5 ms·

Reverse-engineering an early calculator chip with four-phase logic

81 points by chipsa 6y ago

kens 6y ago
Author here for your calculator questions :-)
- Waterluvian 6y ago
  I see "alarm" and "error" on the display. What's the difference?
  - kens 6y ago
    "alarm" is a low-battery alarm, while "error" indicates an overflow error.
- clairity 6y ago
  awesome article! especially love deep dives like this into subjects where i have only rudimentary understanding. i see that the 4-phase logic is time-based. are there similar logic systems that are voltage-based, so instead of a high and low, you had very high, high, low, and very low? (i'm thinking a decimal computer would be neat but maybe too difficult given the relative benefits.)
  - deleted 6y ago
    [deleted]
  - kens 6y ago
    Ignoring analog computers, it's much much easier to build a binary circuit than a multi-level circuit. However, I know of two systems with 4-level logic. One is von Neumann's IAS machine (1952) and copies (ILLIAC, JOHNNIAC, MANIAC, etc). For addition, it used an analog vacuum tube circuit to add the voltages from the two input bits and carry, so you ended up with something like 0V, 20V, 40V, or 60V. Then a threshold circuit converted this to the binary sum and carry bit outputs. Another example of 4-level logic is the Intel 8087, the math coprocessor chip used in the IBM PC. The microcode for math computations was too big to fit on the chip, so Intel used a 4-level ROM, with four transistor sizes for each storage location. So each "bit" gave you four different voltages. Comparators converted the four voltages into two bits. The result was twice the ROM density, allowing the microcode to fit. (In actuality, it was less than twice the density because of the additional circuitry.) Finally, flash memory uses multiple voltage levels to store up to 4 bits per cell. As far as decimal computers, there were lots of decimal computers in the 1950s and 1960s for business use. However, they used BCD (binary-coded decimal) or timed pulses, rather than 10 voltage levels.
    - clairity 6y ago
      neat, and thanks for the historical context! i didn't realize 4-level logic had been used in mass-market chips like the 8087.
  - exmadscientist 6y ago
    You do see multi-level signalling in some communication interfaces, such as MIPI-C (3-level) or PAM4. But it's usually a massive headache not worth bothering with.
- contingencies 6y ago
  What does Autonetics mean?
  - kens 6y ago
    Autonetics is a company name, a division of Rockwell. I don't think it means anything other than sounding high-tech, like cybernetics.
    - contingencies 6y ago
      Figured as much. Seems they mostly worked on military flight control, Boeing acquired them and they a have a mini history here: http://bos.ocgov.com/legacy3/newsletters/pdf/Honoring_the_Legacy_of_Autonetics.pdf http://bos.ocgov.com/legacy3/newsletters/pdf/Honoring_the_Le... Update: Found an oral history reference. The name was coined under the Eisenhower administration, along with it's etymological sibling Rocketdyne. Bad ass. https://airandspace.si.edu/research/projects/oral-histories/TRANSCPT/ATWOOD2.HTM https://airandspace.si.edu/research/projects/oral-histories/... Their inventions apparently include fundamentals to the field such as automated inertial navigation for flight control (1950), automated celestial navigation, and redundant flight control computers. They initially used memory based on the deferred transit of audio through crystal.
deleted 6y ago
[deleted]
dboreham 6y ago
"showing its hand-drawn layout" To me the traces look more like they were laid out with tape, not hand-drawn. Back in the day we used a kind of thin black masking tape that was sold in various widths for this purpose. It had the property that you could form it into a bend radius that looks just like the traces on this board.
- klyrs 6y ago
  What do you make of the X-shaped trace between the -7044 and -7042 chips? I'd expect tape to have constant width, not an irregular island like that. But then, you've got more experience with this.
  - exmadscientist 6y ago
    That could go either way. Not having any teardropping there at the four-way junction will lead to etch traps. Those aren't such a problem any more, but they sure were 50 years ago. So any competent layout person, no matter the method, would have eliminated them. I agree that this doesn't look quite like the tape layouts I'm used to, but I only did tape on bare copper for one-offs, never tape on photofilm. Of course, the rules for those processes are somewhat different even if the basic mechanism is the same. Also, those aren't "7044" and "7042" chips -- those are the date codes, weeks 44 and 42 of 1970, respectively. The IC marks are the top lines, "NRD2256" or "AC2261".
    - colejohnson66 6y ago
      It is interesting how all the date codes are 2 weeks apart. It almost looks like part numbers with the pattern (at first glance). As this calculator is 20+ years older than me, can someone enlighten me as to why the dates would be like that? The only thing I can think of is that the manufacturing runs were done sequentially instead of all at once. Was that a common thing for ASICs back then? I’m also curious why calculators were such a common thing then, yet they still cost over $1000 (in today’s money). I get it makes calculations quicker, but $1000 just seems excessive. Was it just the cost of manufacturing justified by the cost savings (in time) for businesses who bought them?
Stratoscope 6y ago
After the Tymshare diaspora, one of my friends worked briefly at Four-Phase Systems. They were at the corner of 280 and De Anza Blvd in Cupertino, where Apple's Infinite Loop buildings are now. The main thing I remember from a visit is how dreary the building was: musty dark interior hallways with low ceilings, Silicon Valley from the seventies. The tech was interesting though; their AL1 processor was based on four-phase logic like this calculator chip. https://www.google.com/search?q=four-phase+systems&tbm=isch https://www.google.com/search?q=four-phase+systems&tbm=isch https://en.wikipedia.org/wiki/Four-Phase_Systems https://en.wikipedia.org/wiki/Four-Phase_Systems
analog31 6y ago
Interestingly, the phase diagrams look a lot like the diagrams for controlling a charge-coupled-device (CCD) image sensor. While CCD's are being supplanted by CMOS sensors, they still have some specialized applications.

81 points by chipsa 6y ago

kens 6y ago

Author here for your calculator questions :-)

Waterluvian 6y ago
I see "alarm" and "error" on the display. What's the difference?
- kens 6y ago
  "alarm" is a low-battery alarm, while "error" indicates an overflow error.
clairity 6y ago
awesome article! especially love deep dives like this into subjects where i have only rudimentary understanding. i see that the 4-phase logic is time-based. are there similar logic systems that are voltage-based, so instead of a high and low, you had very high, high, low, and very low? (i'm thinking a decimal computer would be neat but maybe too difficult given the relative benefits.)
- deleted 6y ago
  [deleted]
- kens 6y ago
  Ignoring analog computers, it's much much easier to build a binary circuit than a multi-level circuit. However, I know of two systems with 4-level logic. One is von Neumann's IAS machine (1952) and copies (ILLIAC, JOHNNIAC, MANIAC, etc). For addition, it used an analog vacuum tube circuit to add the voltages from the two input bits and carry, so you ended up with something like 0V, 20V, 40V, or 60V. Then a threshold circuit converted this to the binary sum and carry bit outputs. Another example of 4-level logic is the Intel 8087, the math coprocessor chip used in the IBM PC. The microcode for math computations was too big to fit on the chip, so Intel used a 4-level ROM, with four transistor sizes for each storage location. So each "bit" gave you four different voltages. Comparators converted the four voltages into two bits. The result was twice the ROM density, allowing the microcode to fit. (In actuality, it was less than twice the density because of the additional circuitry.) Finally, flash memory uses multiple voltage levels to store up to 4 bits per cell. As far as decimal computers, there were lots of decimal computers in the 1950s and 1960s for business use. However, they used BCD (binary-coded decimal) or timed pulses, rather than 10 voltage levels.
  - clairity 6y ago
    neat, and thanks for the historical context! i didn't realize 4-level logic had been used in mass-market chips like the 8087.
- exmadscientist 6y ago
  You do see multi-level signalling in some communication interfaces, such as MIPI-C (3-level) or PAM4. But it's usually a massive headache not worth bothering with.
contingencies 6y ago
What does Autonetics mean?
- kens 6y ago
  Autonetics is a company name, a division of Rockwell. I don't think it means anything other than sounding high-tech, like cybernetics.
  - contingencies 6y ago
    Figured as much. Seems they mostly worked on military flight control, Boeing acquired them and they a have a mini history here: http://bos.ocgov.com/legacy3/newsletters/pdf/Honoring_the_Legacy_of_Autonetics.pdf http://bos.ocgov.com/legacy3/newsletters/pdf/Honoring_the_Le... Update: Found an oral history reference. The name was coined under the Eisenhower administration, along with it's etymological sibling Rocketdyne. Bad ass. https://airandspace.si.edu/research/projects/oral-histories/TRANSCPT/ATWOOD2.HTM https://airandspace.si.edu/research/projects/oral-histories/... Their inventions apparently include fundamentals to the field such as automated inertial navigation for flight control (1950), automated celestial navigation, and redundant flight control computers. They initially used memory based on the deferred transit of audio through crystal.

deleted 6y ago

[deleted]

dboreham 6y ago

"showing its hand-drawn layout" To me the traces look more like they were laid out with tape, not hand-drawn. Back in the day we used a kind of thin black masking tape that was sold in various widths for this purpose. It had the property that you could form it into a bend radius that looks just like the traces on this board.

klyrs 6y ago
What do you make of the X-shaped trace between the -7044 and -7042 chips? I'd expect tape to have constant width, not an irregular island like that. But then, you've got more experience with this.
- exmadscientist 6y ago
  That could go either way. Not having any teardropping there at the four-way junction will lead to etch traps. Those aren't such a problem any more, but they sure were 50 years ago. So any competent layout person, no matter the method, would have eliminated them. I agree that this doesn't look quite like the tape layouts I'm used to, but I only did tape on bare copper for one-offs, never tape on photofilm. Of course, the rules for those processes are somewhat different even if the basic mechanism is the same. Also, those aren't "7044" and "7042" chips -- those are the date codes, weeks 44 and 42 of 1970, respectively. The IC marks are the top lines, "NRD2256" or "AC2261".
  - colejohnson66 6y ago
    It is interesting how all the date codes are 2 weeks apart. It almost looks like part numbers with the pattern (at first glance). As this calculator is 20+ years older than me, can someone enlighten me as to why the dates would be like that? The only thing I can think of is that the manufacturing runs were done sequentially instead of all at once. Was that a common thing for ASICs back then? I’m also curious why calculators were such a common thing then, yet they still cost over $1000 (in today’s money). I get it makes calculations quicker, but $1000 just seems excessive. Was it just the cost of manufacturing justified by the cost savings (in time) for businesses who bought them?

Stratoscope 6y ago

After the Tymshare diaspora, one of my friends worked briefly at Four-Phase Systems. They were at the corner of 280 and De Anza Blvd in Cupertino, where Apple's Infinite Loop buildings are now. The main thing I remember from a visit is how dreary the building was: musty dark interior hallways with low ceilings, Silicon Valley from the seventies. The tech was interesting though; their AL1 processor was based on four-phase logic like this calculator chip. https://www.google.com/search?q=four-phase+systems&tbm=isch https://www.google.com/search?q=four-phase+systems&tbm=isch https://en.wikipedia.org/wiki/Four-Phase_Systems https://en.wikipedia.org/wiki/Four-Phase_Systems

analog31 6y ago

Interestingly, the phase diagrams look a lot like the diagrams for controlling a charge-coupled-device (CCD) image sensor. While CCD's are being supplanted by CMOS sensors, they still have some specialized applications.