Fix me now - I/O letters #35

Garbage! You can fix anything as long as you have the right tools. Like sawdust for a shafty gearbox, or a screwdriver to turn back odometers. While our man Dan Rutter can't reccommend you try any of the above 'tools', he can convince you that the logitech MX700 is one nice device to win for IOOTM.

IOOTM: Mad scientist department

I: I have been studying some chemistry, and have found out about exothermic reactions and endothermic reactions. Exothermic gives out heat and endothermic takes in heat. What if a custom liquid cooling rig was built which used an endothermic reaction? If suitable chemicals were found, could this be a very easy and cold method of removing that unwanted thermal problem?
Gabe Haines

O: Sure, this'd work, for as long as your chemical supply lasted. You'd need a couple of buckets of reagents, and you'd need to keep topping them up, and you'd need to do something with the waste, too.

One simple endothermic reaction -- actually, it's not technically a reaction at all -- is the one used in commercial 'instant cold packs' for treating injuries. The packs get cold when you squeeze them, because you've ruptured a seal inside that lets some ammonium nitrate (more often used as fertiliser or explosive. . .) dissolve in some water. Many things will cool water when you dissolve them in it, but ammonium nitrate is especially good at it, absorbing 26.2 kilojoules of heat per mole. One mole of ammonium nitrate weighs 80 grams.

One joule is one watt-second, so 26.2 kilojoules is 26,200 watt-seconds, which is the amount of energy a 100 watt heater (like an overclocked CPU) will emit in 262 seconds. An ammonium nitrate and water cooling system for a 100 watt load would, therefore, go through about 18.3 grams of ammonium nitrate per minute.

Of course, you could set up an evaporator arrangement to reclaim the ammonium nitrate (and distil back the water, as well, if you were feeling really clever). This is left as an exercise for the reader.

If you're going to use something consumable to cool a CPU -- presumably because you're shooting for a ludicrous overclocking record -- you might as well do it the traditional way and use liquid nitrogen. No messy mixing and metering required; just seal a foam cup to the top of your processor, and pour in more LiN2 whenever it looks like boiling dry!


Insufficient entropy

I: How do computers (which can only calculate formulas) generate random numbers? They can't cycle through a list and stop at a random point in the list, because that requires a random number first. Is there a formula that can generate a random number?
John Guest

O: They don't (well, almost all of them don't). They generate 'pseudo-random' numbers.

Pseudo-random numbers may have a random distribution -- generate a million ones and zeroes and you'll get pretty much the same number of each, as you should -- but there will always be non-random patterns to the output -- repeating sequences that wouldn't exist in truly random data.

This is because a normal computer has no source of true randomness to use as the input to a pseudo-random number generator (PRNG) program. PRNGs can twiddle 'seed' data into superficially-random-looking numbers as big as you like, but they can only produce output as random as the seed. Given the same seed, a PRNG will always give the same output.

The quick'n'easy way to seed a PRNG is to feed it some time-based number. If you seed a PRNG with the number of milliseconds that have passed since midnight, for instance, you'll get output with a repeating 24 hour pattern.

This doesn't matter if the PRNG is just generating 'random' events in a computer game (though the very non-random spraying of bullets in games like Counter-Strike can be used to players' advantage), but it's important for things like cryptography. If the random numbers used for an encryption key aren't actually very random and an attacker knows it, they can restrict a brute force keyspace search to only those areas of the keyspace that the encoding computer could have actually used. This can result in surprisingly easy decryption of apparently highly secure data.

A PRNG seeded by something genuinely random will do a much better job. SGI's famous but now defunct 'lavarand' project used cameras looking at lava lamps as the randomness source, for instance. It's mutated into 'LavaRnd' ( www.lavarnd.org ), which is less hilarious but more useful.

Via's Nehemiah-core C3 CPUs also have a proper random number generator (RNG) built in; some software is now starting to support it.


Pixel fever

I: Why is it that my laptop (Dell Inspiron 8200) has a native resolution of 1,600 x 1,200 with only a 15in screen, and yet I can't find a single LCD desktop monitor that can handle a resolution this high? Even the best 19in Sony screens will only go as high as 1,280 x 1,024. I've been thinking (dreaming?) about replacing my 21in Sony Trinitron (also running at 1,600 x 1,200) with a couple of LCD flat screens -- but not at the cost of a reduced desktop area. 
Simon Hobman

O: Yes, 1,600 x 1,200 desktop LCDs exist -- they're all at least 20in, though, and generally sell for at least $3000.

Just off the top of my head (he lied, after a brief burst of research), there's NEC's 20.1in MultiSync LCD2080UX, and IBM's ThinkVision L200p 20.1in (a cheapie, at only $2300-odd!), and Hitachi's CML200B, and Samsung's SyncMaster 213T.

Apple have widescreen displays that fit the bill, too; the 23in Apple Cinema HD Display is 1,920 x 1,200, and the cheaper 20in Apple Cinema Display still manages 1,680 x 1,050.

1,600 x 1,200 desktop LCDs could be made a bit cheaper if they used laptop panels instead of the larger, coarser-pixeled desktop ones, but desktop monitors are typically viewed from further away than laptop screens, and the monitor makers don't want to bet that people will be willing to pay for pixels they can barely perceive.

Display devices will have invisibly small pixels in the fullness of time, but right now it's a pain to make everything big enough to be legible on a small, hyper-resolution screen at desktop distances.

Earthy inquiry

I: Power leads have three prongs on the plug -- positive, neutral and earth. My understanding is that the earth is for safety reasons. Most metal appliances have the earth attached to the casing to let wayward current go to ground instead of through the user, however there is no such connection on metal computer cases. Is there one inside the power supply?
Thomas Crossman

O: Yes, there is. The earth contact on the power cable connects to the PSU casing, and thence to the computer chassis via the PSU casing's connection to the chassis, and several other paths. All of the black wires coming out of the PSU are connected to the same earth, and normally end up connected to the chassis via motherboard mounts, drive casings and so on.

It's just possible to foul this up if you've got a fancy lacquered PSU, mount it with low-conductivity anodised screws, put fibre washers on top of the standoffs under your motherboard, and mount drives in plastic fan cages or rubber-suspended noise isolators.

If you've got little or no path to earth from the computer chassis, then a PSU failure that puts 240V on some output wire or other can result in a nasty belt if you touch the computer or a device attached to it.

Live-case problems are much more likely to be caused by a defective power lead or mis-wired wall socket, though.


Woes * 2

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