I recently tore apart my old laptop and scavenged a few parts from it, namely a fingerprint scanner and a synaptics touchpad which I want to use with my raspberry pi.

All are 4 wire devices so assuming them to be serial devices, I am planning to order Serial to USB converter chips.

However it seems there are quite a few options on the chips available.

Please suggest whether I should buy a PL2303HX or a CP2102 based converter.

I don't need any control signals to interact with my devices so either will work for me.

Is there any benefit of using one over the other?Known issues with a chip maybe?

Also if you have any other chip suggestions , that would be great!

  • 1
    Assumptions are dangerous! I agree that based on the amount of wires, those device probably talk serial, but serial comes in many flavors. Immediately assuming that it will be RS232 on TTL level might be incorrect. For example I2C also uses only 4 wires (2 wires power and 2 wires data/clock), RS485 (2 wires power, 2 wires differential data), CAN bus (2 power, 2 data), to name only a few of a much longer list. So I think it might be a good idea to investigate a little more before buying anything.
    – ikku
    Nov 9, 2012 at 21:32
  • Many built-in laptop peripherals are connected via internal USB connections. Nov 10, 2012 at 2:03
  • @ikku-perhaps its better that I directly connect them to the gpio and test them?
    – SteveIrwin
    Nov 10, 2012 at 17:00
  • 1
    No, I would not do that. I would check any type of identification on those modules to get an idea of what you're dealing with. Type numbers, numbers on the IC's etc, run those through Google and big chance you find useful information. You can only blow up those GPIO pins once and they are pretty sensitive.
    – ikku
    Nov 11, 2012 at 22:40

2 Answers 2


The Raspberry Pi has on-board serial. No USB converter required. It registers as /dev/ttyAMA0. The pins for it are on the GPIO header. GPIO 14 (pin 8) is the transmit pin (TXD) and GPIO 15 (pin 10) is the receive pin (RXD).


When deciding which chipset to use to bridge in a serial signal, it is critical to ensure that you are not driving a 5V signal into a 3.3V receiver. According to Sparkfun TTL Voltage Levels (click here) 2V is interpreted as high: you can drive a 5V PL2303 device with a 3.3V sensor and expect to receive data. This is fine if your sensor is read-only: if you try to drive the sensor from the PL2303, then you will need to avoid driving a 3.3V sensor with 5V.

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