Later, when the bipolar diodes were replaced with Schottky diodes, which store negligible charge when on, and when diode clamps were used over the inverting transistor, so that it no longer reached deep saturation and it no longer stored a big charge, TTL was no longer the optimal implementation and some of the so-called Schottky TTL families were actually DTL circuits, not TTL, but they had retained TTL as a marketing term, as this had become almost synonymous with bipolar logic integrated circuit. TTL could never be used in discrete circuits, because it is based on bipolar transistors with multiple emitters and/or multiple bases, which have never been available as discrete parts.