4 ms·

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.

by mrob 3y ago

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.

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.

by 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.