Firmware/Serial Terminal

Firmware Serial Terminal

Open Firmware contains 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-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 a build of Open Firmware after svn 3568,
• a USB serial adapter,
• a USB cable, type A male to type B female,
• an XO to act as target.

Preparation:

• 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.

Starting:

• start the program:

ok serial

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.

Logging

How to record a log of the session:

ok serial-log u:\file.log

Remote

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

• follow the preparation steps above,
• configure Open Firmware networking,
• start a telnet server:

ok telnetd

• note the IP address displayed,
• connect with telnet to the server, and start the serial program,

% telnet 10.0.0.1
ok serial

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:

ok use-uart

• start the program:

ok serial

• connect a crossover cable,
• 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
GND (orange)	GND (orange)	ground
RX (red)	TX (green)	display data, target to host
TX (green)	RX (red)	keyboard data, host to target

The 3.3V (black) pin is not connected.

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

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 already started. 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 serial program has not been started, 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 until the serial program has been started.

References

• terminal.fth
• mailing list post,
• git repository via web, (before it was merged),
• git://dev.laptop.org/users/quozl/olpc-firmware-terminal.git git repository (before it was merged),