Serial adapters

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This page contains information about a +3.3V serial/RS-232/USB adapter that is used in low level laptop development.


OLPC has two different serial adapters for obtaining serial output from the laptop: the original +3.3Vdc serial/RS-232 level converter that came with the ´´ATest´´ boards, and a new multi-purpose serial adapter which supports +3.3V serial to RS-232 or USB.

XO Serial Ports

The XO laptop has two serial ports provided for low level debugging. One (J1) is connected to the serial console on the main processor (Southbridge, actually). The second (CN24) is connected directly to the embedded controller on the motherboard.

The signals levels are 0v/3.3volt TTL.

XO Serial Ports - electrical interface

Console/Serial connector - J1

The J1 pinout of the serial connection on the laptop is:

1+3.3V is provided by the laptop on this pin, for use by the serial adapter
marked on PCB with white circle near the J1 designator.
2TX serial data
3RX serial data
4 GND return for both data and power
marked with a small number 4.

EC Serial connector - CN24

In addition, the EC serial connector (CN24) has an additional pin (5), used to provide a clock for programming the serial ROM in recovery mode. This pin expects a +3.3V compatible 65 MHz clock signal, if used. The EC serial port may be used for monitoring by connecting a laptop/serial adapter cable to the first four pins of the connector.

1+3.3V is provided by the laptop on this pin, for use by the serial adapter
2TX serial data
3RX serial data
4 GND return for both data and power
5 G3.3v 65 Mhz clock signal (optional).

The Pinout of the EC serial connector is identical to that of J1 except for the extra pin 5. The 4 pin connector works fine when plugged up to the EC serial port if its justified to Pin 1.

XO Serial Ports - physical interface

Both connectors are very close (though not quite identical) to a 1.25 pitch MOLEX picoblade. The latter will fit perfectly. See the image below:


The authoritative drawing for from the manufacturer can be found at File:J1 CN24 male.pdf and File:J1 CN24 female.pdf. The dimensions are listed as CKT 4 for the J1 and CKT 5 for CN21 - both one row.

Generally the plastic connector is sold separate from the metal socket. And you assemble those by crimping a wire to the socket and then inserting it into the connector. The type of socket used requires a special crimping tool.

Some vendors sell ready made 150mm or 300mm cables; with a socket on one and and a tinned lead on the other - which means you can pospone buying the expensive crimp tool for a bit.

Molex numbers

Datasheets: (wire to board header) and (terminal and receptacle).

Right angle Molex Pico Blade 1.24 (0.49") PCB 90 degree headers (male, connector) and matching cable sockets (female); SMD is used on the board; the non SMD version can be made to fit and is slightly easier to solder; and much easier to source.

Connectorpinsboard header (male)receptacle on cable (female)
J1 4 53048-0410 (non SMD) 51021-0400
4 53261-0471 (SMD) 51021-0400
CN24 5 53048-0510 (non SMD) 51021-0500
5 53261-0571 (SMD) 51021-0500

Along with matching socket's (above female connectors are just the 'plastic receptacles'), see item 50125/50133 ( depending on wire thickness.

Technical drawings of the board socket: and for the receptacle (J1) or (CN24)

Digikey numbers

Equivalent digikey numbers (of above MOLEX-es):

Connectorpinsboard header (male)receptacle on cable (female)
J1 4 WM7622CT-ND WM1722-ND
CN24 5 WM7623CT-ND WM1723-ND

See DigiKey page 1 and DigiKey page 2 for more details.

Depending on the wire thickness - you will need 4 or 5 sockets to wire things up - which have digikey number WM1775-ND (22AW) or WM1142CT-ND (24AQ). The Crimp tools are WM9931-ND, WM9978-ND, WM9932-ND or WM9933-ND depending on the wire.

RS-Component numbers

RS Components only has the receptacles:

Connectorpinsreceptacle on cable (female)
J1 4 279-9162 (
CN24 5447-6580(

And a 300mm socket to socket premade wire can be found at # 279-9522 ( - or alternatively you can order individual sockets and crimp these.

Multipurpose Serial Adapter

New Serial Adapter

The new adapters are able to do the conversion from LVTTL (3.3V TTL) to RS232 and USB. The also have a mode for converting RS232 to USB. In the RS232 modes they are able to work with both straight and NULL modem cables. Because of this they are a bit more complicated than the ATest adapters.

This complexity is in the form of 2 jumper blocks on the adapters. Blocks J4 and J6.

J4: Mode select block

J4: Mode select block

J4 selects between the 3 modes of operation.

Mode 1: TTL<->USB

  • Jumper 1&2, 5&6

Mode 2: RS232<->USB

  • Jumper 3&4, 7&8

Mode 3: TTL<->RS232

  • Jumper 2&4, 6&8

Note: in the same PCB this conventions are clearly marked

J6: Null Modem block

New Serial Adapter

In the RS232 modes 2 & 3, J6 will swap the TX and RX lines. Normally changing these jumpers is not necessary.

  • RS232 NULL Modem: 1&2, 5&6
  • RS232 Normal: 3&4, 7&8

To make the adapter appear like a serial port you would find on a PC you would use normal wiring. Connection between the adapter and the PC would use a NULL modem or crossover cable. Just like you would use if you were connecting 2 PC's to each other.

If you are using a serial cable that is wired to be straight through then you would use NULL modem wiring.

If you are using Mode 1 then the settings for J6 do not make any difference because the DB9 is not used.


The device is powered from either the 3.3V from the laptop or by USB. Both may be plugged up at the same time.

Schematics and Board Layout

Original Serial Adapter

The original adapter that came with the ´´ATest´´ boards

Original Serial Adapter

was very simple and only does +3.3V serial to RS232 voltage level conversion.


The TTL<->USB chip is a a FT232R made by FTDI and has drivers for all major operating systems. On Windows it will show up as a COMn device and on Linux as a /dev/ttyUSBn device. Where 'n' is a number. On PC systems this will usually start with COM5 and on Linux ttyUSB0.

On a Macintosh, download and install the driver (see next section) using the instructions at


Windows and MacOs


  • The ftdi driver is included the 2.6.x series of kernels and is included by most distributions plug it in and it should just "work".

Flow Control

In the USB modes the hardware flow control pins are wired so that they are de-asserted. Therefore the hardware flow control setting does not matter. The RS232 flow control lines on the DB9 were left unconnected and can float such that the flow control is asserted. So in RS232 modes hardware flow control must be disabled.

Quirks and Known Problems

Conflict with Braille TTY (brltty) software

Debian (and derivative) distros come with the brltty package installed. This package conflicts with using the device as a plain serial port. Upon insertion the brltty driver takes over the device and disconnects the /dev/ttyUSBn device. The solution to this is to remove or disable the brltty package. On Debian (and derivatives) you can do this with

sudo apt-get remove brltty