Hackaday TTLers

Where DIY DIP/SSI/MSI CPU makers meet and discuss other TLA (three letters acronyms)

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It started simply...
I can't keep track of all the awesome "discrete" CPU designs on my own project. There is a list of such projects but it is "curated". Why not make my own list and invite like-minded hackers ?
If you have a similar project here, drop me a message and I'll add you to the contributors.
The group evolved towards the exploration of all the transistor-based technologies and the associated ideas. So we study how to make basic logic building blocks in standard and exotic ways.
The "Team" is a selective who's who of people who designed their own discrete computer or developed advanced logic circuits.
Come and chat with us on the open forum if you have ideas or questions !

For practical reasons (it's impossible to list everything on the 'net), the "project" is mostly about gathering people from HaD who built their CPU (or at the very least digital electronic devices). Here are some external links for those who just can't get enough:

Feel free to suggest or add links of the same kind :-)

PS: the project's logo comes from Wikipedia

PPS: let's not forget the two lists and but note they are subject to curator delay (and taste).

1. Dynamic RAM with single MOSFET per bit ?
2. Bizarre DTL Logic Levels - The Discrete Component PDP-8
3. The Electronics of IBM Standard Modular System Logic
4. ECL or CTL : what's the fastest topology for discrete gates ? [updated]
5. TTL inside
6. Direct Coupled Transistor Logic
7. Interactive Simulations of DEC R-Series Logic
8. Why is ECL faster ?
9. Bipolar XOR gate with only 2 transistors
10. Video Explaining DEC R-Series DTL
11. The rule of 50 (or so)
12. Bipolar transistors are ANDN gates !
13. The return of CTL
14. From XOR to MUX
15. From MUX to Latch
16. Project proposal : Ring oscillators zoo !
17. Analog Multiplexer Logic
18. Another RTL/DCTL latch
19. My own try at a RingO9 with 2N2369A
20. unexpected frequency doubler or rectifier
21. More 2369, with more caps !
22. RingO9 v2 : closer to CDC specs !
23. RingO9 v2 : with caps !
24. Beyond 2ns with 2N2369A
25.   .
26.   .

DEC R-Series Logic Flip Flop.mp4

An animation of the operation of a DEC R-Series Logic Flip-Flop from a PDP-8 or early PDP-11

MPEG-4 Video - 2.04 MB - 07/04/2018 at 23:10


DEC R-Series Logic Inverter.mp4

An animation of the operation of a DEC R-Series Logic Inverter from a PDP-8 or early PDP-11

MPEG-4 Video - 692.53 kB - 07/04/2018 at 23:09


DEC R-Series Logic Inverter.txt

Import this circuit schematic description into this website (requires JavaScript):

plain - 1.19 kB - 07/04/2018 at 23:09


DEC R-Series Logic Flip Flop.txt

Import this circuit schematic description into this website (requires JavaScript):

plain - 3.73 kB - 07/04/2018 at 23:09


  • Dear Marcel

    Yann Guidon / YGDES06/08/2020 at 01:47 1 comment

    Hello @Marcel van Kervinck,

    I'm sorry you had to go away. It was a chance to chat with you and I hope Hackaday will preserve your work and logs for ever. You are sorely missed.

    I'll finish this short log by quoting him in a private discussion, when he reacted to my surprise that he wasn't a TTLer before :

    Those who use TTL are not in a hurry

    (june 16th, 2017)

  • Beyond 2ns with 2N2369A

    Yann Guidon / YGDES05/23/2020 at 03:19 6 comments

    In his log Pushing RTL to <2 ns Propagation Delay, Tim alluded that a combination of base capacitor and base-collector diode could reach 2ns of transition time per inverter.

    Well, @Tim, it wasn't that hard after all ;-) How about 1.73ns at only 5V ?

    OK it's ugly (bad baaaad probing) and the CDC levels are pretty much destroyed...

    But it's FAST and even more POWER EFFICIENT !

    I get 32MHz at 5.2V and only 77mA, or 44mW per gate, a 4.5× improvement compared to  Tim's 200 mW :-)

    What's my secret ? Not much, it's explained in the previous logs ;-)

    • ample capacitor decoupling
    • low Rb (150 Ohms)
    • finely chosen Cb (68 pF)

    But this log has a newcomer : a Schottky diode. Spoiler alert : I didn't pay much attention, I found 2 reels of SMB and SMA-packaged low voltage diodes in my drawers. I don't even remember where/how/when/why I obtained them but here they are !

    This version uses a ROHM RB751V-40 Schottky barrier diode in a tiny tiny package (almost 0603). It's limited to 20mA which matches well because the higher the current, the larger the junction, the more capacitance...

    I also have MBR0520 diodes but the higher current rating potentially increases the capacitance, which would create more problems.

    The 2N2369A is prevented from "switching hard", which has a welcome effect : less current is drawn ! At 1V the circuit sips only 3mA instead of 6mA... By 2V the difference is mostly erased, though, but at low voltages, that circuit is crazy efficient :-)

    Impressive !

    Here is the raw data for the summary graphics :

    1     6  8.292    9.653    3  12.461
    1.1   8  9.191   10.900    5  14.344
    1.2  10  9.942   11.955    7  15.976
    1.3  11  10.576  12.861    8  17.374
    1.4  13  11.140  13.663   10  18.648
    1.5  15  11.609  14.363   12  19.720
    1.6  16  12.001  14.961   14  20.700
    1.7  18  12.344  15.517   16  21.507
    1.8  19  12.636  16.029   17  22.351
    1.9  21  12.867  16.480   19  23.084
    2    23  13.053  16.916   20  23.754
    2.1  24  13.182  17.265   22  24.290
    2.2  26  13.286  17.634   24  24.875
    2.3  28  13.343  17.959   25  25.428
    2.4  30  13.364  18.268   27  25.951
    2.5  31  13.354  18.559   29  26.436
    2.6  32  13.317  18.818   31  26.875
    2.7  34  13.253  19.076   32  27.297
    2.8  36  13.170  19.307   34  27.702
    2.9  38  13.068  19.531   36  28.077
    3    40  12.952  19.749   38  28.432
    3.1  41  12.826  19.949   40  28.756
    3.2  42  12.689  20.140   41  29.063
    3.3  44  12.552  20.311   43  29.335
    3.4  46  12.408  20.472   45  29.583
    3.5  48  12.253  20.630   47  29.832
    3.6  49  12.097  20.779   48  30.050
    3.7  51  11.938  20.921   50  30.260
    3.8  53  11.729  21.060   52  30.467
    3.9  55  11.580  21.182   54  30.645
    4    56  11.426  21.296   56  30.820
    4.1   n   n      21.400   57  30.978
    4.2   n   n      21.500   59  31.126
    4.3   n   n      21.590   61  31.265
    4.4   n   n      21.681   63  31.384
    4.5   n   n      21.766   65  31.507
    4.6   n   n      21.851   66  31.619
    4.7   n   n      21.933   68  31.722
    4.8   n   n      22.010   70  31.824
    4.9   n   n      22.077   72  31.918
    5     n   n      22.144   73  31.997 

    And the gnuplot commands :

    set xlabel 'V'
    set ylabel 'MHz'
    set y2label 'mA'
    set xr [1:5]
    set yr [6:36]
    set y2r [0:90]
    set key right bottom
    set y2tics 3
    plot \
     "ringo9v2_0-68pf-RB751.dat" using 1:2 axes x1y2 title             "0pF current in  mA" w points pt 7, \
     "ringo9v2_0-68pf-RB751.dat" using 1:3           title           "0pF frequency in MHz" w lines, \
     "ringo9v2_0-68pf-RB751.dat" using 1:4           title          "66pF frequency in MHz" w lines, \
     "ringo9v2_0-68pf-RB751.dat" using 1:5 axes x1y2 title "Schottly+66pF  current  in  mA" w lines, \
     "ringo9v2_0-68pf-RB751.dat" using 1:6           title "Schottly+66pF frequency in MHz" w lines

    From there we can also plot the power/frequency curves with the following script :

    set key right bottom
    set xlabel 'V'
    set xr [1:5]
    set yr [0:2]
    set ylabel 'mW/MHz'
    plot "ringo9v2_0-68pf-RB751.dat" using 1:(($2*$1/$3))/9  title  "0pF" w lines, \
         "ringo9v2_0-68pf-RB751.dat" using 1:(($2*$1/$4))/9  title "66pF" w lines, \
         "ringo9v2_0-68pf-RB751.dat" using 1:(($5*$1/$6))/9  title "Schottly+66pF" w lines

     The result is self-explanatory :-)

    These curves were measured on this simple board :

    What else is there to say ?

    It's not the end of the adventure, of course, because it's only a ring oscillator and the diodes have destroyed the saturating...

    Read more »

  • RingO9 v2 : with caps !

    Yann Guidon / YGDES05/22/2020 at 16:01 2 comments

    a few minutes of soldering bring the base capacitors to the board... and once again Tim's curves are confirmed !

    The data :

    V     mA     MHz
    1      6    9.263
    1.1    8   10.481
    1.2    9   11.560
    1.3   11   12.456
    1.4   13   13.268
    1.5   15   13.974
    1.6   16   14.592
    1.7   18   15.166
    1.8   19   15.692
    1.9   21   16.156
    2     23   16.603
    2.1   24   16.985
    2.2   26   17.381
    2.3   28   17.731
    2.4   30   18.056
    2.5   32   18.360
    2.6   33   18.634
    2.7   35   18.899
    2.8   37   19.150
    2.9   39   19.386
    3     40   19.617
    3.1   42   19.828
    3.2   44   20.036
    3.3   46   20.223
    3.4   47   20.406
    3.5   48   20.585
    3.6   50   20.750
    3.7   52   20.906
    3.8   54   21.057
    3.9   56   21.191
    4     57   21.322
    4.1   59   21.435
    4.2   61   21.552
    4.3   63   21.666
    4.4   65   21.771
    4.5   66   21.877
    4.6   68   21.976
    4.7   70   22.073
    4.8   72   22.163
    4.9   73   22.251
    5     75   22.337

    The script :

    set xlabel 'V'
    set ylabel 'MHz'
    set y2label 'mA'
    set xr [1:5]
    set yr [6:24]
    set y2r [0:90]
    set ytics 1
    set y2tics 10
    set key right bottom
    plot "ringo9_2_47pF.dat" using 1:3 title "v.2 47pF Frequency in MHz" w points pt 7,  \
         "ringo9_2_47pF.dat" using 1:2 axes x1y2 title "v.2 47pF total current in mA" w points pt 7,  \
         "ringo9_2_sans.txt" using 1:3 title "v.2 sans cap Frequency in MHz" w lines, \
         "ringo9_2_sans.txt" using 1:2 axes x1y2 title "V.2 sans cap current in mA" w lines

     Once again the capacitor is a simple yet very effective means to go faster, yet the power curve is not affected (in a meaningful, significant way). So the efficiency is much better than v1 :-)

    At 5V the circuit easily reaches 22MHz, or 2.5ns per inverter !

    But is it necessary to go THAT fast ?  Where is the sweet spot again ? I don't think it's a good idea to run at 5V because the speed is only marginally better for a very significant increase in power draw (42mW/gate, or 1.8mW/MHz). So maybe 5V would be reserved for special cases and places that need a serious fanout.

    • 2V : 46mW => 5.1mW/gate, or 0.3mW/MHz/gate
    • 2.5V : 80mW => 8.8mW/gate, or 0.48mW/MHz/gate
    • 3V : 120mW => 13.3mW/gate, or 0.68mW/MHz/gate
    • 3.3V : 152mW => 16.8mW/gate, or 0.83mW/MHz/gate
    • 5V : 375mW total, 42mW/gate, or 1.8mW/MHz

    It would be wise to stay under the 1mW/MHz/gate, 0.5mW/MHz/gate would be even better but the fanout would be insufficient. The standard voltage 3.3V would be a good compromise but let's wait for the results with the other cap values and the diodes !

    Anyway : Going from 2.5V to 3.3V brings only 10% more speed while the power  almost doubles !

    But what is the right capacitor value ?

    the datasheet specifies < 4pF for the gate charge. So the capacitor must be higher than that to cancel the effect. So maybe 47pF ?

    OTOH I saw a speed difference that is similar between 27pF (PCB v.1) and 47pF (PCB v.2) so there would be a diminishing return, which can only be spotted by plotting the V/F curve with various capacitances.

    The smallest capacitors I have are 10pF so that's a good start. I can then add 18pF in parallel to give 28pF. Adding 47pF again will give another trace...

    The results for 10pF are below :

    From this graph, we can only suppose that the next increase would be to 220pF...

    Meanwhile, the current graph has not changed so I don't show it anymore.

    Testing with 280pF gives a pretty unexpected curve, but good to know anyway :

    After a promising start at very low frequency, the 47pF curve is already winning at 1.4V. I now have to check at 100, 68 and 33pF if there is another local maximum...

    The 100pF curve is disappointing : why is it worse than the 280pF ?

    What is so special about the 47pF I tried ? Did I fry a part ?

    Trying with 33pF caps shows interesting results as well, close to the 47pF.

    Apparently the 2×33pF combination has a very light advantage up to 3.5V : that's still good to take and much better than other values.

    Now trying 68pF gives a result very close to 2×33pf. So close that gnuplot almost mixes the colors, unless you zoom a lot.

    So 68pF wins by a tiny margin, but that's all I intended to find out :-)

    The dataset :

    1    8.292    8.563   9.116    9.263   9.623   9.653    9.391    9.863
    1.1 9.191 9.57...
    Read more »

  • RingO9 v2 : closer to CDC specs !

    Yann Guidon / YGDES05/22/2020 at 01:57 5 comments

    I'm already back with another ring oscillator ! and @Tim will love this one even more.

    The precedent one gave me some headaches due to the bad PCB design, I used a single-sided board and couldn't solder anything on the other side... ma que stupido !

    I de-soldered the transistors and made a new board with more headroom. Aaaaaand...

    13.30MHz @ 2.42V / 30mA => that's 4.2ns per inverter !

    Vcollector @ 2.5V => 13.33MHz - plateau at 1.2V

    The new parameters are not far from the previous one :

    Rb = 150 Ohms, Rc = 470 Ohms

    The change of Rb seems to have helped a bit : I now see the collector voltage saturated and not reaching Vcc (between 0.15 and 1.25V). Vb ranges from 0.18V to 0.9V => I'm now near the levels defined by CDC !

    Here is the waveform at the base : the 2N2369 is driven hard at 800mV ! Discharging it however seems to take some time...

    The other change is the ample decoupling, 6×100nF + 3×10nF, I don't know if it helps but you're never too safe with that because later, I might unexpectedly scramble the local CB channels ;-)

    Yet I don't see how/why I gained 30% speed with the same transistors (I replaced one by error) and almost the same resistors (ok the base resistor has lost 25% of its value... but it's worth it right ?)

    Did I mistake a resistor somewhere ? Was one of the transistors "too slow" ? Is there a wrong resistor value in the first RingO ?

    Something else is interesting : I'm now at 30mA but the last "record" was at held at 50mA so something serious is going on here ! Efficiency has jumped too !

    The signal falls in about 5ns on the 200MHz scope, which is close to the limits. There is some overshoot, very likely caused by the ground clip and the limited BW of the whole system.

    Another good sign is that the falling edge (at the collector) is now the fast one, in 5ns :-) (we were puzzled that the rising edge was the fast one on the other board, might have been mistaken for the base ? nah...)

    The rising edge takes about 12 ns to completely reach 1.1V and this will get only longer with more loading. But in 8ns, 1V is reached.

    At 2.5V and 470 ohms shorted in DC to 0V, the collector current is drawing 5mA (approx.)

    Add to this the other current source (the base capacitance and the transistor might have 10mA transients... So once again it's in line with the CDC specs :-)

    The base current is defined by (Vc - Vb) / Rb = (1.2 - 0.85) / 150 => Ib = 2.3mA (at 2.5V, during DC ON) => in line with the expected values :-)

    The circuit alternates between 5mA and 2.5mA, this averages to 3.7mA/9.3mW per inverter (FO1).

    The delay :

    This plot is from the base and collector of the same transistor, so we see the latency of the signal : about 5ns between the middle point of the rising edge on the base and the middle point of the falling edge of the collector. It takes about 8ns from the start of the base's rising edge to the end of the collector's falling edge...

    The reverse however takes more time, due to RC loading.

    The V/F curve :

    V     mA   MHz
    1      6   8.292
    1.1    8   9.191
    1.2   10   9.942
    1.3   11  10.576
    1.4   13  11.140
    1.5   15  11.609
    1.6   16  12.001
    1.7   18  12.344
    1.8   19  12.636
    1.9   21  12.867
    2     23  13.053
    2.1   24  13.182
    2.2   26  13.286
    2.3   28  13.343
    2.4   30  13.364
    2.5   31  13.354
    2.6   32  13.317
    2.7   34  13.253
    2.8   36  13.170 
    2.9   38  13.068
    3     40  12.952
    3.1   41  12.826
    3.2   42  12.689
    3.3   44  12.552
    3.4   46  12.408
    3.5   48  12.253
    3.6   49  12.097
    3.7   51  11.938
    3.8   53  11.729
    3.9   55  11.580
    4     56  11.426

    Clearly : something important has happened :

    set xlabel 'V'
    set ylabel 'MHz'
    set y2label 'mA'
    set xr [1:4]
    set yr [6:18]
    set y2r [0:120]
    set ytics 1
    set y2tics 10
    set key right bottom 
    plot "27pf.dat" using 1:3 title "v.1 27pF Frequency in MHz" w points pt 7,  \
         "27pf.dat" using 1:2 axes x1y2 title "v.1 27pF total current in mA" w points pt 7,  \
         "ringo9_2_sans.txt" using 1:3 title "v.2 sans cap Frequency in MHz" w lines, \
     "ringo9_2_sans.txt" using 1:2 axes x1y2 title "V.2...
    Read more »

  • More 2369, with more caps !

    Yann Guidon / YGDES05/21/2020 at 16:55 0 comments

    Back to the workshop for the revenge of the return of the son of the Vintage Ring Oscillator !

    This time with a new ally : the ceramic capacitor !

    TL;DR : it's 60% faster !

    But the V/F curve might look different from @Tim 's experiments...

    As usual, give me some time to dump and analyse all the yummy data, come back often on this page ;-)

    Back to the circuit : it starts with the exact same board as last time.

    Ring oscillator  with 9 levels of low-grade 2369 (according to their hFE).

    Rb = 220, Rc = 470, like before.

    1nF to decouple a pair of transistors.

    But this time I add more capacitors : 100nF on the power input and 27pF to short each base resistor ! As usual, it's a step by step modification to help with understanding the effect of every change.

    From the beginning, starting at about 10MHz, I saw the incremental increase of frequency : about 500KHz for each capacitor I added. I tested very often because I didn't want to spend any time spotting soldering error.

    After a while I had the 9 capacitors wired and *bim* 16MHz without effort !

    Some tuning later, a lot of blowing, and the best frequency I got was 16.8MHz !

    That's at least 50% better than without the capacitors.

    16.81MHz at 2.45V !

    (yes I know the date is wrong)

    15M81Hz @ 2V : near the sweet spot...

    14MHz@1.5V only :-) still nice looking...

    Here is the V/F curve data:

     V     mA      MHz
    1      10      9.15
    1.1    14     10.22
    1.2    16     11.90
    1.3    20     12.78
    1.4    23     13.52
    1.5    25     14.12
    1.6    28     14.63
    1.7    31     15.02
    1.8    34     15.35
    1.9    36     15.62
    2      40     15.85
    2.1    41     16.13
    2.2    45     16.35
    2.3    48     16.53
    2.4    50     16.62
    2.5    54     16.65
    2.6    56     16.51
    2.7    59     16.41
    2.8    63     16.32
    2.9    65     16.24
    3      69     16.15
    3.1    72     16.09
    3.2    74     16.05
    3.3    78     16.02
    3.4    81     16.00
    3.5    83     15.98
    3.6    87     15.96
    3.7    89     15.94
    3.8    93     15.93
    3.9    96     15.93
    4      99     15.93

    I made more points because I want to plot a better curve than the previous one.

    More precision is hard because it's temperature sensitive and the can is connected to the collector so it must radiate quite a lot of HF...

    The 16MHz region spans from 2.06V to 3.3V and then the curve is mostly flat.

    The peak frequency moved to 2.45V : still not far from 2V.

    16.8MHz amounts to 3.3ns per inverter, down from 5.5ns with just a capacitor.

    Beware however !

    • 27pF is just a random value in the drawer : more tests are needed to estimate and choose the best value. So it's still not optimal.
    • This is very sensitive to the load ! the frequency drops quickly when it is loaded.
    • Temperature affects the speed by a few %.
    • No clamp diode yet... later :-)

    The power/frequency curve is clearly  changed as well and this is where it's the most interesting :-D

     V    mA    MHz      mW     mW/MHz   delta
    1     10    9.15     10      1.092
    1.1   14   10.22     15.4    1.507   0.415
    1.2   16   11.90     19.2    1.613   0.106
    1.3   20   12.78     26      2.034   0.421
    1.4   23   13.52     32.2    2.381   0.347
    1.5   25   14.12     37.5    2.655   0.274
    1.6   28   14.63     44.8    3.062   0.407
    1.7   31   15.02     52.7    3.508   0.446
    1.8   34   15.35     61.2    3.986   0.478
    1.9   36   15.62     68.4    4.379   0.393
    2     40   15.85     80      5.047   0.668
    2.1   41   16.13     86.1    5.337   0.29
    2.2   45   16.35     99      6.055   0.718
    2.3   48   16.53    110.4    6.678   0.623
    2.4   50   16.62    120      7.220   0.542
    2.5   54   16.65    135      8.108   0.888
    2.6   56   16.51    145.6    8.818   0.71
    2.7   59   16.41    159.3    9.707   0.889
    2.8   63   16.32    176.4   10.808   1.101
    2.9   65   16.24    188.5   11.607   0.799
    3     69   16.15    207     12.817   1.21
    3.1   72   16.09    223.2   13.871   1.054
    3.2   74   16.05    236.8   14.753   0.882
    3.3   78   16.02    257.4   16.067   1.314
    3.4   81   16.00    275.4   17.212   1.145
    3.5   83   15.98    290.5   18.178   0.966
    3.6   87   15.96    313.2   19.624   1.446
    3.7   89   15.94    329.3   20.658   1.034
    3.8   93   15.93    353.4   22.184   1.526
    3.9   96   15.93    374.4   23.502   1.318
    4     99   15.93    396     24.858   1.356

    The power estimate is not very precise because the integrated ampere-meter has only so many digits... The delta column has some "noise" in it but this is useful anyway !

    Gnuplotting gives nice results, sure !

    Frequency vs voltage, Current vs voltage curve :

    set xlabel 'V'
    set ylabel 'MHz'
    set y2label 'mA'
    set xr [1:4]
    set yr [6:18]
    set y2r [0:120]
    Read more »

  • unexpected frequency doubler or rectifier

    Yann Guidon / YGDES05/20/2020 at 03:10 0 comments

    So I was Falstad'ing some ECL/differential amplifier topologies and playing with the resistor values ratios...

    I found some strange behaviours with this single-ended circuit when the collector and emitter resistors are equal.

    At 5V the turning point is at 3.1V, so I created a sine wave centered around 3V with +/- 1V peaks. The output looks like a rectified version...

    The effect disappears when the ratio of the resistors is modified. This might be a desired effect or an unwanted behaviour, and since I'm playing with ECL topologies, I want to avoid this so I need to understand what is going on.

    This is important because I would like to save a transistor at the common emitter node so the resistor value must be well chosen. It's good to know that a 1/1 ratio is BAD, and changing it affects the kink point...

    But OTOH it opens up potential for fun, such as sound effects :-P

  • My own try at a RingO9 with 2N2369A

    Yann Guidon / YGDES05/16/2020 at 12:15 17 comments

    Edit: See also More 2369, with more caps !

    With a sporadic and limited access to the workshop (at last !) I can finally try new ideas ! I have meanwhile received 9K PMBT2369 in SMD but I decided to use the old stock of 50pc 2N2369A in metal can, that was waiting in a small bag that I received from various sources... What can be closer to the CDC era ? (A motor-generator ? :-P)

    This is a "mixed bag" with at least 2 sources or makers, some with golden legs, and I decided to test them. Just because I now have a better tester and it's good to see if/how the different types differ...

    Most "golden" parts fall in the lower bins and the tinned ones have overall the best gain. I made 3 bins :

    • < 60 (lowest is 46)
    • < 84
    • higher (a few up to 114 and one at 119)

    and then I use the lower gain ones to build the RingO, with 9 parts to give a low-enough frequency that makes 'scoping reasonable.

    I could have made a > 100 bin but

    • I just wanted to have a look at the spec spread
    • I wanted to weed out the lemons (and use them first to establish a baseline)
    • the temperature sensitivity makes it moot.

    After Tim's experiments, I chose from my parts bins :

    • Rc = 470 (so I can test from 2V to 5Vcc) in 0805
    • Rb = 220 (that's what I have in stock right now, close enough)

    Afterthought : I should have tried 100 Ohms for Rb. Or even 47/50 ohms maybe....

    After-afterthought : or 330 ohms (see near the end)

    And the soldering iron was turned on !

    • For the sake of simplicity I omitted the caps. They used too much room. Next time I'll look at the SMD stock.
    • I added 1nF to decouple every pair of transistor (that's 5nF but spread to ease HF transients)
    • I found some partial reels of SMD Schottky diodes but once again, decided to not use them yet.

    So I wanted to establish a baseline for speed and more importantly : explore the power vs speed envelope because... Tim found that a LOT of power was wasted. I would like to get a gate that is still "pretty fast" and yet consumes at least 10 times less power.

    For the measurements I used a 200MHz digital scope with 10x probe. The output waveform is pretty nice and quite square-y :-) No funky feature is noticed, it's plain old RTL and I didn't bother to measure the rise/fall time because the measurement circuit is not optimised.

    Still it's very telling.








    Rise time is about 5ns. Which is odd since I expected that RC would dominate it.

    In fact something else is happening : it seems that the transistor has a harder work to totally saturate and keep Vce sat to a sufficiently low value. This in turn reduces the frequency because the transistor turns off later.

    At 5V the circuit doesn't seem to get hot, maybe thanks to the help of the metal cans and the resistors directly soldered to thick metal that can spread the heat.

    You can find a curve that is similar to what Tim found already :

    Vcc  Total   Freq.
    5V   123mA   6.98MHz  <= never mind.
    4.5V 109     7.37
    4V    97     7.77
    3.5V  81     8.61
    3V    69     9.42 <= why waste so much power ?
    2.5V  55    10.16
    2.25  47    10.48
    2V    40    10.55 <= sweet spot !
    1.75  33    10.48
    1.5V  26    10.06
    1.25  18     9.13
    1V    11     7.34  <= wow, that's still good :-D

    The frequency is measured by the scope but not finely calibrated. My HP freqmeter wouldn't accept the raw signal, something to do with ringing and probe impedance, I'll check that later. Still you can find the important features.

    With my choice of parts, I find that the sweet spot is in the 1.75V-2.25V range, as roughly expected, so I'm pretty happy ! But there is more to that curve.

    Let's now compute and deduce some numbers :

    Vcc  mA Total Freq. mW Total mW/MHz
    5     123     6.89    615      89
    4.5   109     7.37    490      66.5
    4      97     7.77    388      50     5V/4V=> *1.8
    3.5    81     8.61    283      32     
    3      69     9.42    207      22     4V/3V=> *2.3
    2.5    55    10.16    137      13.5 
    2.25   47    10.48    106      10.1
    2      40    10.55     80       7.6   3V/2V=> *2.9
    1.75   33    10.48     57       5.43
    1.5    26    10.06     39       3.88
    1.25   18     9.13     23       2.52
    1      11     7.34     11       1.5   2V/1V=> *5

    The power/speed ratio decreases faster...

    Read more »

  • Another RTL/DCTL latch

    Yann Guidon / YGDES04/24/2020 at 19:36 0 comments

    Discussions on the chat with @Tim and his reseach into weird latches made me look back at my MUX-based latch. The early idea was not working but later revisions did, once I created a fully functional MUX2.

    So here is a new version :

    This one works down to 2V. It uses the same values for all resistors, it has a /Q output as well, a single-phase clock input, and only 5 transistors.

    So far there are 5 transistors but for a whole register, the clock transistor can be shared among several bits.

    At one point, I had to add a 1G resistor to prevent totally bonkers behaviour of the simulator between the two PN junctions of the transistors in series, ignore it for real circuits. In rare cases however, Falstad indicated 25GV at that node...

    This time, I understood that there is one case that looks wrong but has no real effect on the circuit : when C is low and Din is high and feeds current into the /Q node, but it does not matter. In previous versions I added diodes to prevent current from flowing back but when absent, the working supply voltage can go lower and the noise immunity is better (or it can dissipate less power).

    The /Q output might not be required. In this case, the 1K resistor that feeds the base of the output transistor could be omitted. This would also speed the circuit up a bit.

    At some points I have problems with Falstad's sim (how surprising !) because the circuit wouldn't want to hold the High state on Q. I added a 470p capacitor to hold a bit of charge during the clock transition but later retries (and a full screen refresh) work without it... The value is approximate and the transistors' inherent capacitances would normally do the trick and add the tiny delay. Of course, a full simulation with xSPICE will work better :-)

    Of course Falstad is a bit buggy but in practice, it's a lot like in real circuits, which are not perfect either. If you can make your circuit work in many conditions in Falstad's sim, the real circuit has good chances to work in real life.

    The next step is obviously to create a DFF with a pair of these cells but in practice, timing is everything and with discrete computers, the clock strobe can be made very short, or successive latches can be strobed by clock signals with a little delay...

  • Analog Multiplexer Logic

    Tim04/10/2020 at 15:46 2 comments

    Not all weird discrete logic has to be based on ancient components.

    Most projects in discrete logic families focus on recreating ancient circuit styles (like RTL, DTL, DCTL or, as a bastard abberation, LTL) with the components that are still available today. It turns out that many of the specialized transistors are long gone. How about doing it the other way: Pick a minimal building block that is easily available today and base logic on that? 

    Browsing distributor listings I found an interesting category of small devices: Analog switches and multiplexers.

    Basic building Block: The 2:1 Multiplexer

    An example of an 2:1 analog multiplexer is shown above. These are not digital devices, but actual analog switches. The connection between B and A will be low ohmic when it is active and assume a very high resistance when deselected. This means that it can be used in both directions.

    There is an abundance of 2:1 switches available in very small SOT-363 packages (above) from different sources for prices that rival that of discrete transistors. A short listing of some of the devices I found on LCSC is in the table below:

    PartManufacturerPrice (100+)
    NC7SB3157On Semi$0.046
    74LVC1G3157Diodes Incorporated$0.037
    BL1551Shanghai Beiling$0.03
    CH443KJiangsu Qin Hang$0.036
    SGM3157Youtai Semiconductor Co$0.036

    Spice Model

    Unfortunately I was not able to find any spice model of these devices that is suitable for LTspice. So I made my own behavioral model as shown below, next to the entity symbol. Many parasitics are not considered here. Neither is the delay that the control logic is causing, so it can only be seen as a crude approximation.

    Note the biasing resistor on the output, which is very important to prevent LTspice from getting stuck in a metastable state. Spice does not really like switches...

    Building basic logic gates

    Most basic two-input gates can be realized with one or two analog multiplexers. It's interesting to note that the MUX is more accomodating to positive logic. Inversions typically require adding an additional multiplexer.


    AND Gate

    The OR gate can be realized in a very similar manner. NOR and NAND require an additional inverter.

    XOR Gate

    XOR can be realized by an multiplexer that selectes between an inverted and non-inverted version of the secondary input. XNOR is realized by swapping multiplexer inputs.


    Latches are the achilles heel of any logic family. Building a latch with a digital multiplexer is actually fairly easy and can be done with a single multiplexer by routing the output back to one of the inputs. However, this is not so easy with analog multiplexers, as they only act as a switch without any buffering or amplification.

    Instead, we will revert to a dynamic latch as shown below.

    The first multiplexer acts as a path gate. If the clk is high, the input data will be routed to the output where the storage capacitor is charged. If the clk is low, the output will be connected to a floating input, so that the charge on the capacitor is held. The second multiplexer acts as an output buffer.

    Obviously this is a bit tricky in operation as some of the charge will dissipate through leakage into the buffer control input and internal leakage in the multiplexer. A sufficiently high clock is required to allow cyclicated refreshing of the latch content.

    The figure above shows simulation results of the latch in operation. Since loading of the capacitor causes a current surge on the input, spikes are seen on the input signal. Proper buffers and dimensioning of the storage capacitor is necessary.

    Counter design

    To verify the functionality in a more complex circuit, I designed an 8 bit counter in spice. You can see some of the output traces above. I hope that some of the transient spikes disappear once real-world parasitics are added.


    This looks like a potential approach to build discrete...

    Read more »

  • Project proposal : Ring oscillators zoo !

    Yann Guidon / YGDES03/14/2020 at 01:30 14 comments

    We TTLers love to test technologies, play with parts and explore new (or old !) realms. And one of the first things we do when we get our hands on a new transistor is see how fast they can go !

    For example:
    #Ring Oscillators: Fairchild DTL 949 by @Dana Myers
    CBJT ring oscillator by @Ted Yapo
    Relay ring oscillator
    I2L ring oscillator by @Dana Myers
    2N2369 ring oscillator / #Discrete Bipolar Logic (RTL/LTL) /  Speed optimisation of pmbt2369 ring oscillator  by @Tim
    LTL ring oscillator by @Tim
    Emitter-Coupled Logic ECL ring oscillator by @Dana Myers
    74AC02 gated ring oscillator by @Ted Yapo
    BC548B 5 Stage Ring Oscillator by @agp.cooper
    Ring oscillators on Silego GreenPAK 4 by @whitequark ...
    (add yours here !)

    These days I'm contemplating "tasting" BFP740 (44GHz GBW but not in stock so far) and 2N2369 gates (I have a fistful but not enough to make anything interesting)...

    I propose to create a new project/page where we gather all the ring oscillators experiments, sort them by technologies, discuss on measurement details (and gotchas) and agree on a standard "size" to help tally and compare speeds, efficiencies etc.

    I was thinking that with my BFS480 (rated at 7GHz) I would need 9 inverters in series to have a reasonably observable waveform and a frequency that my HP5335A could accurately follow.

    Is anybody interested ?

View all 25 project logs

Enjoy this project?



Yann Guidon / YGDES wrote 06/26/2020 at 14:04 point

More ECL reverse engineering !

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Yann Guidon / YGDES wrote 05/20/2020 at 00:41 point

Some yummy ECL reverse-engineering there :

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Yann Guidon / YGDES wrote 04/24/2020 at 23:02 point

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George wrote 04/16/2020 at 12:17 point

SN7400N and К155ЛА3 are 2 legends of different sides of the earth.

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Yann Guidon / YGDES wrote 03/14/2020 at 17:17 point

Our friend @Artem Kashkanov  in München museum of computers :

Thanks for sharing :-)

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Artem Kashkanov wrote 03/14/2020 at 17:58 point

Huh... Now I need to add normal subs :D

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Yann Guidon / YGDES wrote 03/14/2020 at 18:12 point

of course !
you won't let us miss this awesome visit, right ? ;-)

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Yann Guidon / YGDES wrote 03/05/2020 at 00:46 point

I've come across a type/family of CMOS logic gates called "TSPC"

"True Single Phase Clock" latches

I can't yet wrap my head around it but it's fascinating...

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Tim wrote 03/05/2020 at 05:50 point

Isn't that the, more or less, standard way of doing edge triggered flip flops in CMOS?

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Yann Guidon / YGDES wrote 03/05/2020 at 06:19 point

it seems to be a whole family of design techniques for dynamic latches.
The static latches (using flip-flops, feedback, muxes...) are more common in the litterature. I know CMOS design stuff for more than 20 years and have heard about "domino logic" and other "precharged" gates but they are usually frowned upon, unless you have total control of the fab process...

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Tim wrote 02/27/2020 at 06:25 point

Since there is a lot of discussion about exotic bipolar logic here... I have never seen any mention of I²L:

It's much more useful in an integrated circuit, though.

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Yann Guidon / YGDES wrote 02/27/2020 at 09:43 point

yes I discovered it here too :-)

I wish there were discrete transistors with multiple collectors...

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Tim wrote 03/01/2020 at 10:35 point

In I²L they actually use emitter diffusions for the collectors. Will result in a low hfe, but it means that the problem actually reduces to finding discrete multi-emitter transistors.

One of the original papers* on I²L actually shows an implementation example where light is used for the current injection.


(why on earth is the nesting depth of comments limited!?! Can't respons to Yann)

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Yann Guidon / YGDES wrote 03/01/2020 at 15:17 point

@Tim that would be a very interesting read if you can find it :-)

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Yann Guidon / YGDES wrote 03/05/2020 at 00:48 point

Thanks @Tim !!!

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Yann Guidon / YGDES wrote 03/05/2020 at 07:00 point

Now if only that document was not paywalled :-(

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Yann Guidon / YGDES wrote 02/17/2020 at 18:56 point

In case you didn't know already ;-)

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Tim wrote 02/14/2020 at 21:50 point

I wrote an article about fast bipolar switching transistors for RTL. Looking at the transistors used on most of the RTL and DTL projects, this should be of interest to you:

Actually I made neat progress on RTL and DTL/LTL designs. Will try to publish more here

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Ken Yap wrote 02/14/2020 at 22:15 point

Thanks, that CDC6600 article is an interesting read even though I have no intention to experiment with old logic technologies. Also thanks for the blog post on 3¢ MCUs. The Padauk looks interesting now that there is toolchain, although the saving over 20¢ STM8S is not yet compelling for my uses.

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Tim wrote 02/14/2020 at 22:25 point

Yeah, I also thought I would just play around with ancient circuit design a bit... :) 

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Yann Guidon / YGDES wrote 02/15/2020 at 21:25 point

Welcome Tim !
and thanks for bringing the CDC6600 back to the stage's front (i'm also a Cray admirer ;-) )

Oh I I thought it was the 2369 that was used by CDC, I'm not sure I've heard of the 709...

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Tim wrote 02/16/2020 at 05:12 point

Thanks! Well, it took me quite a while to connect all the dots, although all the information is out there... Originally I found the 2369 via Nexperias product selector and was happy to have something that performed much better than the vanilla 847/3904. Good thing I did some simulation before diving into a build.

The knowledge about switching transistors seems to be something very obscure nowadays, otherwise it's hardly explainable why all RTL/DTL projects on the web insist on using suboptimal transistors? The 2369 comes in the same form factors and at almost the same price as the alternatives.

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Yann Guidon / YGDES wrote 02/17/2020 at 18:58 point

At least now the search engines have one more reference to provide when asked about your interrogations :-)

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Warren Toomey wrote 03/26/2019 at 02:50 point

(I posted a question but I think in the wrong area... try again!) I'm building a 16-bit tri-state program counter using 7400-family chips. I'm stuck with four 74HC161 4-bit counters and two 74HC241 8-bit buffers. Anybody know a way to reduce the chip count here, with TTL-level DIP devices? Need to increment, load, hold value & tri-state.

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Alastair Hewitt wrote 03/26/2019 at 11:04 point

The 74ALS561 is a 4-bit counter with tri-state. This would eliminate the need for the two buffers.

If you didn't need to load then the 74HC590 gives you an 8-bit counter with tri-state in one chip.

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roelh wrote 03/26/2019 at 11:41 point

But that's expensive, almost $6 at Mouser...

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Julian wrote 07/11/2018 at 06:15 point

Just thought I should leave this here: can't say I've ever seen these before...

... 74181s *in a narrow DIP package* :)

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Dave's Dev Lab wrote 07/11/2018 at 17:00 point

funny you should post that, as i picked up two of these at a surplus shop last week. it was the first time i had seen the 181 in that package!

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Olivier Bailleux wrote 02/11/2018 at 08:39 point

Do you know the ?

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Yann Guidon / YGDES wrote 02/13/2018 at 05:40 point

who doesn't ? :-)

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Dylan Brophy wrote 02/13/2018 at 06:15 point

true :-D

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Yann Guidon / YGDES wrote 02/13/2018 at 16:10 point

Actually, it is listed, but with the old address. I'm updating the details page now.

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Frank Buss wrote 11/06/2017 at 00:01 point

I designed a simple CPU some years ago, optimized for running Forth:
The prototype worked in a FPGA.

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Yann Guidon / YGDES wrote 11/06/2017 at 00:14 point

you cheated then ;-) what keeps you from building it out of discrete transistors like the #AYTABTU - Discrete Computer  or #ED-64: A discrete 8-bit computer ? :-P

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Frank Buss wrote 11/06/2017 at 00:43 point

I guess I could do this, but this would be another week long project, and I have already so many unfinished projects :-)

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tomtibbetts wrote 11/05/2017 at 21:54 point

Hi, maybe this is not the correct forum.  But I have an issue with ringing on clock pulses.  I am building a SAP 1 computer on PCBs ( and the clock circuit is producing clock pulses that sometimes have a bit of ringing on both the rising and trailing edges of the pulse.  it doesn't happen all the time but there does seem to be a pattern to it.  One of the clocks is used for the sequencer and the other is for clocking all the registers.  Because of the ringing, the counters sometimes get double clocked and will skip a count.  What would be causing this?  Any help is appreciated.  Thanks

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Yann Guidon / YGDES wrote 11/05/2017 at 22:55 point

It would be a good idea to detail everything in a log (or more) on the project's page, because so far, the only answer I can give is to try to add a series resistance, add a 100ohms adjustable in series and check the bounces with a scope to see which impedance matches your tracks ?
Also : make sure you have a balanced and clean clock tree to prevent crazy matching problems.

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Marcel van Kervinck wrote 11/06/2017 at 12:15 point

I found this document helpful, specifically figure 17.

For my computers I found there is some more ringing with FETs (74HCT) than with bipolars (74LS). But it never caused misbehaviour and I see no clipping, so I'm still with architecture (a) on my boards. I was prepared to add series resistors (option c) as a possible counter measure.

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Yann Guidon / YGDES wrote 09/23/2017 at 13:53 point

@256byteram , @Peter Bosch , @Tony Robinson  and @John Croudy  are still invited to the project :-)

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Yann Guidon / YGDES wrote 09/23/2017 at 02:38 point

Oh I missed that one :-D

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Bill Rowe wrote 07/11/2017 at 20:23 point

Hi: Is there such a thing as a practical logic minimizer?  Something where you would feed in a truth table with N inputs and M outputs and get out a configuration of 7400/4000 chips that would implement it?

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Ed S wrote 07/11/2017 at 20:28 point

Maybe Project Icestorm would do it? I know that with Xilinx' tools it's possible to get a logic netlist - not 7400, but logic gates. See

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jaromir.sukuba wrote 07/12/2017 at 08:24 point

Take a look at Logic Friday -

You enter logic table, software spits out schematics made of logic gates of your choice. I used that thing when designing my #Fourbit

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Dr. Cockroach wrote 07/12/2017 at 08:51 point

Thanks Jaromir for the link to Logic Friday, looks like a tool I can use for my project :-)

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Bill Rowe wrote 07/12/2017 at 10:51 point

Excellent - thanks.  I'll have a look.

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Yann Guidon / YGDES wrote 06/08/2020 at 01:54 point seems to be dead ?

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agp.cooper wrote 09/23/2017 at 06:48 point

I use Logic Friday. It is a GUI for Expresso. It has a 16 input/output limit so beyond that you need to go directly to Expresso (I use a bat/cmd file). It optimises for generic gates packages or die-area.

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Yann Guidon / YGDES wrote 01/11/2017 at 21:00 point

The list is growing. It feels good to be surrounded by so many ace designers !

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