The Information Train

The increased application of software-controlled digital electronics hinders the understanding of how things work. The information train is a scientific experiment exhibit that physically demonstrates how computers communicate. It comprises a network in which a model Lego train acts as a physical carrier transferring a picture's pixels from one computer to the other. The sending end computer scans a simple picture, and directs a model train to send that pixel to the receiving end computer. This is done by sensing the approaching train and switching a rail junction depending on whether a pixel is on or off. The train carries on its top a piece that rotates depending on the train’s route, thus carrying the data between the two computers. At the receiving end, two sensors detect the shape’s orientation allowing the receiving-end computer to reassemble the picture bit-by-bit, pixelby-pixel. The receiving-end computer is a One Laptop per Child (OLPC) XO-1 machine, programmed using EToys. This provides further opportunities for motivated adventurous children to interact with the experiment’s implementation. When I was a kid I had a pretty good idea of how most appliances in our home worked. The phone was a circuit that physically connected the microphone at each end with the speaker at the other end. The record player’s needle picked up the sound from the grooves of a rotating disc and converted it to current through a magnetic coil. Even the TV was a relatively simple affair: two electromagnets had a ray scan the picture while it was modulated to turn parts of the screen white. How things change... Nowadays to give a similarly realistic picture to our generation’s children I have to talk about analog to digital conversion, CPUs, flash memory, compression, psychoacoustic coding, packet routing, pixels, color perception, and liquid crystal displays. Or lie. Yet, there is still value in understanding the basic principles of modern communication technologies, even at the cost of brave simplifications, for this will seed in our children the interest in the world surrounding them, the willingness to explore it, and, maybe, the ambition to pursue a personally and socially rewarding career in science and engineering. Figure 1 Experimental setup In response to this goal, I created a scientific experiment exhibit that physically demonstrates how computers communicate with each other by setting up a network in which a model Lego train transfers a picture's pixels from one computer to the other (Figure 1). In brief, the sending end computer (on the figure’s left) scans a simple picture from left to right and from top to bottom, and directs a model train (on the front) to send that pixel to the receiving end computer (on the right). This is done by sensing the approaching train and switching a rail junction (front-left) depending on whether a pixel is on or off. The train carries on its top a horizontally-mounted L-shaped piece, hinged in a way that allows it to rotate so that it protrudes from the train’s left or right side. This part carries the data between the two computers. Depending on the track to which the junction sends the train, the train will pass close to a column on its left (red) or right hand side (blue), thereby rotating the L-piece to the corresponding direction. At the receiving end, a sensor (yellow, on the right) detects the train’s passing, and a second one (green-blue, on the front right) checks to see the shape’s orientation. Based on that input the receiving-end computer reassembles the picture bit-by-bit, pixel-by-pixel. The exhibit is based on readily available components. The use of a large-scale (Duplo) Lego train provides a robust, accessible, and configurable platform to which children can easily relate to. The sending and receiving computers are laptops. In contrast to desktop computers, these are self-contained, and can therefore easily communicate their sending or receiving function by placing them near the positions of the track related to it. One of the laptops is a One Laptop Per Child (Lee 2006) XO-1 model, thus demonstrating the machine’s educational potential. The interfacing parts are bespoke circuits based on cheap electronic components. Although the same functionality could have been achieved using Legoprovided black-box components, like WeDo or Mindstorms, the chosen alternative is more open and affordable. Most modern PCs lack simple general purpose input output ports. In particular, USB ports require complex interfacing hardware and device drivers. I therefore repurposed ports provided for other purposes. Nevertheless, the USB ports proved useful for providing a relatively clean 5V supply needed to power the sensors. Sender implementation Figure 2 The sending computer Figure 3 Sender motor control circuit On the sender side (Figure 2), interfacing takes place through the laptop’s parallel printer port. This supports eight output bits and five input bits used for signaling conditions like “printer busy” or “out of paper”. A motor is used to control the rail junction’s position through a rack and pinion assembly. The motor control circuit (Figure 3) uses two of the printer port’s output bits to control the junction’s motor, according to the following table. Bit A Bit B Motor 0 0 Stopped 0 1 Forward 1 0 Reverse 1 1 Not allowed This is done by using a Darlington transistor array integrated circuit (Texas Instruments 1976) to control the windings of two relays, which in turn switch externally-supplied current to the motor. One of the relays controls the motor’s power and the other its rotational direction. The Darlington transistors were wired in pairs to increase their current driving