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Never trust a system that seems to be working

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Re: Never trust a system that seems to be working

#281

Post by kortex »

jallmann wrote: post #255
electroly wrote: post #43

This is why 4-20 mA is a common signaling standard in industrial automation. 4 mA means zero and 20 mA means one. 0 mA means broken transmitter !

Also issue with golang (+ protobufs) where you can't distinguish a populated zero value from the default "not present" value.

This drives me absolutely bonkers. We had a situation in a machine learning context where we should have been treating the output of a detector as NaN (it was deliberately opting out), but instead it was falling back to zero.

Last I checked (this was years ago) the way to deal with this is to wrap it in a sub-message (which are actually nullable), which is just kinda gross.

Re: Never trust a system that seems to be working

#282

Post by bombcar »

ddingus wrote: post #265
Gordonjcp wrote: post #243

Earlier quoted context omitted.

Pretty much every car made since the 1980s has had dual-circuit brakes with usually one front and one rear wheel cylinder on each circuit. Volvo being Volvo took it further with their 2-series cars in the 1970s and 1980s, where the front discs had four-pot calipers configured as two sets of two-pot calipers. Each braking circuit had one half of each front caliper and one rear caliper, so in the event of one half of t…

There is one subtle difference between automatic transmissions and manual ones and that is the manual one will allow a shift into a lower gear even when engine damage is likely. An automatic one will not, and will hold the current gear until car speed falls within shifting range. Ask me how I know :)

Yeah, newer ones can be too smart for your own good (but not theirs) - older ones had no problem with you slamming it into reverse or park at freeway speeds - ask me how I know!

Re: Never trust a system that seems to be working

#283

Post by mananaysiempre »

sneak wrote: post #269
mananaysiempre wrote: post #156

Earlier quoted context omitted.

Also why a standard keyboard has a Break key: used to be that a telegraph station would have a bell that rang if it detected a broken connection (a “break condition”), then people figured that deliberately inducing a break was a handy way to alert the remote operator. I think the first automated telegraph systems actually used current-loop signalling, but I’m not sure about the first practical ones; in any case, the…

I have a standard keyboard and it has no break key.

To clarify, by a “standard” keyboard I mean a (usually US) QWERTY PC keyboard with 101 to 105 keys (depending on the amount of Win/Super and Menu keys and the presence of an additional punctuation key to the left of Z). Those have three rows of three keys each above the arrows (not counting any “media keys” such as Power); the topmost one of those is PrintScreen / SysRq, Scroll Lock, and Pause / Break, though some of those labels may be omitted. I honestly have never seen a full-size PC keyboard without those. (How am I supposed to hit Ctrl-Break or Win-Break otherwise?)

Strictly speaking, there is no Break key in the same sense that there is no SysRq key—there are only special Break and SysRq scancodes that AT and PS/2 keyboards send instead of Pause and Print Screen when Ctrl or Alt respectively is depressed (thus the combined labels), while USB keyboards don’t even have those, they just send normal modifier sequences. But I don’t think that’s what you had in mind :)

Re: Never trust a system that seems to be working

#284

Post by thaeli »

bombcar wrote: post #129
nordsieck wrote: post #125

Earlier quoted context omitted.

> Some military trucks are designed the opposite way on purpose - because they’d rather have no brakes and be able to move than be stuck on the battlefield. Do you have any details on that? Or a source I could read? I'm pretty curious now. I drove trucks in the military - specifically, FMTV, HEMTT, HMMWV, and 5 tons. None of them had the reverse brakes you describe.

The deuce/half air-over-hydraulic brakes fail open (loss of air pressure leaves you with hydraulic brakes alone, loss of hydraulic leaves you with no brakes at all). However, there is a failure mode that leaves the brakes full on if the air cylinder jams. At the time it was an acceptable trade-off; later military vehicles have more in common with trucks than tanks, I suspect.

Losing brake boost in a failure condition is the typical failure mode in most vehicles with hydraulic brakes. A hydroboost failure in modern medium trucks reduces braking performance just as much as an air pack failure in the deuce and is just as scary..

The brakes, unboosted, are supposed to still be capable of stopping the vehicle. This is also why hybrid and electric vehicles with regenerative braking, that normally use brake-by-wire with a pedal feel simulator, fail through to using the PFS as a direct unboosted hydraulic cylinder.

Losing all brakes in a hydraulic circuit failure is just because those trucks are so old they only have single circuit hydraulic brakes. Part of the A3 upgrades was changing over to a modern dual circuit hydraulic system.

So basically, the issue is that medium trucks are in a weird in-between. Not heavy enough to require full air brakes, but heavy enough that some of the safety assumptions we make about light vehicles are potentially less true.

Re: Never trust a system that seems to be working

#285

Post by endianswap »

throwawayallday wrote: post #271
TaylorAlexander wrote: post #218

As I and others have previously pointed out [1] foone uses they/them pronouns and feels particularly frustrated when their posts get featured on HN, because users here seem unable to honor this simple preference of theirs. It looks like they deleted their tweet out of frustration with todays latest round of misgendering. I hope people on HN will learn to respect all members of our community. Yes that involves not ass…

Quoted post unavailable.

> no one is getting dehumanized

[citation needed]

Re: Never trust a system that seems to be working

#286

Post by beckingz »

csours wrote: post #206
SilasX wrote: post #138

Earlier quoted context omitted.

Similarly, the joke that the engineer’s second worst nightmare is, “it doesn’t work but it should”, while their worst nightmare is, “it works but it shouldn’t”.

I used to do industrial tech support and this gives me the raging anxieties.

I have an exceptional ability to fix things: often I can fix things before determining why they are not working.

This is immensely frustrating because I like to understand mechanical and electrical devices and take pride in my craft, so if I fix something too easily I don't trust that the fix will hold without a mental model of the thing.

Re: Never trust a system that seems to be working

#287

Post by thaeli »

rkagerer wrote: post #212
hedora wrote: post #61

Earlier quoted context omitted.

It is also why white traffic lights no longer mean "go". White means "the red glass lens in front of the incandescent bulb shattered again". Related, from train signals: > The invention of the track circuit in 1872 by William Robinson permitted automatic block signals. Most simply, a low voltage battery current runs up one rail and down the other. As long as current flows from the battery through one rail to the rela…

Would they have breaks (gaps) to segment the track into distinct stretches of conductors? How far down the line would the supply voltage run (or how far away would a train complete the circuit) before resistance losses degrade the system?

Yes, insulators were (and still are) used between track blocks.

The maximum length of a block with this system (versions of which are still used today) varies greatly depending on local conditions. I've seen 2k to 10k feet as a general performance range.

Re: Never trust a system that seems to be working

#289

Post by cafeinux »

sneak wrote: post #269
mananaysiempre wrote: post #156

Earlier quoted context omitted.

Also why a standard keyboard has a Break key: used to be that a telegraph station would have a bell that rang if it detected a broken connection (a “break condition”), then people figured that deliberately inducing a break was a handy way to alert the remote operator. I think the first automated telegraph systems actually used current-loop signalling, but I’m not sure about the first practical ones; in any case, the…

I have a standard keyboard and it has no break key.

It's also named "Pause key", but one could argue that if your keyboard doesn't have any of these two keys, it cannot really be called "standard".

Re: Never trust a system that seems to be working

#290

Post by greycol »

zoover2020 wrote: post #179
ianferrel wrote: post #159

Earlier quoted context omitted.

>I wonder why would US & Canada go a different way with regards to so many small things (think turn signals on cars being red for american cars as another example)? In many cases the US were the first to mass-manufacture things to some kind of standard, so the question is really: why did the rest of the world choose a different standard. Often they have a good reason (better design, maybe), but the US also has a good…

And standards don't adapt? To me this feels wrong and backwards

When ATMs were first implemented they would dispense money before or at the same time as giving your card back.

This resulted in premature conclusion errors. They went to the ATM to get money. They got their money so they left... forgetting their card.

When ATMs were updated they fixed the design error. Now the card popped out and had to be removed before money would dispense. This resulted in a spike of people leaving with money hanging out of the ATM because they had been trained that removing their card was the end of the task. (Which is why money now gets sucked back into the machine if it hasn't been removed fast enough).

The point of the anecdote (other than that you hire HCI experts before implementing an interface) is that implementing a new and objectively better system doesn't necessarily result in an objectively better outcome when replacing an incumbent worse system.

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