NXT Plotter

Thursday, May 10th, 2007

Völker Nils from Germany has made this impressive NXT plotter. Nice images and video on his site. The programming was done in nxc, and is described in detail here. Take a look at the way he made a homebrew long gear rack using modelling clay!

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News of the Month!

Thursday, May 10th, 2007

For those who are not very familiar with the nitty-gritty of the NXT digital I2C bus, I wish to highlight the significance of the ‘Homebrew NXT port expander’ published on NXTasy.

As many have experienced the major limitation in the NXT is the number of sensor and motor ports. The only solution so far was using another NXT, communicating with Bluetooth with the first NXT (or RCX communicating with other means, like published a while ago). HiTechnic has been developing an active sensor multiplexer for some time (a picture of the prototype is here). This multiplexer will allow connecting 4 sensors – either passive (e.g. touch sensor), active (e.g. light sensor) or digital (e.g. sonar), to a single NXT port.

The solution presented by xw25adc is different – his port expander connects in series just the four ‘digital’ wires from the 6-wire NXT port – the +4.3V, the ground and the two digital I2C wires. Well, I’m getting too technical… Let’s stop that.

So what this so innovative about xw25adc work? To connect more then one digital sensor to a one port (i.e. a common ‘I2C bus’) you must have the sensor recognize that other sensors are connected. Why? Because the total of all sensors’ pull-up resistors must equal 82kOhm. If you naively connect two digital sensors, the pull-up resistance equals to two 82kOhm resistors in parallel which gives 82kOhm/2=41kOhm and the sensors will not function.

Anyone who read mindsensors.com manuals might notice a feature called ADPA: Supports Auto Detecting Parallel Architecture (ADPA) for sensors. This means that ACCL-Nx can coexist with LEGO or third party digital sensor on the same NXT port. This enables user to employ several sensors on the same port without the need of external sensor multiplexer, reducing the overall size.” This feature is just what one need to solve the previous problem – activating the ADPA on mindsensors.com sensors (it is shipped turned off) make them sense and recognize if more sensors are wired to the same port, and change the pull-up resistance to compensate this. xw25adc MUST have activated the ADPA feature, and used the compass sensor “advanced” I2C commands to change his ‘address’ from the default value 2 to 4. As far as I know, this is the first demonstration of this feature in action. Now some may ask - Wow! Can I do it too? Well, I can tell you that mindsensors.com have a prototype of a similar port expander in hand. In a few days I will post an image of this prototype when it reaches me. I’m currently working on a simple PC/Mac application that will ‘manage’ sensors – change ADPA on and off, change address etc. If I get some help from Lego/NI this application might even incorporate into the ‘Tools’ menu in NXT-G (no promises…).

One final technical detail – what differs between HiTechnic sensor multiplexer and mindsensors.com multiplexer? The former also allow connection of passive and active sensors, while the later supports only digital sensors (actually, only digital sensors which do not require 9V supply). This requires active components to regulate which sensor’s analog readout is transfer to the NXT at any moment. For these reasons, the HiTechnic multiplexer is expected to be more expansive (and it require 9V supply) then the passive mindsensors.com ‘I2C expander’. 

Guy Ziv
NXTasy.org

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