RaspberryPi Serial Comms & PHP

I’ve been playing around with serial comms on 2 Raspberry Pi computers for the last couple of days.

And I now understand a little of the black-art required to get this working, including how to send serial data via a PHP web page.

So I thought I’d write up the details before I have time to forget it all again!

There are two pins available on the Pi header for simple serial comms: pin 8 (transmit data) and pin 10 (receive data). On Raspbian this port or device appears as: /dev/ttyAMA0.

Configuration

But before you can use this serial port, you may need to modify a couple of files on the Pi (modify as root user). We basically need to free-up this port for our use, and stop it being used for something else.

So type in a terminal:-

sudo pcmanfm

Now navigate to /boot/cmdline.txt and open with LeafPad. Look for any string entries containing: /dev/ttyAMA0

For example:-

console=ttyAMA0,115200

…and carefully delete them. You should end up with a series of strings/commands separated by spaces, for example:-

dwc_otg.lpm_enable=0 console=tty1 root=/dev/mmcblk0p2 rootfstype=ext4 elevator=deadline rootwait

Save changes and navigate to: /etc/inittab

Open file and look for an entry containing /dev/ttyAMA0 which you should just comment out like this:-

#Spawn a getty on Raspberry Pi serial line
#T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100

Finally, to allow any PHP code on our web server to control this serial port we need to add the user to the dialout group. So in a terminal:-

sudo usermod -a -G dialout www-data

Note: www-data is the user running on my web server. Take a look at my earlier posts to see how my web server is configured using lighttpd and PHP.

Now re-boot.

Serial comms test

We need to test that serial comms is working, before we try and do anything clever with PHP. Open a terminal and install minicom:-

sudo apt-get install minicom

Carefully fit a jumper wire or link to connect pin 8 to pin 10.

In a terminal type:-

sudo minicom -D /dev/ttyAMA0 -b 9600

You should see a “Welcome to minicom…” message.

If you press <ctrl><a> you should get an information bar at the bottom of the terminal. Press <ESC> to clear the bar, and then type something, e.g. hello!

Your message should be displayed, which shows that your loop-back test was successful (i.e. you sent characters out via TXD and got them back via RXD.

Not convinced? Then remove the link between pins 8 and 10, and then try again. As you type, the screen does not show the result..

If you have a second Pi, you can run minicom on both systems, and connect pin 8 from one Pi to pin 10 to the other.

Make sure you also link the two Pi common/gnd together. Now you can test by sending a message from one Pi to the other.

 

For more detail: RaspberryPi Serial Comms & PHP