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enter image description here

What does the x that's wired to pin 3 and 6 mean?

Does it mean it should be wired to gnd?

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    \$\begingroup\$ "not used" in this configuration. \$\endgroup\$ – Piotr Kula Jun 25 '11 at 7:57
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Some schematic programs use this as a symbol to show that the pin is specifically not connected to anything.

However, this looks to be a schematic from Eagle. Eagle requires no such symbol. Output or passive pins are allowed to not be connected without any electrical rules check error. You can make a symbol and footprint for anything you want in Eagle, so only the designer knows for sure what this means. It could mean a test point, but most likely not since there is no component designator. A bunch of test points on the board are pretty useless unless labeled, so I don't think that's it. Most likely whoever created that schematic was used to a different schematic program and felt compelled to put Xs there to indicate not connected. In Eagle, you simply don't connect anything and don't give it a name and it won't be connected anywhere.

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  • \$\begingroup\$ I'm not 100% sure, but I think that Proteus too can display very similar schematics and requires termination for simulation to work. \$\endgroup\$ – AndrejaKo Jun 24 '11 at 18:45
  • \$\begingroup\$ If you go to Eagle's default parts library and search for 'test point' you come up with a few options that have X's as their schematic symbol. I will check on the exact name that I'm referring to when I get home from work. \$\endgroup\$ – NickHalden Jun 24 '11 at 18:52
  • \$\begingroup\$ Nope nevermind, not quite an 'x'. TEST-POINT2 is what I was talking about but it's a little different. \$\endgroup\$ – NickHalden Jun 25 '11 at 16:33
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    \$\begingroup\$ The reason to have a symbol for not connected is to distinguish between "Not connected because it doesn't need to be", and "Not connected because the designer forgot". \$\endgroup\$ – Optimal Cynic Oct 7 '11 at 8:08
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    \$\begingroup\$ I always put an X on unconnected pins. This tells readers of the schematic (including me in 6 months time) that the pin is intentionally unconnected, and I didn't just forget it. \$\endgroup\$ – Rocketmagnet May 28 '12 at 13:39
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It looks like the pin is not used. They are probably just indicators for test points.

If you're using this schematic to design a PCB, just do not make any traces coming out of those pins.

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    \$\begingroup\$ Agreed. Definitely not ground, probably meant as "leave unconnected". \$\endgroup\$ – zebonaut Jun 24 '11 at 18:17
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    \$\begingroup\$ ...Who down-voted this answer? I'm pretty sure nothing is wrong here. \$\endgroup\$ – NickHalden Jun 24 '11 at 18:24
  • \$\begingroup\$ I don't know who downvoted it or why. I was just about to upvote it to cancel the downvote when I noticed the downvote was gone. Apparently whoever did it reversed the vote or cancelled it. I didn't know you could do that. \$\endgroup\$ – Olin Lathrop Jun 24 '11 at 18:48
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    \$\begingroup\$ Haha yeah, I mean I don't care about losing the 2 rep points but I would like to learn if I say something incorrect. \$\endgroup\$ – NickHalden Jun 24 '11 at 18:50
  • \$\begingroup\$ I was lashing out, my bad. \$\endgroup\$ – dhsieh2 Jun 25 '11 at 1:24
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Others already said it indicates that the pin is left unconnected. I would mark pins that way as a check that they've been accounted for. On a finished schematic all IC pins should either be connected somewhere or have this marking.

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The purpose to me says clearly that the pin is left floating.

It may be a clear indicator from the designer to himself and others that these pins should be left floating in this design. It may otherwise be unclear whether the pin should be connected (forgotten, removed by accident etc.). This is an explicit sign the pin is left floated.

In Eagle you don't need to do anything with unused pins, so this would be the only useful explanation to me.

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It is an unused/unconnected pin.

enter image description here

In some EDA suites (I use Altium Designer, this is also likely true in other packages), the "not-connected" symbol has an additonal role. In this case, it is a No-ERC Directive, and as such, it tells the design compiler to not verify the pin's net against the design rules. As such, you may see similar directives even in situations where there is a valid connection.

This is useful in odd cases, such as if you are powering a small, low-power sensor off a MCU output pin. Normally, the schematic checker will complain that you have an output connector, and a power pin on the same net, since this is normally something you do not want to do. In this case, it lets you override the checking, and suppress this error message.

Alternatively, it is also useful in a more traditional role if you have a device with an input pin, that has an internal pull-up. Normally, you want to avoid leaving floating inputs, so the design compiler warns you if you do so. In this case, you want to suppress this warning, since there is an internal pull-up, so you tag the pin with a No-ERC directive.

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that X means connection with a pins headers

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    \$\begingroup\$ That's absolutely wrong. It means "no connection". \$\endgroup\$ – Federico Russo May 28 '12 at 5:46

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