RPi Serial Connection - Revision history

IanH: Separate section for level conversion

2013-04-19T07:53:21Z

Separate section for level conversion

IanH: Serial port programming link

2013-04-18T20:31:55Z

Serial port programming link

IanH: Clarify this section - we're setting up the PC end here, not the Pi

2013-04-17T19:45:15Z

Clarify this section - we're setting up the PC end here, not the Pi

IanH at 15:08, 1 April 2013

2013-04-01T15:08:20Z

IanH: /* Windows terminal set-up */

2013-04-01T14:57:57Z

‎Windows terminal set-up

IanH: /* Windows terminal set-up */

2013-04-01T14:46:14Z

‎Windows terminal set-up

IanH: /* First Dialog */

2013-04-01T14:41:34Z

‎First Dialog

IanH: Note about reboot

2013-04-01T12:31:00Z

Note about reboot

IanH: Hopefully more beginner-friendly

2013-04-01T12:25:04Z

Hopefully more beginner-friendly

IanH: Partial edit, will prettify later

2013-03-30T18:12:47Z

Partial edit, will prettify later

@@ Line 5: / Line 5: @@
 * Connecting to a microcontroller or other peripheral which has a [http://en.wikipedia.org/wiki/Serial_port serial] interface. This can be useful if you want the Pi to control another device.
-== Connection to a PC ==
+== Connections and signal levels ==
-−
-'''Note that the Raspberry Pi cannot be wired directly to an RS232 serial port. To do so, some additional electronics are required! '''
 [[File:Adafruit-connection.jpg|300px|thumb|right|Adafruit serial cable connected to Pi]]
-A UART is available in the Broadcom SoC, and its RxD and TxD lines are available on the [[RPi_Low-level_peripherals|GPIO header]]. However, these use 0 and 3.3V logic levels, not the +/-12V used by [http://en.wikipedia.org/wiki/RS-232 RS-232] serial ports. If you wish to connect one of these, you need a board or adapter to convert the signal levels. See [http://codeandlife.com/2012/07/01/raspberry-pi-serial-console-with-max3232cpe/ this tutorial] for one example on how to build a 3.3V to RS-232 level converter with a breadboard, a MAX3232CPE IC and five 0.1 uF capacitors.
+The Raspberry Pi serial port consists of two signals (a 'transmit' signal, TxD and a 'receive' signal RxD) made available on the [[RPi_Low-level_peripherals|GPIO header]]. To connect to another serial device, you connect the 'transmit' of one to the 'receive' of the other, and vice versa. You will also need to connect the Ground pins of the two devices together.
-If your PC has a USB port, it is possible to connect a USB-to-serial cable which uses 3.3V logic levels (e.g. the [http://www.adafruit.com/products/954 Adafruit 954] or the [http://www.ftdichip.com/Products/Cables/RPi.htm FTDI TTL-232R-RPI] cables) directly to the GPIO pins.
+The Broadcom chip at the heart of the Pi uses 0 and 3.3V logic levels, not the +/-12V used by [http://en.wikipedia.org/wiki/RS-232 RS-232] serial ports found on some older PCs. If you wish to connect one of these, you need a board or adapter to convert the signal levels. See [http://codeandlife.com/2012/07/01/raspberry-pi-serial-console-with-max3232cpe/ this tutorial] for one example on how to build a 3.3V to RS-232 level converter with a breadboard, a MAX3232CPE IC and five 0.1 uF capacitors.
-If you have a serial connection using 5V logic levels, you should ideally have a circuit to convert between the voltage levels. See [http://www.element14.com/community/groups/raspberry-pi/blog/2012/07/18/look-ma-no-display-using-the-raspberry-pi-serial-console 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, and the 5V TxD can be connected via a [http://en.wikipedia.org/wiki/Voltage_divider voltage divider] to the GPIO RxD. A 5V to 3V resistor divider can be built from a 2K2 (top) and a 3K3 (bottom) resistor.
+If you wish to connect your Pi to a PC with a USB port, the simplest option is to use a USB-to-serial cable which uses 3.3V logic levels (e.g. the [http://www.adafruit.com/products/954 Adafruit 954] or the [http://www.ftdichip.com/Products/Cables/RPi.htm FTDI TTL-232R-RPI] cables). These can be simply plugged in directly to the GPIO header (see illustration).
-When this is done, you will need to set up a [http://en.wikipedia.org/wiki/Terminal_emulator terminal emulator] program on your PC as described below.
+You can connect the Pi to a PC using a USB-serial cable, or (if it has an RS232 port) a level-converter circuit - see above for details. When this is done, you will need to set up a [http://en.wikipedia.org/wiki/Terminal_emulator terminal emulator] program on your PC as described below.
 ===Console serial parameters===
@@ Line 109: / Line 112: @@
 ==Connection to a microcontroller or other peripheral==
-The TxD and RxD 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.
+The TxD and RxD signals can also be connected directly to similar signals on a microcontroller board like the Arduino provided the signals are all at 3V3 levels. Some Arduino variants use 5V levels, and should have level conversion (see 'Connections and Signal levels' above). Even 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.
 If your microcontroller uses 5V logic levels, level conversion is usually necessary - see 'Connecting to a PC' for details.

@@ Line 139: / Line 139: @@
   ps aux | grep ttyAMA0
 to search for <code>getty</code> processes using the serial port.
 ===Handshaking lines===

@@ Line 30: / Line 30: @@
 ===Linux terminal set up===
-You will need to know the ''port name'' of your PC's serial port. On Linux this will be:
+If your PC is running Linux, you will need to know the ''port name'' of its serial port:
-* Built-in (standard) Serial Port: the Linux standard is  '''/dev/ttyS0''', '''/dev/ttyS1''', and so on, but on the raspberry pi, it is /dev/ttyAMA0
+* Built-in (standard) Serial Port: the Linux standard is  '''/dev/ttyS0''', '''/dev/ttyS1''', and so on
 * USB Serial Port Adapter: '''/dev/ttyUSB0''', '''/dev/ttyUSB1''', and so on.
 ** Some types of USB serial adapter may appear as '''/dev/ttyACM0''' ...
 You will need to be a member of the ''dialout'' group to access this port (for later releases the required group is ''tty''). You can check which is needed with:
-  ls -l /dev/ttyAMA0
+  ls -l /dev/ttyUSB0
 and you will see something like "crw-rw----T 1 root dialout ..." which says root can 'create,read,write' and the group xxx can 'read,write' to the port and everyone else cannot access it.
@@ Line 72: / Line 72: @@
 Start ''GtkTerm'', select Configuration->Port and enter the values above in the labeled fields.
 ===Windows terminal set-up===

@@ Line 8: / Line 8: @@
 '''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 [[RPi_Low-level_peripherals|GPIO header]]. However, these use 0 and 3.3V logic levels, not the +/-12V used by [http://en.wikipedia.org/wiki/RS-232 RS-232] serial ports. If you wish to connect one of these, you need a board or adapter to convert the signal levels. See [http://codeandlife.com/2012/07/01/raspberry-pi-serial-console-with-max3232cpe/ this tutorial] for one example on how to build a 3.3V to RS-232 level converter with a breadboard, a MAX3232CPE IC and five 0.1 uF capacitors.
@@ Line 103: / Line 105: @@
 You can then log in as you would with a keyboard and screen.
-==Unwanted serial garbage input==
+=== 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.

@@ Line 74: / Line 74: @@
 ===Windows terminal set-up===
-Users of Windows Vista or later will need to download a terminal program, for instance [http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html putty], or [http://en.wikipedia.org/wiki/Tera_Term TeraTerm]. Users of XP and below can choose between using ''putty'' and the built-in ''Hyperterminal.''
+Users of Windows Vista or later will need to download a terminal program, for instance [http://www.chiark.greenend.org.uk/~sgtatham/putty/download.html PuTTY], or [http://en.wikipedia.org/wiki/Tera_Term TeraTerm]. Users of XP and below can choose between using ''PuTTY'' and the built-in ''Hyperterminal.''
 ===Boot messages===

@@ Line 79: / Line 79: @@
 You will need to know the number of your serial (COM) port. Built-in ports are COM0 to COM3, and USB-attached ports are COM4 and upwards.
-==First Dialog==
+===Boot messages===
-If you get the prompt below, you are connected to the Raspberry Pi shell!
-  prompt> #
-First command you might want try is "help":
+If your connection is set up correctly, when the Pi is booted you should see many messages as the system comes up:
-  prompt> # help
+ Uncompressing Linux... done, booting the kernel.
-If you get some output, you are correctly connected to the Raspberry Pi! Congratulations!
+and so on. Eventually, you should see a login prompt:
 ==Unwanted serial garbage input==

@@ Line 116: / Line 116: @@
   #T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100
-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.
+Finally you will need to reboot the Pi for the new settings to take effect. Once this is done, you can use <code>/dev/ttyAMA0</code> like any normal linux serial port, and you wont get any unwanted traffic confusing the attached devices.
 The above instructions have been verified on Raspbian 'wheezy'; other distributions may be set up differently. To double-check, use

@@ Line 101: / Line 101: @@
 The Broadcom UART appears as <code>/dev/ttyAMA0</code> under Linux. There are several minor things in the way if you want to have dedicated control of the serial port on a Raspberry Pi.
-Firstly, the kernel will use the port to output messages during boot, and for kernel debugging. These are caused by the kernel parameter console=ttyAMA0,115200 set in /boot/cmdline.txt
+* Firstly, the kernel will use the port as controlled by kernel command line contained in <code>/boot/cmdline.txt</code>. The file will look something like this:
-next is the login prompt, set in /etc/inittab with the line T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100
+You must be root to edit this (e.g. use <code>sudo nano /boot/cmdline.txt</code>). Be careful doing this, as a faulty command line can prevent the system booting.
-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
+* Secondly, after booting, a login prompt appears on the serial port. This is controlled by the following lines in <code>/etc/inittab</code>:
-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
+You will need to edit this file to comment out the second line, i.e.
-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
+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.
 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 [https://github.com/rewolff/bw_rpi_tools/tree/master/gpio  gpio_setfunc]
 {{Template:Raspberry Pi}}

@@ Line 93: / Line 93: @@
 ==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.
+The TxD and RxD 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
+If your microcontroller uses 5V logic levels, level conversion is usually necessary - see 'Connecting to a PC' for details.
-the first is kernel boot messages at startup, those are caused by the kernel parameter console=ttyAMA0,115200 set in /boot/cmdline.txt
+===Preventing Linux using the serial port===
 next is the login prompt, set in /etc/inittab with the line T0:23:respawn:/sbin/getty -L ttyAMA0 115200 vt100