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Apollo 11 vs. USB-C Chargers (2020)

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Re: Apollo 11 vs. USB-C Chargers (2020)

#51

Post by jcalvinowens »

>> others point out that the LVDC actually contains triply-redundant logic. The logic gives 3 answers and the voting mechanism picks the winner.

This is a very minor point... but three of something isn't triple redundancy: it's double redundancy. Two is single redundancy, one is no redundancy.

Unless the voting mechanism can somehow produce a correct answer from differing answers from all three implementations of the logic, I don't understand how it could be considered triply redundant. Is the voting mechanism itself functionally a fourth implementation?

Re: Apollo 11 vs. USB-C Chargers (2020)

#52

Post by bpye »

adgjlsfhk1 wrote: post #50
vlovich123 wrote: post #12

Is the weight/cost calculus sufficiently improved now that it’s cheaper to shield the processor in it’s entirety rather than trying to rad harden the circuitry itself (much more expensive due to inability to use off the shelf parts & limits the ability to use newer tech)? If I recall correctly this was one of the areas being explored by the mars drone although not sure if Mars surface radiation concerns are different…

one thing worth remembering is that a bigger computer runs into a lot more radiation. the cortex m0 is about .03mm^2 vs about .2m^2 for the Apollo guidance computer. as such, the m0 will see about 6 million times less radiation.

Aren't the smaller transistors going to be more susceptible to damage and bit flips though?

Re: Apollo 11 vs. USB-C Chargers (2020)

#53

Post by lazide »

jojobas wrote: post #20
vlovich123 wrote: post #12

Is the weight/cost calculus sufficiently improved now that it’s cheaper to shield the processor in it’s entirety rather than trying to rad harden the circuitry itself (much more expensive due to inability to use off the shelf parts & limits the ability to use newer tech)? If I recall correctly this was one of the areas being explored by the mars drone although not sure if Mars surface radiation concerns are different…

Aren't high energy space particles a pain in a way that the more shielding you have, the more secondary radiation you generate?

It depends on the type of shielding. For gamma radiation, lead only is a definite problem this way. As is neutron and high speed charged particles/cosmic rays.

Water less so.

Re: Apollo 11 vs. USB-C Chargers (2020)

#54

Post by lmm »

hinkley wrote: post #37
AnotherGoodName wrote: post #10

Pretty much all USB chips have a fully programmable CPU when you go into the data sheets. It feels silly for simple hid or charging devices but basic microcontrollers are cheap and actually save costs compared to asics.

I still want to see postgres or sqlite running straight on a storage controller some day. They probably don’t have enough memory to do it well though.

Booting Linux on a hard drive was what, 15 years ago now?

Re: Apollo 11 vs. USB-C Chargers (2020)

#55

Post by [deleted] »

jcalvinowens wrote: post #51

>> others point out that the LVDC actually contains triply-redundant logic. The logic gives 3 answers and the voting mechanism picks the winner. This is a very minor point... but three of something isn't triple redundancy: it's double redundancy. Two is single redundancy, one is no redundancy. Unless the voting mechanism can somehow produce a correct answer from differing answers from all three implementations of the…

[deleted]

Re: Apollo 11 vs. USB-C Chargers (2020)

#56

Post by SV_BubbleTime »

somethingsaid wrote: post #32
jojobas wrote: post #21

Earlier quoted context omitted.

That sounds way too low. Modern fly-by-wire planes are said to have 12-way voting.

If you read the link it’s actually two cpu cores on a single cpu die each returning a string. Then 3 of those cpus send the resulting string to the microprocessors which then weigh those together to choose what to do. So it’s 6 times redundant in actuality.

That’s not 6x though.

It’s a more solid 3x or 3x+3y, which… if you had a power failure at a chip doesn’t take a 6x to make it 5x. It makes it 4x with the two remaining PHY units because two logical cores went down with one error.

The x being physical units, and the y being CPUs in lockstep so that the software is confirmed to not bug out somewhere.

It’s 6x for the calculated code portion only, but 3x for CPU and 1-3x for power or solder or circuit board.

I know it’s pretty pedantic, but I would call it the lowest form for any quality, which is likely 2-3x.

Re: Apollo 11 vs. USB-C Chargers (2020)

#57

Post by gurchik »

p-e-w wrote: post #36
jojobas wrote: post #21

Earlier quoted context omitted.

That sounds way too low. Modern fly-by-wire planes are said to have 12-way voting.

I don't understand this. If two or more computers fail in the same way simultaneously, isn't it much more likely that there is a systemic design problem/bug rather than some random error? But if there is a design problem, how does having more systems voting help?

Having at least 3 computers allows you the option to disable a malfunctioning computer while still giving you redundancy for random bit flips or other environmental issues.

Re: Apollo 11 vs. USB-C Chargers (2020)

#58

Post by GuB-42 »

p-e-w wrote: post #36
jojobas wrote: post #21

Earlier quoted context omitted.

That sounds way too low. Modern fly-by-wire planes are said to have 12-way voting.

I don't understand this. If two or more computers fail in the same way simultaneously, isn't it much more likely that there is a systemic design problem/bug rather than some random error? But if there is a design problem, how does having more systems voting help?

It is possible for a random error to affect two computers simultaneously, if they are made from the same assembly line, they may fail in exactly the same way, especially if they share the same wires.

That's the reason I sometime see that for RAID systems, it is recommended to avoid buying all same disks at the same time, because since they will be used in the same way in the same environment, there is a good chance for them to fail at the same time, defeating the point of a redundant system.

Also, to guard against bugs and design problems, critical software is sometimes developed twice or maybe more by separate teams using different methods. So you may have several combinations of software and hardware. You may also have redundant boards in the same box, and also redundant boxes

Re: Apollo 11 vs. USB-C Chargers (2020)

#60

Post by scaredginger »

coder543 wrote: post #47
hinkley wrote: post #38

Earlier quoted context omitted.

Analog computers have to be rebuilt if it turns out the program is wrong though, don’t they?

In the context of this thread, I believe even a digital computer would have to be rebuilt if the program is wrong... :P Unless you typically salvage digital computers from the wreckage of a failed rocket test and stick it in the next prototype. If the FCC is wrong, kaboom.

I'm pretty sure you can perform tests and find defects without actually flying the rocket
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