If you have a USB adapter and would like to use it when working with Arduino or Raspberry Pi you may wonder if it is safe to use with a 3v3 system.
Purchasing these on eBay can be cheaper than buying from someplace like Adafruit but the listings often have no documentation on if the chip can be used with 3.3v devices.
You can measure some of the pins to make your own determination. The unit I was working with several pins labeled:
I measured voltage between the GND pin and the TX pin which gave around 1.8 – 2.5V without any data being transmitted. When I sent data to the device the voltages went intermittently to around 3.3 volts or maybe 3.4 volts.
But I wanted to make sure that the chip was actually setup for 3.3v and so I found the [datasheet for the PL-2303HX](http://www.adafruit.com/datasheets/PL2303HX.pdf)
Make sure to get the correct datasheet for the chip you have – some of the chips are slightly different, like the HXD version.
On page 11 of that datasheet it showed the pinout diagram – notice the small circle indicating “pin 1” which should correspond to the small dot on the actual chip:
Pin 4 is labeled `VDD_325` and the note says:
> RS232 VDD. The power pin for the serial port signals. When the serial port is 3.3V, this should be 3.3V. When the serial port is 2.5V, this should be 2.5V. The range can be from 1.8V~3.3V.
Use this information at your own risk, but what I believe this datasheet is expressing is that if pin4 measures 3.3V then the TX pin will transmit using 3.3V and if it is 5V then it will transmit using 5V.
When I measured pin4 on my `PL-2303HX` it showed 3.3V and I was able to use it with a raspberry pi with no problems.
