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Using a serial interface with U-boot and libreCMC

A serial interface (console) provides a means of debugging or unlocking features hidden from the user. Most computing devices have a serial interface whether it is broken out by the manufacture or not.

In the case of devices that run libreCMC, a serial console is used to configure U-boot and debug libreCMC if the network interface can't be brought up. These instructions outline the basics of using a serial console with a USB to serial cable.

The basics

There are a few different standards for serial interfaces, but here we are providing just the basics. There are 3 different connections that we care about : Transmit (TX), Receive (RX) and Ground (GND). Depending on the serial cable, the color coding could be different. The common coloration is RX is Green, TX is white and GND is black*.

These three wires will be connected to a pin header on the board or might need to be soldered to pads. Many device manufactures are kind enough to provide a header and indicate where the serial interface is. Look for silkscreen labeling with : GND, TX, RX. Sometimes additional work is needed to break out a stable serial interface.

  • Many serial cables ship with the color corresponding to the connection on the board, so TX and RX are effectively reversed.

Some routers use the following standard:

        |            |     |
        | .   .   .  |  .  |              <- Inside of the router (you may need to do some soldering)
          |   |    \     \
          *  GND   RX    TX

In this case, transmit is in the marked off box.

        |                 |
        | .   .   .    .  |             
      * |_________________|

          |   |    \     \
        GND   RX    TX   Vcc

Some might use a dot to indicate GND, etc...

TPE-R1100 example:

alt text

Warning: NEVER connect the red wire to the device / router! You will most likely fry it because many routers use 3.3V for Vcc instead of the 5V provided by USB.

Getting a console

In order to communicate with the device, we need a terminal application to interact with the serial port provided by the USB to serial cable. In this case, we are going to use GNU Screen. Make sure that the GNU Screen package is installed and make sure that your user is part of the dialout group.

Before we can use the terminal application, we need to know the baud rate at which the serial console will be provided. With most devices running libreCMC and u-boot, this most likely will be 115200

Putting it all together

1) Plug in / wire up the serial cable to the board.

2) Plug the USB part of the serial cable into an available USB port.

3) run screen /dev/ttyUSB0 115200 or (if you are NOT part of the dialout group) sudo screen /dev/ttyUSB0 115200 Please note that the serial device might have a different name (ttyUSB(n), ttyS0, etc...).

4) Power on the device you are connecting to.

5) You should see ledgible ASCII text scroll by.

If the device gets all the way through the boot process, the libreCMC banner will appear with a shell prompt:

                    ____  _____  ____
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 | (_) |__ _ _ ___ | |   | | | || |
 | | | '_ \ '_/ -_)| |__ | | | || |__
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