Interface Roomba-USB

In order to diagnose/repair a Roomba, it is necessary to do some test and measurements.

Even if each Roomba has a diagnostic program on-board, often is necessary to have more control and a better feedback, with also some measurements.
To do so the basic tool every Roomba-hacker needs to have is a cable to connect the Roomba with a PC.

There are many ways to buy/build such cable.
This article describe how I did it, just to give ideas. Is not necessarily the best way but it worked for me.

Material


We started from a consumer SIM USB card reader.  These readers are available on ebay and similar places for few dollars.
Here a picture of the electronic board (they usually come in some kind of plastic box).

These USB SIM card readers usually are based on a chip capable to convert the USB port into a serial one, the Prolific PL-2303.
Then a cable with a minidin connector is needed and eventually a PCB minidin.
In my specific case I used :

• USB SIM card reader
• 8 pin minidin PCB outlet
• 8 pin minidin cable (at both ends)

Preparation


The first thing to do is to remove from the USB SIM card some useless components, like the SIM card connector, a 74HC04 and a Crystal (3.5 Mhz).

At this point is necessary to identify on the PL-2303 the pins we need to connect.

• Pin 1 PL-2303
  TXD -> connected to the Roomba RXD (pin 3 minidin)
• Pin 3 PL-2303
  RTS -> connected via a transistor to the Roomba Power pin (pin 6 minidin 7 - pin 5 minidin 8)
• Pin 5 PL-2303
  RXD -> connected to the Roomba TXD (pin 5 minidin 8 - pin 4 minidin 8)
• Pin 7 PL-2303
  GND - pin 6/7 minidin 7 - pin 7/8 minidin 8)

Here the schematic of what we want to do :

It is very important to pay attention of the type of minidin used.In the schematic I used the original Roomba minidin 7, so the pinout is DIFFERENT from the minidin 8 !

On the net is possible to find information about the pinout and specifications, like here.

Here some pictures of cable preparation.