Electronics > Projects, Designs, and Technical Stuff
Another reason to hate "soft touch" power switches
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Richard Crowley:
When I was managing the E-test lab (on-wafer evaluation of test-dice during process development) we had a problem with mains power interruptions for a period. The hits were pretty short so that most equipment would keep running, although after scrambling the operation. 

After many very valuable test wafers were ruined, I got a bunch of power contactors and installed one in each equipment rack.  So that power for the rack was provided through the contactor, but it would drop-out with any power interruption.  At the local surplus store, I found some nice green N.O. "START" buttons, and red N.C. "STOP" buttons. and built them into a nice 1RU panel at the top of each rack.

That gave us the opportunity to go around to each automated test station and re-start it properly vs. letting the zombie equipment to go on a rampage.
SiliconWizard:

--- Quote from: IDEngineer on July 18, 2019, 09:57:41 pm ---
--- Quote from: SiliconWizard on July 18, 2019, 09:41:41 pm ---Well as for me, again I don't really mind those soft power buttons. As long as they are implemented correctly (which granted is not always the case).
--- End quote ---
Granted, but my question is: What value do they ADD that justifies their complexity, cost,

--- End quote ---

Well, as for the "value", I think we already covered that. Additionally, they add a more "modern" look and feel. You are of course entitled to personally find those values are of no value to you.

As I said earlier though, I think you're mistaken in thinking they add complexity and cost. In many cases, in modern equipment, I'd say they are pretty cheap to add, and that they are actually much simpler overall than having to deal with proper handling of sudden power-on and power-off, not just for the graceful shutdown cases, but also any time a specific power supply sequencing is required, and other reasons. Anything that is less expensive/or more flexible to deal with in software than purely in hardware.

Depending on the implementation, upon soft shutdown, they can do as little as putting the whole instrument in a "sleep" mode (thus not necessarily shutting down the main power supplies but merely shutting down everything that was already meant to be enabled/disabled by software and then put the display(s) and CPU(s) to sleep as well. They can additionally partially shut the main power supplies down (usually only partially). This last thing doesn't cost anything much either these days. Many DC/DC power modules used in those instruments already contain control signals to shut the PS down (as with computer PSUs), so the cost is practically nothing.


--- Quote from: IDEngineer on July 18, 2019, 09:57:41 pm ---and the added downside of often being implemented INcorrectly?

--- End quote ---

Well, this is a good point. But a "hard" power switch can be implemented incorrectly in many ways as well due to reasons exposed before...
Again, I think most manufacturers will favor this solution for several reasons, but the main one being that everything that depends more on software than hardware is considered much more flexible and much easier to fix if there are bugs, so in that respect, that's more of a comfortable option for the manufacturer than for the user.

I personally don't mind much, but I think there should be some sense of proportion when designing a lab instrument. Whereas I expect a modern scope to be much like a dedicated computer, thus I expect it to function much like one (except I still expect it to boot faster than a typical computer!), for a simple lab power supply, I would find that silly.

Now when a manufacturer has a whole range of instruments, that still makes sense to use a common OS, for instance, and common subsystems in all of them when possible. So again what's a benefit for the manufacturer is not necessarily a direct benefit for the users.
IDEngineer:

--- Quote from: SiliconWizard on July 19, 2019, 12:45:51 pm ---...they are actually much simpler overall than having to deal with proper handling of sudden power-on and power-off, not just for the graceful shutdown cases, but also any time a specific power supply sequencing is required, and other reasons. Anything that is less expensive/or more flexible to deal with in software than purely in hardware.
--- End quote ---
Here's how it pencils out:

1) Some devices don't care about power loss. There is no timing requirement to losing power, regardless of reason. They need take no special action to handle loss of power.

2) The remaining devices do care about power loss, and must design to accommodate it, again regardless of reason (intentional or unintentional).

Device type (1) can use either type of power switch because it literally doesn't matter. Device type (2) MUST handle power loss because it can occur for lots of reasons unrelated to the power switch - and once it must handle unexpected power loss, it can automatically handle a hard switch because that is no different from a power company event.

The moment someone suggests there is a "middle ground", where a soft switch is needed because the OS "needs to do something special when power is lost", we're right back at #2 above: What happens when power is lost at the wall? The device can either accommodate that (in which case it's type 1) or it can't (in which case it's type 2), and in both cases a properly designed instrument does not require a soft switch.

That leaves one middle ground: An IMproperly designed instrument that expects power to "play nice" 100% of the time, where they assume power is never lost unexpectedly, and so the "Engineers" on that project leave a gaping hole that causes inconvenience (or worse) when power is nevertheless lost without warning. I would consider such a "middle ground" to be a broken design because a properly designed instrument should anticipate, and gracefully handle, predictable events like power loss. And those that do immediately put themselves into type 2 above, where a hard switch works as well as a soft one.


--- Quote ---Anything that is less expensive/or more flexible to deal with in software than purely in hardware.
--- End quote ---
That's precisely the point: Graceful handling of power loss IS a software event. It is software which is notified that power has been lost, and reacts by saving important non-volatile information. Yes, there's a hardware cost for the AC-loss detection circuitry but that's vanishingly small in any modern instrument's BOM budget, and once that's in place you get exactly what you described: "less expensive/or more flexible to deal with in software", to the extent that if it's later realized that some important piece of data wasn't being preserved that can be corrected with a firmware update.

Bottom line: Power loss either matters or it doesn't. If it does, you have to handle the unexpected loss of power anyway. And once you've done that, a soft switch doesn't add anything (the OS is already getting notified of power loss, so there's no additional information conveyed by actuating a soft switch).
Dundarave:
Back in the day, one had to “gracefully” shut down gear so that cached data would be written out to slower permanent storage, i.e. hard drives, etc.  Caching, and thus the need to be careful about blowing away the cache before it get written, is essentially a hack to mitigate read/write wait times.

Given that almost everything is now solid-state and the data speeds are so fast, what kind of test gear would still require such a process?  If a scope can save giga-samples per second in real time, surely it can save your trace color selection in the time between “Ok” and when the power goes out...
rrinker:
 I do not like leaving ANYTHING on when I am not there - so everything on my bench is plugged in to a power strip that has a nice handy remote on/off hard switch with a pilot light, which I have mounted up under the corner next to where my socket is for wrist straps.
 For my basement/model train room, I have all the wall outlets where everything plugs in to controlled by a central switch to shut off the whole thing when I am not there.
 About the only thing I leave running 24/7 is my main desktop computer and my server, and even those get shut down when I am away for any extended period of time like I am now (no worries, I posted about being away on another forum and someone mentioned security. Well, I have two of the most evil, vicious guard Pugs ever known, plus my house is not empty just because I happen to be gone).

(the Pugs are so vicious they might trip you in attempting to be the first to get scratches and treats even if you are a complete stranger)
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