Did you know that some cases ground the front-panel IO?

[VirtualLarry]

This tidbit comes from Rudy Toody in this thread:
http://forums.anandtech.com/showthread.php?t=2078381

Apparently, on some cases, the power and/or reset switches, have to be plugged in with a specific polarity. They are not just SPST switches connected to the motherboard, but rather, one of the pins is grounded! This means that if you connect the power switch backwards, the mobo will never turn on, or will turn on and then turn off in a few seconds.

I never would have guessed this if it hadn't been pointed out to me. Apparently, many Antec cases share this design "feature".

 

[Fayd]

so when putting the computer together, follow the recommended wiring diagram for how to wire the front panel connector. how the hell hard is that?

 

[BTRY B 529th FA BN]

so when putting the computer together, follow the recommended wiring diagram for how to wire the front panel connector. how the hell hard is that?

When the I/O panel & the panel wiring isn't clearly marked with "+" & "-" signs it gets a little more difficult

 

[nitromullet]

When the I/O panel & the panel wiring isn't clearly marked with "+" & "-" signs it gets a little more difficult

...but the wires themselves are color coded. They are generally white and some other color - white is negative, the color positive.

 

[Modelworks]

If the case is following the UL listed rules any exposed metal has to be grounded. On cases like the Antec 900 that have the ports where the outer ground shell can be touched by a hand it has to be grounded . Now if the case is connecting the actual switch to case ground through that ground connection and not using a wire going back to the motherboard that is a violation of ATX spec.

The only way to tell is use a meter, unplug the switch wire from the motherboard and check the resistance between both pins and the case ground, it should be infinite on both wires.

 

[EarthwormJim]

I thought this was assumed...

I can't recall a single computer case that I've dealt with where polarity did not matter on the switches.

 

[MJinZ]

If it doesn't work, flip it around...

We need an article and thread on this?

 

[VirtualLarry]

The only way to tell is use a meter, unplug the switch wire from the motherboard and check the resistance between both pins and the case ground, it should be infinite on both wires.

Read the thread I linked. Someone did that, and it was not infinite on one of the wires.

 

[VirtualLarry]

I thought this was assumed...

I can't recall a single computer case that I've dealt with where polarity did not matter on the switches.

Conventional wisdom was that both the power and reset switches aren't polarized. In fact, some mobos make this assumption, as they don't give a + and - for those pins.

 

[BTRY B 529th FA BN]

...but the wires themselves are color coded. They are generally white and some other color - white is negative, the color positive.

... but that is not common knowledge

 

[EarthwormJim]

Conventional wisdom was that both the power and reset switches aren't polarized. In fact, some mobos make this assumption, as they don't give a + and - for those pins.

Not really sure where you're getting conventional wisdom. Every single instruction I've ever read for assembling a PC has suggested that the user observe polarity on the switches, and not just the LEDs.

I myself have accidentally inserted the switch connectors in backwards on many occasions, only to have the computer not power up when I press the power button. This is using Lian-Li, Coolermaster, Silverstone, Antec, Enermax, and generic brand cases.

I realize momentary switches themselves do not have any polarity, but the switches are housed in metal, and every piece of metal on a case is grounded (or should be).

I'm guessing that you've always installed the switch connectors correctly every time you've assembled a computer. Kudos to you since I always seem to reverse something

... but that is not common knowledge

It can be deduced with logic then...

If every front panel connector (pwr, reset, pwr led, hdd led) has one white wire, I'd think a person could guess that white is common and thus ground.

 

[wiretap]

Actually, that is common knowledge. (at least if you've ever looked at a user manual or have built at least one computer)

 

[Modelworks]

Read the thread I linked. Someone did that, and it was not infinite on one of the wires.

Then they did something wrong or there is a problem with their wiring. I had to clean out my antec 900 case today and just to double check I unplugged the power and reset connection and tested both for a ground connection, neither had one which is as it should be according to atx specs.

The wires have to be unplugged from the motherboard in order to test them. You cannot test them while attached because the motherboards ground will cause one of the wires to read a ground connection.

 

[Modelworks]

I realize momentary switches themselves do not have any polarity, but the switches are housed in metal, and every piece of metal on a case is grounded (or should be).

The switches internal connections are not grounded to the case. There is not a single manufacturer of switches that makes such a switch. You would have to jumper one of the switch pins to the switch body, which is usually plastic not metal.

I'm guessing that you've always installed the switch connectors correctly every time you've assembled a computer. Kudos to you since I always seem to reverse something

It doesn't matter what polarity the power or reset switches are connected. Motherboards use active low CMOS switches to determine status. The pins stay high unless brought low with a 100 ohm ground connection , it isn't supposed to be a direct short to ground. If you have a case that has a pin of the switch connected directly to ground I would email the manufacturer, that is an ATX I/O spec violation. Tell them to read intel document A29286-001 , page 17


LED it will not hurt to wire backwards, they just will not work. Really the only thing you have to worry about damaging a pc connecting is the usb connectors, but they are keyed so they cannot be inserted wrong unless you force it.