Hardwiring your NXTs... with LEGO
To get around this problem, I looked into ways to hardwire the NXTs together; there are some pretty amazing homebrew solutions out there that various people have come up with. However, as I lack the technical/electronics skills required to make such devices, I looked for a way to do it purely with NXT pieces. I came up with an interesting two-way serial communications cable that simply uses two light sensors to send and recieve data.
The two sensors are connected facing each other with a shield around them to block out ambient light. Each sensor is connected to a different NXT. One NXT sends data to the other by blinking its light sensor on and off, which the other light sensor picks up. The key advantage I found was that these blinks can be picked up over very short intervals of time (~30 milliseconds). This makes simple communications quite fast - perhaps faster than Bluetooth.
Here's a video demonstrating the "LightComm" cable. One NXT sends the position and power level for the motor to the other NXT, and then both NXTs move their motors accordingly. In the video, the NXT beeps when it starts sending each message, to give you an idea of the speed of the communication link. In the last part, I opened the casing around the light sensors so you can see the blinks.
One advantage of this cable is that you can customize the method of communication to be the most efficient for each application. One method would be to have the light blink on and off like a digital link to send "bits" of information. In the above video, I used an analog method, where the data is sent by the time duration of each blink. The light blinks a total of two times. The first blink gives the target position of the motor, and the second gives the power level. Amazingly, the recieving light sensor can accurately measure the duration of the light to milliseconds! Thus, each millisecond in the duration of the blink represents a degree in the target position or a unit of power. As you can see in the video, the message is usually transmitted with an accuracy of about +/- 2 degrees, which should be ample for most applications.
-Jonathan
Comments
see also:
http://thenxtstep.blogspot.com/2010/01/high-bay-warehouse-video-is-finally.html
and
http://www.youtube.com/watch?v=WIp_oIj4FLE
they also use a light protocol since BT over more then 8 nxt is not working.
Here is a photo and description
http://tinkernology.blogspot.com/2010/06/communicating-with-light.html
...you can also do this (under certain environments) with the NXT beeping and detecting it with a sound sensor (useful to save a port for "one way" communication). Or physically linking two motors together (requires two motor ports, but fun to watch). Or using a motor and a rotation sensor (one-way again).
I've not tried it the way described here actually - I've always gone for a "digital" solution, as sending messages is then much less time-dependent, faster, and can be self-correcting (look up self-correcting codes). An analog solution like this is interesting, and I wonder if you could take it further - use NXT 2.0 color sensors, so you have three possible colors as well as on/off states.
it's great that the sensor can detect it in milliseconds!
Of course, you could connect your NXTs using port 4 in high-speed mode using a standard NXT cable. I tried this using someone else’s source code, but didn’t get to do anything with it yet.
Yes, I was aware of the Motor Multiplexers, but they're so expensive that I wanted a different method.
I saw some stuff about using port 4 for communication, but I didn't know there was source code already out there.
6 way communication with light sensors