6 ms·

Resistor–transistor logic

Physkal 3y ago
Nice to see more hardware articles.
squarefoot 3y ago
I first encountered RTL technology when i was a kid, probably 14 or 15, rummaging at a local electronics surplus shop, looking for cheap boards to salvage parts from, and saw this odd board full of small modules in which components were sandwiched between the main board and the many mini PCBs it contained, each one, as I later discovered, made a RTL logic gate. It cost almost nothing, so i bought it and salvaged a good number of transistors from it, all marked with the SGS-ATES logo and internal numbering that made them impossible to identify, although they all worked in the non critical experiments a kid might try in the very early 80s.
- MarkusWandel 3y ago
  Wasn't by any chance one of these, was it? https://www.oldcalculatormuseum.com/c-programma101.html https://www.oldcalculatormuseum.com/c-programma101.html One of my "shoulda preserved that" regrets is demolishing (as a teenager) a nonworking Programma 101 for the bits and pieces. The interleaved rows of these modules made for very dense circuit packing for the discrete component era.
  - squarefoot 3y ago
    Not identical but quite similar: the mounting style and board arrangement was the same, although in my case all transistors were in TO-18 metal case, all resistors 1/4 watt, no inductors, diodes were very rare and the small board were green FR4 just like the main board.
JKCalhoun 3y ago
I would also refer to those interested in RTL to the late Don Lancaster's "RTL Cookbook": https://archive.org/details/RTL_Cookbook_Don_Lancaster https://archive.org/details/RTL_Cookbook_Don_Lancaster
- systems_glitch 3y ago
  Came here to say this. Excellent book. Dead tree format is often pretty cheap, too.
demondemidi 3y ago
Today's kids don't understand biasing the base current, it's all "CMOS-this" and "fan-out" that. Grumble grumble grumble.
- sadhorse 3y ago
  The future is now, old man.
BitterPanda 3y ago
I remember I tried to make a modular full-bit adder with NPN transistors, diodes and resistors. I was a bit foolish expecting NPN transistors to act as simple switches and I was in way over my head. But I learned a lot, since I went from this is a logic gate to can we build this as a circuit to holy f*ck this is actually how people did it. A very interesting project
BitterPanda 3y ago
I remember I tried to make a modular full-bit adder with NPN transistors, diodes and resistors. I was a bit foolish expecting NPN transistors to act as simple switches and I was in way over my head. But I learned a lot, since I went from this is a logic gate to can we build this as a circuit to holy f*ck this is actually how people did it. A very interesting project
artemonster 3y ago
Its funny how integration inverted the „BOM“. In discrete systems, transistors were the most expensive components and resistors/caps were dirt cheap, so something like r-t-l made sense. On the chip, though, resistors and caps are „expensive“ due to huge area that they require and so its sometimes better/cheaper to emulate resistors and even capacitors with transistors
- CableNinja 3y ago
  They dont emulate, they tune the transistor properties to get the required resistance or capacitance. Its pretty wild. They could use inbuilt caps and resistors, but, theres no need when tuning a transistor value yields the same result. Iirc, the capacitance is dealt with by the gap distance of transistors, resistance by the lead
  - artemonster 3y ago
    by emulation I have meant this: https://en.wikipedia.org/wiki/Switched_capacitor https://en.wikipedia.org/wiki/Switched_capacitor
irdc 3y ago
Note that for hobbyists RTL is still interesting when building a logic circuit from discrete components.
- 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.
Stratoscope 3y ago
RTL was my first experience with digital circuit design. When I was a teenager in the 1960s, my neighbor was a sales engineer at Motorola. He saw me put up an antenna on the roof and guessed correctly that I was into ham radio. So he gave me a big Motorola databook of RTL components and said, "Let me know if you ever need anything, and we can send you some free samples." The book had had gates, flip-flops, everything. You could get an entire JK flip-flop in a little tin can the size of a TO-5 [1] (but with more leads)! I wanted to build an iambic Morse code keyer. This has two separate switches for your thumb and forefinger. Press with your thumb and you get one or more "dits". With your forefinger, you get a stream of "dahs". Squeeze them together and you get alternating dits and dahs, thus the "iambic" name. I designed the circuit, built it, and it worked great! Until I actually hooked it up to my transmitter (which was of course the point of the whole exercise). I didn't have much of a clue about RF shielding, and RTL wasn't very robust around RF energy. So the interference from my transmitter made the circuit go crazy! I never did figure out how to fix it, so I had to go back to a straight key. [1] https://en.wikipedia.org/wiki/TO-5 https://en.wikipedia.org/wiki/TO-5
d_tr 3y ago
Not to be confused with the other "RTL" which stands for "register-transfer level". [1] [1] https://en.wikipedia.org/wiki/Register-transfer_level https://en.wikipedia.org/wiki/Register-transfer_level
thriftwy 3y ago
Taking a course in digital circuits design made me appreciate that I get to work with software, as well as say thanks to those who actually supply circuits for that software to run on.
- kstrauser 3y ago
  My favorite semester in college was when I had discrete math and digital circuit design at the same time. Classroom: learn about Karnaugh maps. Lab: build them. That was incredibly satisfying.
retrac 3y ago
The main reason for RTL (and then DTL) at the time was conserving transistors. Discrete ones were quite expensive. In the mid-1960s roughly speaking, in modern dollars, a discrete transistor was about $10, while a semiconductor diode was about $1 and a resistor was, like today, about $0.05. The cost of the transistors dwarfed concerns about the cost of the boards or other components. This is a flip-flop circuit from a PDP-8. [1] It's diode-transistor logic, primarily. It's for 3 bits. There are only two transistors used per flip-flop to store the bits. A modern designer would use more transistors to route the inputs/outputs and for addressing the flip-flop. [2] The PDP-8 designers used capacitors and diode and resistor networks, to selectively pass pulses, using transistors only when truly necessary to invert or boost a signal. Several dozen diodes, caps and resistors, plus pulse logic (which complicates the whole design) were still cheaper than a few more transistors. With early integrated circuits, the yield was terrible. Most of the devices didn't work. It's much easier to build up a working resistor than a working diode on an IC, and it's easier to build a diode than it is a transistor. Also, with a very large process size, there's little concern about the heat of resistors. When we start increasing speed and shrinking the die, the power dissipation becomes considerable. Even by the late 1960s the heat of RTL at high speed limited how densely you could pack in it an IC. This is the same reason why, much later, CMOS would displace NMOS, despite requiring 2x as many components on chip. It runs cooler so it can be packed much more densely and run at a higher clock. (If you absolutely needed speed, from the 1950s right into the 1990s, you would use ECL, a kind of logic that doesn't switch on/off so the transistors never enter full saturation; like a differential amplifier, the current flow is constant.) [1] https://homepage.cs.uiowa.edu/~dwjones/pdp8/UI-8/map/R211schematic1965.GIF https://homepage.cs.uiowa.edu/~dwjones/pdp8/UI-8/map/R211sch... [2] https://ars.els-cdn.com/content/image/1-s2.0-S0026269216301574-gr2.jpg https://ars.els-cdn.com/content/image/1-s2.0-S00262692163015...
- hwillis 3y ago
  > a resistor was, like today, about $0.05. Sourcing on that, for no particular reason (also, since 1965 the dollar has inflated by ~10x): Through-hole types that would have been used for this are around .5-1 cents in 5k quantities, and ~5 cents in units of 10+ (right on the money! I wouldn't have known that offhand.): https://www.digikey.com/en/products/filter/through-hole-resistors/53?s=N4IgbCBcoJYCZRAWgIwAYBMIA0IAOALojiAQJ54CmicAzgMYgC%2BTQA https://www.digikey.com/en/products/filter/through-hole-resi... SMD/chip resistors, which are used for most things except relatively small scale products, are .2-.5 cents in bulk: https://www.digikey.com/en/products/filter/chip-resistor-surface-mount/52?s=N4IgjCBcpgnAHLKoDGUBmBDANgZwKYA0IA9lANogAMIAugL73EBsyIAlgCZQgC0YVAEwhiABwAuPESHEBPUfh6dcaRkA https://www.digikey.com/en/products/filter/chip-resistor-sur... Note that obviously all these prices are pretty rough- digikey is not the cheapest source, and the context of the industry has changed a lot.
ShuffleBoard 3y ago
FWIW this is how async paradigms will be looked back upon a few decades from now. Droll, pathos-laden gawk-bait.
- throwawaylinux 3y ago
  Are you talking about asynchronous logic? I thought that already is pretty much a curiosity.
  - ShuffleBoard 3y ago
    Not the worst parallel to draw, I reckon, especially in the wake of Copilot, GPT-3, etc. And I'm sure that circuit designers who'd invested the majority of their career in that async paradigm, only to see it facing obsolescence due to 'inexorable' tooling (digital EDA being that era's Copilot/GPT) advances, would've also downvoted an analogous comment :D

Nice to see more hardware articles.

I first encountered RTL technology when i was a kid, probably 14 or 15, rummaging at a local electronics surplus shop, looking for cheap boards to salvage parts from, and saw this odd board full of small modules in which components were sandwiched between the main board and the many mini PCBs it contained, each one, as I later discovered, made a RTL logic gate. It cost almost nothing, so i bought it and salvaged a good number of transistors from it, all marked with the SGS-ATES logo and internal numbering that made them impossible to identify, although they all worked in the non critical experiments a kid might try in the very early 80s.

MarkusWandel 3y ago
Wasn't by any chance one of these, was it? https://www.oldcalculatormuseum.com/c-programma101.html https://www.oldcalculatormuseum.com/c-programma101.html One of my "shoulda preserved that" regrets is demolishing (as a teenager) a nonworking Programma 101 for the bits and pieces. The interleaved rows of these modules made for very dense circuit packing for the discrete component era.
- squarefoot 3y ago
  Not identical but quite similar: the mounting style and board arrangement was the same, although in my case all transistors were in TO-18 metal case, all resistors 1/4 watt, no inductors, diodes were very rare and the small board were green FR4 just like the main board.

JKCalhoun 3y ago

I would also refer to those interested in RTL to the late Don Lancaster's "RTL Cookbook": https://archive.org/details/RTL_Cookbook_Don_Lancaster https://archive.org/details/RTL_Cookbook_Don_Lancaster

systems_glitch 3y ago
Came here to say this. Excellent book. Dead tree format is often pretty cheap, too.

demondemidi 3y ago

Today's kids don't understand biasing the base current, it's all "CMOS-this" and "fan-out" that. Grumble grumble grumble.

sadhorse 3y ago
The future is now, old man.

BitterPanda 3y ago

I remember I tried to make a modular full-bit adder with NPN transistors, diodes and resistors. I was a bit foolish expecting NPN transistors to act as simple switches and I was in way over my head. But I learned a lot, since I went from this is a logic gate to can we build this as a circuit to holy f*ck this is actually how people did it. A very interesting project

BitterPanda 3y ago

artemonster 3y ago

Its funny how integration inverted the „BOM“. In discrete systems, transistors were the most expensive components and resistors/caps were dirt cheap, so something like r-t-l made sense. On the chip, though, resistors and caps are „expensive“ due to huge area that they require and so its sometimes better/cheaper to emulate resistors and even capacitors with transistors

CableNinja 3y ago
They dont emulate, they tune the transistor properties to get the required resistance or capacitance. Its pretty wild. They could use inbuilt caps and resistors, but, theres no need when tuning a transistor value yields the same result. Iirc, the capacitance is dealt with by the gap distance of transistors, resistance by the lead
- artemonster 3y ago
  by emulation I have meant this: https://en.wikipedia.org/wiki/Switched_capacitor https://en.wikipedia.org/wiki/Switched_capacitor

irdc 3y ago

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

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.

Stratoscope 3y ago

RTL was my first experience with digital circuit design. When I was a teenager in the 1960s, my neighbor was a sales engineer at Motorola. He saw me put up an antenna on the roof and guessed correctly that I was into ham radio. So he gave me a big Motorola databook of RTL components and said, "Let me know if you ever need anything, and we can send you some free samples." The book had had gates, flip-flops, everything. You could get an entire JK flip-flop in a little tin can the size of a TO-5 [1] (but with more leads)! I wanted to build an iambic Morse code keyer. This has two separate switches for your thumb and forefinger. Press with your thumb and you get one or more "dits". With your forefinger, you get a stream of "dahs". Squeeze them together and you get alternating dits and dahs, thus the "iambic" name. I designed the circuit, built it, and it worked great! Until I actually hooked it up to my transmitter (which was of course the point of the whole exercise). I didn't have much of a clue about RF shielding, and RTL wasn't very robust around RF energy. So the interference from my transmitter made the circuit go crazy! I never did figure out how to fix it, so I had to go back to a straight key. [1] https://en.wikipedia.org/wiki/TO-5 https://en.wikipedia.org/wiki/TO-5

d_tr 3y ago

Not to be confused with the other "RTL" which stands for "register-transfer level". [1] [1] https://en.wikipedia.org/wiki/Register-transfer_level https://en.wikipedia.org/wiki/Register-transfer_level

thriftwy 3y ago

Taking a course in digital circuits design made me appreciate that I get to work with software, as well as say thanks to those who actually supply circuits for that software to run on.

kstrauser 3y ago
My favorite semester in college was when I had discrete math and digital circuit design at the same time. Classroom: learn about Karnaugh maps. Lab: build them. That was incredibly satisfying.

retrac 3y ago

The main reason for RTL (and then DTL) at the time was conserving transistors. Discrete ones were quite expensive. In the mid-1960s roughly speaking, in modern dollars, a discrete transistor was about $10, while a semiconductor diode was about $1 and a resistor was, like today, about $0.05. The cost of the transistors dwarfed concerns about the cost of the boards or other components. This is a flip-flop circuit from a PDP-8. [1] It's diode-transistor logic, primarily. It's for 3 bits. There are only two transistors used per flip-flop to store the bits. A modern designer would use more transistors to route the inputs/outputs and for addressing the flip-flop. [2] The PDP-8 designers used capacitors and diode and resistor networks, to selectively pass pulses, using transistors only when truly necessary to invert or boost a signal. Several dozen diodes, caps and resistors, plus pulse logic (which complicates the whole design) were still cheaper than a few more transistors. With early integrated circuits, the yield was terrible. Most of the devices didn't work. It's much easier to build up a working resistor than a working diode on an IC, and it's easier to build a diode than it is a transistor. Also, with a very large process size, there's little concern about the heat of resistors. When we start increasing speed and shrinking the die, the power dissipation becomes considerable. Even by the late 1960s the heat of RTL at high speed limited how densely you could pack in it an IC. This is the same reason why, much later, CMOS would displace NMOS, despite requiring 2x as many components on chip. It runs cooler so it can be packed much more densely and run at a higher clock. (If you absolutely needed speed, from the 1950s right into the 1990s, you would use ECL, a kind of logic that doesn't switch on/off so the transistors never enter full saturation; like a differential amplifier, the current flow is constant.) [1] https://homepage.cs.uiowa.edu/~dwjones/pdp8/UI-8/map/R211schematic1965.GIF https://homepage.cs.uiowa.edu/~dwjones/pdp8/UI-8/map/R211sch... [2] https://ars.els-cdn.com/content/image/1-s2.0-S0026269216301574-gr2.jpg https://ars.els-cdn.com/content/image/1-s2.0-S00262692163015...

hwillis 3y ago
> a resistor was, like today, about $0.05. Sourcing on that, for no particular reason (also, since 1965 the dollar has inflated by ~10x): Through-hole types that would have been used for this are around .5-1 cents in 5k quantities, and ~5 cents in units of 10+ (right on the money! I wouldn't have known that offhand.): https://www.digikey.com/en/products/filter/through-hole-resistors/53?s=N4IgbCBcoJYCZRAWgIwAYBMIA0IAOALojiAQJ54CmicAzgMYgC%2BTQA https://www.digikey.com/en/products/filter/through-hole-resi... SMD/chip resistors, which are used for most things except relatively small scale products, are .2-.5 cents in bulk: https://www.digikey.com/en/products/filter/chip-resistor-surface-mount/52?s=N4IgjCBcpgnAHLKoDGUBmBDANgZwKYA0IA9lANogAMIAugL73EBsyIAlgCZQgC0YVAEwhiABwAuPESHEBPUfh6dcaRkA https://www.digikey.com/en/products/filter/chip-resistor-sur... Note that obviously all these prices are pretty rough- digikey is not the cheapest source, and the context of the industry has changed a lot.

ShuffleBoard 3y ago

FWIW this is how async paradigms will be looked back upon a few decades from now. Droll, pathos-laden gawk-bait.

throwawaylinux 3y ago
Are you talking about asynchronous logic? I thought that already is pretty much a curiosity.
- ShuffleBoard 3y ago
  Not the worst parallel to draw, I reckon, especially in the wake of Copilot, GPT-3, etc. And I'm sure that circuit designers who'd invested the majority of their career in that async paradigm, only to see it facing obsolescence due to 'inexorable' tooling (digital EDA being that era's Copilot/GPT) advances, would've also downvoted an analogous comment :D