cheap serial-UART works, and can power R-Pi


6 posts
by jbeale » Thu Jun 28, 2012 7:17 pm
Just wanted to post my positive results using a cheap USB->UART board. The "PL2303HX auto converter module" sells for US $2.31 including shipping. It does have 3V3 logic levels and works OK with my new R-Pi. http://www.ebay.com/itm/350568364250?ss ... 1439.l2649

I was able to use it to log in and work on the Pi, using the PuTTY terminal emulator on WinXP. http://www.chiark.greenend.org.uk/~sgtatham/putty/

This small USB-UART board comes with four F-F jumper cables to connect its header pins to the RPi GPIO pins. You only need to connect three wires: ground to ground, TxD to RxD, and RxD to TxD. Note this super-cheap board has a USB plug, not a socket, so it plugs directly into a PC, and the provided jumper wires are short. You may want to use a USB extender cable unless your RPi is actually taped to the side of your PC/Laptop/etc.

Then I realized I could go a step further, by disconnecting the microUSB power, and connecting the +5V line from the USB-UART chip to the RPi GPIO. It works! As long as you only need serial comms, this is all you need. The R-Pi in the photo is running (TP1-TP2 voltage shows 4.70 V) although it is not working on any big job.

RPi-simple-wiring.jpg
RPi powered from USB-UART module
RPi-simple-wiring.jpg (57.81 KiB) Viewed 7388 times

Better quality photo: https://picasaweb.google.com/lh/photo/t ... directlink
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by rasbeer » Thu Jun 28, 2012 7:31 pm
I can see this leaves both of the Pi's USB ports free - are there other advantages?
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by jbeale » Thu Jun 28, 2012 7:33 pm
Not needing a separate +5V USB supply, microUSB cable, or any extra wires is nice. Probably this would not work if you were trying to power a lot of other stuff, but for simple applications it seems OK.
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by rasbeer » Thu Jun 28, 2012 8:05 pm
Fair enough - nice you've got it working :)
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by jbeale » Thu Jun 28, 2012 10:17 pm
I should mention one other thing about that particular USB-UART converter. I've been using similar devices with a FTDI chip for talking to CPUs, and in that case WinXP will always assigns the same COMxx port number to a given device, regardless of which USB port on the computer it is plugged into. But with this PL2303HX module, plugging the same device into a different USB port shows up as a different COM number. Also, I purchased two of them, and swapping between two different physical PL2303 devices in the same USB port, still yields the same COM number. So here the com number stays locked to a USB port, whereas with the FTDI chip it is locked to the physical device. The device manager says it is a Prolific USB-to-Serial Comm Port with USB VID 067B and PID 2303.

I don't know if this is better or worse behavior, but at any rate it is different.
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by MrRadish » Thu Jul 05, 2012 1:31 pm
Just ordered one of these. I run headless and I am suffering a number of USB Wireless and fewer but still frequent Ethernet issues. Most definitely the Pi but it would be good to be able to connect my tablet or laptop up directly to find out what is going on without having to relocate and attach a monitor and keyboard - hope it works ;-)
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