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!
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 relay and back through the other rail, the relay remains energized and routes current from another battery to the green lamp of a clear signal. But when a train is present, the circuit is short circuited by the steel wheels and axles of the cars; a broken rail or an open switch will also break the circuit. As a result the relay is opened and the signal displays red. The system is arranged so the failure of any component results in a red indication or a dark signal (which must be interpreted as the most restrictive aspect that signal can display).
https://www.trains.com/trn/train-basics/abcs-of-railroading/...
For example, cars have "fail open" brakes but have independent cylinders so that it is relatively hard to have all four wheels fail at the same time (older cars had single master cylinders) - and one of the tradeoffs is that people want cars to get going immediately and not wait for brake cylinders to "charge up".
As more and more things get computerized, you have more options for "intelligent failover" - anyone who used to drive an old manual car knew you could slow down using the engine - but now the computer could detect brake failure and do things for you to help recover.
And as seen from other comments, the modern US military prioritizes "on-road safety" over "under ongoing attack ability" - partially because of changing strategies I suspect, but also from the realization that the US military kills more members of the US military each year via accidents, etc, than anything else.
You can use engine braking in automatic vehicles too, by simply changing to a lower gear.
An automatic one will not, and will hold the current gear until car speed falls within shifting range.
Ask me how I know :)
This of course meant the entire cars safety and stability control systems, ABS and traction control were all disabled. It drove like a tank.
The fix, obviously, was to replace the brake booster primer pump (hybrid car). It had worn enough to no longer recalibrate. Still perfectly functional, and would have kept working for a long time, bit it wouldn't quite meet the factory expected parameters so it failed it's test.
It appears that ZF 4HP20/4HP22 gearboxes can do this an unlimited number of times without apparently being damaged, but I'm not trying it on either of mine.
And emergency brake that's wholly separate circuit.
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.
At the time it was an acceptable trade-off; later military vehicles have more in common with trucks than tanks, I suspect.
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.
I remember a house at the bottom of a steep mountain grade that got plowed into on a regular basis especially during that time.
EDIT: hmm... seems freezing might still be a way to cause failure?
In other words, the brakes would apply on loss of pressure, instead of via pressure.
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?
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.
The transmitter and receiver are wired in parallel and connected to a power supply through a series resistor. When the receiver receives the signal from the transmitter, it pulls the supply voltage down, which then causes the transmitter to stop transmitting, which then causes the receiver to stop pulling the supply down, and the cycle continues. The end result is that if there's no obstruction, the supply voltage pulses and the system checks for this. If there's any other type of condition, the signal is either 0 or VCC. Basically, the system is designed so that false positives are extremely unlikely since that's the worst case scenario.
There's some capacitors in the transmitter/receiver to control the frequency.
edit: it gets you protection against one of the two failing closed, duh
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 :)
I would be willing to bet that none of the ten most popular/best-selling computers in the country for the last 5-10 years have had a key that says “break”.
I'll take that bet and offer up a break key from my active keyboard if I'm wrong.
For example, on most modern Thinkpads, Pause is Fn+P, and thus Break is Ctrl+Fn+P.
In electrical circuits made with normal conductors and insulators, e.g. copper wires and organic polymer insulators, it is possible to ignore the currents that branch from the wires and close through the insulators, because they are extremely small (e.g. in the femtoampere range), but frequently it is not possible to ignore the voltage drops along the copper wires, which are caused by the resistances of the wires, and which may be in the millivolt range.
As much as I love academia, I really wish there were more instructors with real experience that could say things like, "for math purposes assume a zero-loss conductor" while still emphasizing that real life is not so simple.
We did have a healthy incubation program, where instructors helped companies with product development, so perhaps that is what brought the real-world experience.
Generally, the biggest problem with math, physics and chemistry is that if you miss just one lesson (no matter if you physically miss it, or in worst case miss out understanding a concept for a couple minutes), you can easily end up "left behind" for years. With history, arts, languages, a number of computer science topics or most of biology you can either skip out on a certain subject or catch up easily on your own, but not getting a specific part in math/physics/chemistry and lacking the resources to follow up is setting oneself up for exponential learning gap later on.
The societal problem is that we've not set up schools or universities to deal with this "failure mode". Privileged kids can have after-school teaching or siblings helping out, but most will sooner or later resign and hate these subjects for the rest of their lives - and that is incredibly sad.
Why do I get a sneaking suspicion that few in the machinery of education want this, as it is a fast-track to continuous accountability?
A nice-to-have addition to your table saw is a big red stop button you can reach with your foot so you don't have to take your hands off of your work pieces to shut the saw down if things have gotten more interesting than you expected them to. Three wire control makes this super easy to add (once you've found the big red mushroom-head n/c switch, of course).
[0] https://twcontrols.com/lessons/control-wiring-3-wire-control...
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.