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I want to connect my DIY basic stamp via RS232 MAX232 to the serial port on my laptop. Now since I'm a complete newbie in electronics, what do I do with Ground Pin 5 on the DB9? I assume ground means I have to connect it to some insulator with 0V? Maybe you can also explain why there has to be a GND pin anyway, what is the purpose?

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    \$\begingroup\$ On that diagram it says the GND is the same as the minus side of the power supply. Can that help you find it? \$\endgroup\$ – SDsolar Apr 25 '17 at 4:48
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I don't know the STAMP controllers but in this context 0V = GND. All voltages are relative to some reference point. In this case it is 0V on one device and GND on the other device. They just have different names for the same reference point.

If you start going in to mains electrical then 0V may not be GND as this could be Safety Ground (referred to as GND often ) and 0V could be isolated so be careful.

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  • \$\begingroup\$ Right on. You nailed the answer of what the GND pin is for. Especially for RS-232, which is +/-15V on the signal pins. I am sure there is an obvious place to connect it directly to the MAX232 pin 15 or to the minus side of the P/S. \$\endgroup\$ – SDsolar Apr 25 '17 at 4:44
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I'll try to keep this answer simple and to the point: Connect ground pin directly to a connection which represents the ground on the schematic in the article you posted. Right under the schematic, there's a list of items on it. Ground symbol is number six on the list. It says:

The GND - GROUND Symbol-symbol indicates GROUND, which in this case is the same as the MINUS of the powersupply.

Note that all ground symbols should be connected together. The actual connection on the schematic is replaced by the symbol with the understanding that on the actual device, symbols will be connected. This is done in order to decrease the amount of clutter on the schematic.

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  • \$\begingroup\$ Thanks for the answer, can you explain what purpose the GND pin has? Why does there have to be a GND port in a serial connection, and why in this case it has to be connected to -5V? \$\endgroup\$ – JohnnyFromBF Aug 20 '13 at 6:59

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