RPi Serial Connection

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Revision as of 15:51, 28 March 2013 by IanH (talk | contribs) (Connection to a PC)
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Back to RPi Advanced Setup.

The Serial Port is a low-level way to send data between the Raspberry Pi and another computer system. There are two main ways in which it can be used:

  • Connecting to a PC to allow access to the Linux console
  • Connecting to a microcontroller or other peripheral which has a serial interface

Connection to a PC

Note that the Raspberry Pi cannot be wired directly to an RS232 serial port. To do so, some additional electronics are required!

A UART is available in the Broadcom SoC, and its RxD and TxD lines are available on the GPIO header. However, these use 0 and 3.3V logic levels, not the +/-12V used by RS-232 serial ports. If you wish to connect one of these, you need a board or adapter to convert the signal levels. See this tutorial for one example on how to build 3.3V to RS-232 levels converter with a breadboard, MAX3232CPE and five 0.1 uF capacitors.

If your PC has with a USB connections, it is possible to directly connect a USB-to-serial cable which uses 3.3V logic levels (e.g. the Adafruit 954 or the FTDI TTL-232R-RPI cables) to the GPIO pins.

If you have a serial connection using 5V logic levels, you should ideally have a circuit to convert between the voltage levels. See this tutorial for an example circuit. For prototyping work, provided the connections are kept short you may connect the GPIO TxD directly to the 5V RxD, but the 5V TxD should be connected via a voltage divider to prevent damage to the SoC. A 5V to 3V resistor divider can be built from a 2K2 (top) and a 3K3 (bottom) resistor.

Serial Parameters

The following parameters are needed to connect to the Raspberry. All parameters except Port_Name and Speed are default values and may not need to be set.

  • Port_Name: Linux automatically assigns different names for different types of serial connectors. Choose your option:
    • Standard Serial Port: ttyS0 ... ttySn
    • USB Serial Port Adapter: ttyUSB0 ... ttyUSBn
  • Speed: 115200
  • Bits: 8
  • Parity: None
  • Stop Bits: 1
  • Flow Control: None

The Serial Port is generally usable by the users in the group dialout. To add oneself to the group dialout the the following command needs to be executed with root privileges:

 $useradd -G {dialout} your_name 
  • Super Easy Way Using GNU Screen

Enter the command below into a terminal window

 screen Port_Name 115200
  • Super Easy Way Using Minicom

Run minicom with the following parameters:

minicom -b 115200 -o -D Port_Name
  • Tedious Old-Fashioned Way Using Minicom

Another method to setup minicom is described in the Tincantools Minicom Tutorial

  • GUI method with GtkTerm

Start GtkTerm, select Configuration->Port and enter the values above in the labeled fields.

  • Windows Users

Windows Users above Windows XP must download putty, TeraTerm or a comparable terminal program. Users of XP and below can choose between using putty and Hyperterminal.

First Dialog

If you get the prompt below, you are connected to the Raspberry Pi shell!

 prompt> #

First command you might want try is "help":

 prompt> # help

If you get some output, you are correctly connected to the Raspberry Pi! Congratulations!

Unwanted serial garbage input

Note that on older software by accident the internal pullups of the RxD GPIO pins were not enabled, this could lead to lots of serial garbage being picked up if the GPIO pin was touched, or even if a finger was nearby. In extreme case this could lead to kernel warnings and other problems.

Connection to a microcontroller or other peripheral

The TxD and TXD signals can also be connected directly to similar signals on a microcontroller board like the arduino provided the signals are all at 3V3 levels. It's still a good idea to put 2K2 series resistors in the lines to prevent damage when two outputs are connected together, which could also happen if a GPIO input pin is accidentally programmed as output.

there are several minor things in the way if you want to have dedicated control of the serial port on a raspberry pi

the first is kernel boot messages at startup, those are caused by the kernel parameter console=ttyAMA0,115200 set in /boot/cmdline.txt

next is the login prompt, set in /etc/inittab with the line T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100

kernel debuging is also on the serial port and i dont know what effect it may have on the end-use, so you may want to turn that off also in cmdline.txt, just remove kgdboc=ttyAMA0,115200

once all of those are off, you can use /dev/ttyAMA0 like any normal linux serial port, and you wont get any unwanted traffic confusing the attached devices

some distro's may be setup differently, just double check 'ps aux|grep ttyAMA0' and 'cat /proc/cmdline' to verify ttyAMA0 is not in use anywhere

You can have the RTS0 signal on GPIO 17 (P1-11) if you set it to ALT function 3. Likewise, the CTS0 is available on GPIO 30 (P5-05), if it is set to ALT function 3. You can control the settings of IOs with gpio_setfunc