I'm a university student attempting to make a wall-mounted RFID reader using a Raspberry Pi device and a Parallax RFID reader. I'm still in the process of planning and ordering the components, and thus my questions has to do with the option between a Serial or a USB connection to my Raspberry Pi.

The RFID Card Reader USB uses a USB connection to the Raspberry Pi and obtains its power through the USB socket.

The RFID Card Reader Serial uses a serial connection at 2400 baud (8N1). A batter will be used to power it.

My question:

  • Can the Raspberry Pi power a 5V device through its USB socket?
  • And if this is not a problem, would you recommend using a USB enabled device, or a serial connected one instead?

Here is the documentation for both RFID readers in case anyone needs additional information.

  • Courious: Did you manage to get the setup working? – s.Daniel May 22 '13 at 9:08

The raspberry pi can supply power through its USB ports, but not as much as a normal computer can. According to this question, the practical limit on how much current one can draw from each port is about 100 mA. Looking at your documentation, it looks like the tag reader draws nominally 100 mA and up to a maximum of 200 mA. So it's tough to call. It's possible that the pi could power it, but remotely so and I wouldn't recommend it. The usual solution is to connect the device via an externally-powered hub, but that probably wouldn't be advantageous to you.

As for your question about serial versus USB, it looks like it would be about the same. The USB reader uses a serial protocol through the USB COM port, so it's just a virtual serial device. Connecting your tag reader to the I2C GPIO pins might be more difficult, but it's not more complex computation-wise, as far as I know. So if you thing you can handle the electronics and interfacing of using the serial reader, I would go with that.

Just make sure your voltages going into the pi's pins are correct: it operates at 3.3 V. You may or may not have to adjust for that. See this page here, but exercise extreme caution. If you're shaky on electronics, maybe it's best to go with USB and an externally-powered hub. Then again, there's no better time to learn electronics than pi time!

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