This page is about remote-controlling the old Lego RCX system. For the new NXT system, please click here
The Lego Mindstorms robotics kit provides a cheap and easy way to build simple robot applications.
However, as robot control programs are usually uploaded to the Lego RCX computer to run standalone, it suffers from some
To overcome these problems, I have programmed a small direct interface that remote-controls the
RCX from within a Windows C-program on the host computer. Data exchange with the RCX is done by the
interface, which runs in an extra task in the background and provides simple data exchange using
C-variables. Data is transmitted to and from the robot by setting and reading out these variables.
- control programs are limited by the capacity of the RCX module
- in- and output are limited to the three sensors and three motors of the RCX
- control from within larger applications or external programs is usually not possible
- a Lego Camera exists (Vision Command), but it is not intended to use the video image to control the robot
This interface directly accesses the Lego tower via the serial port or the USB port (which simulates a serial port) directly via Win32 functions, which means that neither spirit.ocx nor MFC are needed. On the RCX, only the standard Lego firmware is needed. It supports direct read- and write-commands via the IR-tower, and these functions are used by the interface.
The second important part for advanced robot control is gaining access to the Lego camera (or any other video camera) from within the robot-controlling VC++ program. For this, the Video-for-Windows standard interface is used (see below).
This tutorial contains the following example programs:
- Roam - a minimalistic windows console application that demonstrates how to use the rcx21.cpp remote interface to access the RCX via serial tower or USB tower
- Videostream - Another minimalistic windows application. It shows how one can read in and display live video in a VC++ program.
- Vehicle - This more advanced application demonstrates how one can combine video input and the rcx remote interface to create a Braitenberg-vehicle-like "bull robot" that is attracted by all red things.
The rcx21.cpp V2.1 Interface (for serial port and USB)
Here is the newest version of the RCX interface program. It supports both the serial port version and the USB version of the Lego tower. Since version 2.0, the robustness of the interface to message loss has been improved, and it now supports to set sensor types and modes (which is needed for light and rotation sensors). Also, RCX_error now reports communication failures.
This is a direct interfacing library, which means that sensor/motor
data is transferred between the RCX and the host computer in realtime.
Downloading programs onto the RCX to make it work stand-alone is not supported.
This library focuses on direct robotic control from C++ programs and uses the
RCX only as an interface between the sensors/motors and the host computer.
- sensor polling can be switched on/off for individual sensors by setting flags
- raw sensor values can be directly read in int variables
- sensors can be set to any sensor type and mode
- motors can be switched on/off via flags
- motor direction/speed is controlled by writing into variables
- RCX sound effects can be triggered
- interfacing between the computer and the rcx is done asynchronously in an
extra task, independent from your program
- in case of transmission problems, the program will retry (without
blocking) until success
Starting and stopping the interface
To start the RCX interface, call the routine
RCX_start() with the port type and port name as parameters, for instance
RCX_start(1, "COM1"); for the serial tower, or
RCX_start(2, "\\\\.\\LEGOTOWER1"); for the USB tower.
To stop the RCX interface, use the following commands:
This signals the RCX interfacing task to quit and waits until it has finished.
Possible values for RCX_sensor_on variables:
0 (polling off) and 1 (polling on).
RCX_sensor_on is alive signal,
RCX_sensor_on is value of sensor 1,
RCX_sensor_on is value of sensor 2,
RCX_sensor_on is value of sensor 3,
RCX_sensor_on is battery power (in mV).
Please note that polling sensors slows the transmissions down, so don't switch on sensors if you don't need them.
RCX_sensor_val contain the current sensor values, if polling for the respective sensor is switched on. The value range depends on the current sensor mode (see below), but is coded as an integer.
Types for sensors. Only the values for sensors 1,2,3 are used.
Sensor types are: 0: raw, 1: touch, 2: temperature, 3: light, 4: rotation.
Modes for sensors. Only the values for sensors 1,2,3 are used.
Valid sensor modes are:
0: Raw - value in 0..1023
1: Boolean - Either 0 or 1
2: Edge count - Number of boolean transitions
3: Pulse count - Number of boolean transitions divided by two
4: Percentage - Raw value scaled to 0..100
5: Temperature in °C - 1/10ths of a degree, -19.8 .. 69.5 (coded as an int)
6: Temperature in °F - 1/10ths of a degree, -3.6 .. 157.1 (coded as an int)
7: Angle - 1/16ths of a rotation, represented as a signed short
Not all of the sensor modes are tested, and some of the values might need additional conversion (for example temperature).
Possible values for RCX_motor_on variables:
0 (off) and 1 (on).
RCX_motor_on is motor A,
RCX_motor_on is motor B,
RCX_motor_on is motor C.
A motor which is switched off will block. If you want a motor to be able to turn passively without much resistance,
switch it on and set its RCX_motor_val to 0 (see below).
Possible values for RCX_motor_val variables:
-8 (maximal power backwards), ... -1, 0 (no power, free float), 1, ... , 8 (maximal power forwards).
Possible values for RCX_sound:
The variable is 0 in the beginning. Setting the variable to a value 1,2,3,4,5 or 6 triggers the according
sound effect (blip, beep-beep, downward tones, upward tones, low buzz and fast
upward tones). After the sound command has been transferred successfully, the
variable is set back to 0 automatically.
For a demonstration how to use the rcx21.cpp interface, please refer to the "roam" example below.
Click to download the RCX V2.1 interface source code (also included in roam.zip and vehicle.zip):
Click to download older versions of the interface source code (for backwards compatibility only):
This is a small Visual C++ console application that demonstrates how to use the RCX interface to interact with the robot.
The behavior implemented in this example program is to go forward until the touch sensor connected to sensor input 1 signals that an obstacle is hit, then turn and continue forward.
Click here to download roam.zip (9 kb)
This application shows how to set up the video for windows interface to enable video input to your VC++ program.
It also shows the incoming video in the window and provides a dialog for selecting a video source.
Uses standard video-for-windows library vfw32.lib.
Click here to download videostream.zip (17 kb)
This more advanced application demonstrates how one can combine video input and the rcx21.cpp interface to create a Braitenberg-vehicle-like "bull robot" that is attracted by red things. The incoming video images are scanned for red
objects, and the robot motors are controlled according to the position and size of the red objects in the robot's view.
Click here to download vehicle.zip (17 kb)
Note: these links point to pages outside of the Max Planck Institute's webspace. We take no responsibility for their content.
This software is provided as-is. Please remember that you use this software on your own risk.
I am currently not working on Lego robotics any more, but you can contact me for questions and comments at: email@example.com
Last updated: September 7th, 2012. (Version of September 15th, 2005)