4 ms·

> The reverse-engineering solves one mystery about the flip flop ... Looking at the reverse-engineered schematic, though, explains that a sharp pulse on the J p

> The reverse-engineering solves one mystery about the flip flop ... Looking at the reverse-engineered schematic, though, explains that a sharp pulse on the J pin will act like the clock, sending a pulse through the capacitor, turning off the transistor, and causing a high output. I assume this behavior is not intentional, and J inputs are expected not to transition as sharply as when I touched it with a ground wire. This paragraph caused chills to run up my spine. The fact that they used AC coupling on the clock line seemed clever because it saved a couple of transistors, but the fact that they depended on J and K always being "slow" was cringeworthy. If J and K were somehow not slow, the device would become "not a proper flip flop." And yet it went to the Moon, so I guess they knew something I don't. One other thought: When Ken grounded the J line, he was attaching it to essentially zero impedance to ground. The clock line will never have zero impedance, but it's reasonable to expect it to have lower impedance than regular signal lines. The sensitivity on J and K might be more a function of signal impedance than rise/fall time per se. Of course the two things are not unrelated. Potato/potahto.

iasay 4y ago
Fresh out of university as an EE in my first job I was surprised to find that the rule was "if it conforms to the tests then it works". Sometimes the tests were poorly designed or were narrow enough only to test the happy paths. Box ticked, ship to customer. I eventually moved to software at which point I discovered that it's even worse here. It's at least 10^6x more difficult to kill people though and you usually don't have to get off your chair to undo the carnage.
- pixl97 4y ago
  > "if it conforms to the tests then it works". Um, but what if someone does something outside of the tests. "Well, then it breaks". Yes, but this is software, how does it break? "Hopefully not badly and insecurely"
  - deleted 4y ago
    [deleted]
- bee_rider 4y ago
  Yes in software we don't even have to worry about trying to get the magic smoke back into the chip (very difficult).
- Aloha 4y ago
  Often its "If it appears to pass the test it ships".
kens 4y ago
Good comments. I don't have details on impedance vs fall time so I don't know how big the safety margin was. But this behavior caused me so much confusion when I was testing the module to reverse engineer it. By the way, this modules wasn't used in flight; it was part of a test box that was used on the ground. The Updata Link box onboard the spacecraft was built with different technology. Just want to avoid confusion :-)
somat 4y ago
Note that this box did not go to the moon, I don't think it would have been in the signal path(if I understand correctly it is a ground unit used to run unit tests on the radio transmission gear). Now as to whether similar loosely specified flipflops were in flight hardware, I have no idea. I also will note that even the much vaunted Apollo guidance computer was a case of worse is better. The Saturn 5, now that had a proper flight computer(the LVDC), designed by IBM, redundant everything, A computer that could not fail. The AGC in contrast not only had no internal redundancy, it was designed to fail (in case of error the system could very quickly reboot and resume running it's program where it left off). However the trade off was that the AGC was half the weight and twice the speed as the LVDC and they could carry two of the things to get the reliability they wanted.
adrian_b 4y ago
Before the flip-flops implemented as monolithic integrated circuits, all the flip-flops used AC-coupled clock inputs that required fast input clock edges and the correct operation of the flip-flops depended critically on various timing parameters. The reason for this is that it saved much more than a couple of transistors. Most flip-flops with relaxed timing requirements are of the so-called master-slave type, and the most frequent implementation variants need either 4 times or 3 times more transistors than are needed for an AC-coupled flip-flop, for which 2 transistors may be enough in its simplest variant. Even the first integrated flip-flops have used schematics similar to the earlier discrete flip-flops, until they were replaced by later models, using more transistors, such as the TTL 54/7472, 54/7473, 54/7474, 54/7476.
- dreamcompiler 4y ago
  Fair enough. I learned this subject during the TTL era after transistors got cheap. Capacitors were "ugly analog things" that were only used for Vcc buffering. I never knew AC coupled clocks happened before DC coupled clocks. Thanks!

iasay 4y ago

Fresh out of university as an EE in my first job I was surprised to find that the rule was "if it conforms to the tests then it works". Sometimes the tests were poorly designed or were narrow enough only to test the happy paths. Box ticked, ship to customer. I eventually moved to software at which point I discovered that it's even worse here. It's at least 10^6x more difficult to kill people though and you usually don't have to get off your chair to undo the carnage.

pixl97 4y ago
> "if it conforms to the tests then it works". Um, but what if someone does something outside of the tests. "Well, then it breaks". Yes, but this is software, how does it break? "Hopefully not badly and insecurely"
- deleted 4y ago
  [deleted]
bee_rider 4y ago
Yes in software we don't even have to worry about trying to get the magic smoke back into the chip (very difficult).
Aloha 4y ago
Often its "If it appears to pass the test it ships".

kens 4y ago

Good comments. I don't have details on impedance vs fall time so I don't know how big the safety margin was. But this behavior caused me so much confusion when I was testing the module to reverse engineer it. By the way, this modules wasn't used in flight; it was part of a test box that was used on the ground. The Updata Link box onboard the spacecraft was built with different technology. Just want to avoid confusion :-)

somat 4y ago

Note that this box did not go to the moon, I don't think it would have been in the signal path(if I understand correctly it is a ground unit used to run unit tests on the radio transmission gear). Now as to whether similar loosely specified flipflops were in flight hardware, I have no idea. I also will note that even the much vaunted Apollo guidance computer was a case of worse is better. The Saturn 5, now that had a proper flight computer(the LVDC), designed by IBM, redundant everything, A computer that could not fail. The AGC in contrast not only had no internal redundancy, it was designed to fail (in case of error the system could very quickly reboot and resume running it's program where it left off). However the trade off was that the AGC was half the weight and twice the speed as the LVDC and they could carry two of the things to get the reliability they wanted.

adrian_b 4y ago

Before the flip-flops implemented as monolithic integrated circuits, all the flip-flops used AC-coupled clock inputs that required fast input clock edges and the correct operation of the flip-flops depended critically on various timing parameters. The reason for this is that it saved much more than a couple of transistors. Most flip-flops with relaxed timing requirements are of the so-called master-slave type, and the most frequent implementation variants need either 4 times or 3 times more transistors than are needed for an AC-coupled flip-flop, for which 2 transistors may be enough in its simplest variant. Even the first integrated flip-flops have used schematics similar to the earlier discrete flip-flops, until they were replaced by later models, using more transistors, such as the TTL 54/7472, 54/7473, 54/7474, 54/7476.

dreamcompiler 4y ago
Fair enough. I learned this subject during the TTL era after transistors got cheap. Capacitors were "ugly analog things" that were only used for Vcc buffering. I never knew AC coupled clocks happened before DC coupled clocks. Thanks!