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I have an USB-UART cable. The connectors listed are Red VCC, Black GND, Green TXD, White cable RXD.

I want to connect it to a Alfa Hornet-UB board (http://www.youtube.com/watch?v=i58XOF4Rfc4) with pins for GND, TX, RX, VDD. If I connect:

Black (GND) - GND
Green (TXD) - TX
White (RXD) - RX
Red (VCC) - VDD

Will this damage the board?

I want to be cautious because I have previously connected a 3v to the VDD pin, which broke the board.

I assume TX is the same as TXD (Transmit), and RX = RXD?

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    \$\begingroup\$ What voltage is the USB UART cable? (5V or 3.3V are likely answers, not clear from that web page) And what's the required board voltage (do you have a page describing it that isn't a video?) \$\endgroup\$ – pjc50 Jan 7 '13 at 16:13
  • \$\begingroup\$ From the manufacturer alfa.com.tw/products_show.php?pc=99&ps=50. The page doesn't seem to specify a voltage for the UART interface. \$\endgroup\$ – jammmie999 Jan 7 '13 at 16:25
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Based on the images used on the manufacturer's page, found two sites WifiPineapple Wiki (very detailed) and Opening The Alfa AP121U (AP121U is the commercial product version of the Hornet-UB board) that use the same image and reference the same board. Both state DO NOT CONNECT THE VDD PIN. So that's the main source of your problems. The WifiPineapple Wiki mentions that any 3.3v uart adaptor will work.

This board is based on the AR9331, an Atheros SoC. Atheros never releases datasheets for the chips, but they have been reversed engineered by OpenWRT users, and these chips are known to use 3.3v.

Oh, and importantly, you want to crossover the uart cable. The Uart Cable TX would connect to the board's RX, and Uart Cable RX connects to the board's TX.

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Yes, VCC and VDD are the same (+). And VEE and VSS are the same (-), just refer to different technologies.

Will this damage the board?

Nope, it is exactly how it is meant to be used. But it's always best to be cautious!

Hang on - what about the voltages?

Without the voltages I can't say for sure.

Do you want to power the board or just communicate to it? If just the communicate, you can leave the positive voltage disconnected and as long as they share a ground and the logic level voltage is the same (3.3v or 5v) you will be able to communicate without an issue. If the logic voltage is different you need to use something like a logic level converter to correct for this. (There are many other options, including voltage drivers, etc.)

The board is powered separately by an external 12v power supply, but I don't know what the UART Interface wants

Ok, then like I said above, don't bother connecting the VCC/VDD. Just stick with ground. As for the uart voltage level, according to the not so helpful links I have found for the USB PL-2303HX device,

PL-2303HX to connect directly to 3.3V ~ 1.8V devices

So to be on the safe side, I would test the the TX line of both the USB and the board. If you only have a multimeter, will probably have to set both devices up to be flooding the transmit line with data and at a slow datarate. Don't have them connected together, just one at a time put the positive probe of the meter on TX and the negative on ground. If the voltage reads above 3.3v then that device is 5v data, if under 3.3v (never exceeding 3.3 or 3.4 if you have a cheap meter,) then it is most likely 3.3v, same goes for the 1.8v but this is fairly rare, and I highly doubt either is.

The reason you need to set a very slow data speed and a lot of data is that the line is at this voltage for such a short period of time, you wouldn't notice it on a multimeter. If you have an osiliscope, then this test will be easier.

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  • \$\begingroup\$ Hang on - what about the voltages? \$\endgroup\$ – pjc50 Jan 7 '13 at 16:12
  • \$\begingroup\$ Sorry, didn't notice the voltage issue. What is the input voltage of the board? \$\endgroup\$ – Garrett Fogerlie Jan 7 '13 at 16:15
  • \$\begingroup\$ Um, I don't know the manufacturer doesn't mention it? alfa.com.tw/products_show.php?pc=99&ps=50 \$\endgroup\$ – jammmie999 Jan 7 '13 at 16:19
  • \$\begingroup\$ @jammmie999 Do you want to power the board or just communicate with it? I updated my post. \$\endgroup\$ – Garrett Fogerlie Jan 7 '13 at 16:24
  • \$\begingroup\$ The board is powered separately by an external 12v power supply, but I don't know what the UART Interface wants \$\endgroup\$ – jammmie999 Jan 7 '13 at 16:29
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As long as the voltages are the same there is no problem. Vcc and Vdd are just other terms, just like Vee and Vss.

USB voltage is 5V, so if your board uses 5V as well this will work fine.

However be aware of little differences between the voltages, as the voltage difference will be used to produce heat.

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    \$\begingroup\$ -1 for answer that is based upon not knowing what voltage comes out of the USB-TTL serial cable and what voltage is used by the MCU board. They may not be the same! The MCU board may also be powered by another means and if so then it would NOT be appropriate to connect the USB cable supply to the MCU's onboard supply - even if the voltages were the same. \$\endgroup\$ – Michael Karas Jan 7 '13 at 16:41
  • \$\begingroup\$ Real problem is that the question is mis-titled. This answers the title of the question, but the actual problem which needs to be solved is different - it has nothing to do with the terms VCC vs VDD, but rather with the difference between common grounding for communication, and sourcing power. \$\endgroup\$ – Chris Stratton Jan 7 '13 at 17:07

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