Sunday, December 27, 2009

Toy decoding: vtech Push and Ride Alphabet Train

So, it's Christmas and you end up visiting people with kids... and they've got a fancy new vtech Push and Ride Alphabet Train. Now, you're the world's worst child minder because you see it and think: how does that work?


Specifically, when you insert one of the 26 alphabet blocks into the side of the train how does it know to say the correct letter? And how does it know which side (letter or word) is facing outwards (so it can say a letter or a corresponding word: "A is for Apple" etc.).

Now it quick examination shows that there are 6 small switches in each block receptacle and that each block has corresponding bits of plastic and holes to make different binary patterns. The top bit (bit 5) seems to be used to indicate which side of the block is showing.

That leaves 5 bits for the alphabet. Of course that means there are 32 possible combinations (actually 31 since 'block not present' indicated by all switches up is important), and 26 letters in the alphabet. So which 5 binary combinations are not needed for the alphabet and what do they do?

First here's the mapping between letters and their five bit patterns. Here 0 = button is depressed by little sliver of plastic, and 1 = button is left up because there's a space in the block.

a 11010 b 00010 c 00011 d 00100 e 00101 f 00110
g 00111 h 01000 i 01001 j 01010 k 01011 l 01100
m 01101 n 01110 o 01111 p 10000 q 10001 r 10010
s 10011 t 10100 u 10101 v 10110 w 10111 x 11000
y 11001 z 00001

As I'm sure you've noticed there's something very odd about this sequence. Letters b through y follow a nice pattern, but what's up with a and z? Here's the same information using decimal to make the problem clear:

a 26 b 2 c 3 d 4 e 5 f 6
g 7 h 8 i 9 j 10 k 11 l 12
m 13 n 14 o 15 p 16 q 17 r 18
s 19 t 20 u 21 v 22 w 23 x 24
y 25 z 1

As you can see it appears that the numbers for a and z are swapped. You'd expect a to be 1 and z to be 26. Now, there could be some clever explanation for this but I'm guessing it's the work of Captain Cock-up.

When I used to write software in a hardware company it was pretty common for there to be mistakes in the hardware design or implementation that had to be fixed in software. I remember one very snowy December outside of Route 128 at an HP works debugging something nasty with an EISA card on which our code was running inside some new HP workstation (pretty sure it was a Series 700 with the native GSC bus and something called the Wax ASIC to provide an EISA bus). Turned out that our hardware wasn't latching things onto the EISA bus with quite the perfect timing that the ASIC needed and corrupt data was hitting the main bus. This is not the sort of thing you want to have happen. The fix was done in software to alter the order of writing (which was done with two 16-bit writes) and a little loop to spin around checking for stability.

So, I bet vtech had a little mistake like that. Somehow the codes for a and z got swapped in software there's a fix.

If you haven't played around with hardware much you might have been surprised that button depressed = 0 (see above). This is actually pretty common because it's typical to connect logic lines going into some logic (especially if it's TTL) to positive 5V (or similar) with a pull-up resistor.

In TTL logic an unconnected pin will float around and try to be high, and so most designers ensure that it is actually high with a pull-up. Then to change the input you connect the input pin to ground via your switch (with no resistance). Thus the input is normally high (which is typically interpreted as 1) and goes low (normally that's 0) when the switch is depressed.

Here's a typical circuit:


The only disappointed me was that the extra 5 combinations of 1s and 0s don't do anything. I was really hoping for an Easter Egg left by the developers.

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If you enjoyed this blog post, you might enjoy my travel book for people interested in science and technology: The Geek Atlas. Signed copies of The Geek Atlas are available.

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