Firmware/Serial Terminal

Jump to: navigation, search

Firmware Serial Terminal

Open Firmware Q7B18, Q4D28, Q3C12, Q2F16 and later contain a serial terminal emulator that can be used for diagnosis of the laptop, and other laptops.

command purpose
serial connect the keyboard and display to the serial port, use ctrl-a k to exit.
serial-log FILE as above, but write all output to the FILE. (not available on XO-1)
use-usb set up serial to use a USB serial adapter, the default.
use-uart set up serial to use the built-in serial port UART.

Using a USB serial adapter

How to use an XO as a serial terminal to another XO, using a USB serial adapter.

You will require:

  • an XO to act as host, with latest release of Open Firmware,
  • a USB serial adapter,
  • a USB cable, type A male to type B female,
  • an XO to act as target.


  • connect the target XO to the USB serial adapter,
  • connect the B end of the USB cable to the serial adapter,
  • connect the A end of the USB cable to the host XO.


  • start the program:

The program supports some key sequences used by screen:

  • ctrl+a k or ctrl+a ctrl+d to exit,
  • ctrl+a ctrl+b to send a break,
  • ctrl+a ctrl+a to send a ctrl+a,
  • ctrl+a C to clear the screen.


How to record a log of the session:

serial-log u:\file.log

The log is written to the file file.log on the USB drive.

This feature is not available on XO-1.

Using back to back serial ports

How to use an XO as a serial terminal (host) to another XO (target), without using any USB serial adapters:

  • on the XO acting as host, get to the Ok prompt, and set up to use the serial port:
  • connect a crossover cable,
  • start the program:
  • turn on the target.

Caution: the use-uart command turns off the Open Firmware serial console, to avoid unpleasant interaction between the Open Firmware ok prompt and any connected device. Connecting the cable before use-uart may result in each laptop screaming at the other repeatedly.

A crossover cable, to join two OLPC-supplied serial cables, consists of the following:

host pin (cable colour) target pin (cable colour) purpose
4 GND (orange) 4 GND (orange) ground
3 RX (red) 2 TX (green) display data, target to host
2 TX (green) 3 RX (red) keyboard data, host to target

The 3.3V (black) pin is not connected.

A crossover cable may also be fabricated from two cable connectors, or using wire soldered to the connector pads.

Using an XO as a network relay to another XO

See Firmware/Remote#Combining_with_serial_terminal for how to use an XO as a network serial adapter for another XO.

How it works - with a USB serial adapter

When a USB serial adapter is plugged in, and the serial program started, then keyboard input is passed by the program to the USB serial adapter, and thence to the target. Data from the target is received and displayed.


How it works - serial ports connected back to back

This diagram shows when serial ports are connected back to back, with the serial program running. Keyboard input on either the host or the target eventually reaches the target command interpreter.


How to fail - doing things in the wrong order

This final diagram shows a pathological data loop. The use-uart command has not been entered, so the Ok prompt from one host is sent as if it is keyboard input into the other host, which sends it back to the first host, and so on.

To avoid this:

  • use a USB serial adapter, or
  • keep the target power off or the cables disconnected until the use-uart command is entered.