Nov 6, 2008

One Kit Wonders: The PunchBot

For the new NXT book coming out this month, I chose to create a robot that reaches back a few decades to use an old method of programming - it's called PunchBot.

Long before we ever had graphical user interfaces (GUI) or even the command prompt (look it up if you don't know what THAT is), early programmers used a flat piece of stock paper called a punchcard to submit instructions to large mainframes (basically the predecessor to what we know as servers - web servers, email servers, etc.). They fed these cards into a slot (sometimes a card feeder could be used) and each card contained a small bit of the larger program. I use the word 'bit' here slightly tongue-in-cheek because that's exactly how early programmers did it - they programmed in binary, using only 1s and 0s (zeroes) to perform basic functions such as ADD, SUBTRACT, MOVE, and more commands. (Do a search for "machine language" for more information.)

It was tedious, error prone, and not as glamorous as programming with a nice drag-and-drop interface like NXT-G. These cards were printed with dozens, sometimes hundreds, of small circles or rectangles. Holes could then be punched in the card - a punch in a particular hole indicated a 1... a non-punched hole indicated a zero.

So on a particular card, if you saw the following:

punch - no punch - punch - punch - punch - no punch - punch - no punch

this equated to

1 - 0 - 1 - 1 - 1 - 0 - 1 - 0

That's an eight-bit sequence! - if this were treated as a binary number, it could be converted to the decimal value of 186. You'll need to do some research on binary math to understand this process, but for now what you need to understand is that programs were written using hundreds, thousands, and sometimes even millions of cards containing punched sequences like the one above. Some cards could hold dozens of sequences and other cards might only hold one or two... as technology progressed (laugh), cards could hold more and more sequences of punched values - this in turn meant that programs took up fewer cards!

My Punchbot will show you how to create your own punchcards, with each card holding five bits - the first bit is always "punched" and is how the robot will find the start of each card (I call it the synchronization bit). The remaining four sections of the card can either be punched or not punched. Four bits means I have a total of 16 possible combinations (again, do some research if you want to understand binary counting).

This means that each card can tell the Punchbot to run 1 of 16 possible mini-programs! (Each mini-program is stored in a Switch block that is configured with 16 tabs - each tab can contain one or more NXT-G blocks meaning you can create complex behaviors for each card.)

For example, a card with 1-1-0-0-1 converts to 9. (Remember, the first bit on the far left is a synch bit and isn't counted BUT it must always be punched so it is represented by a 1.)

If I submit two punchcards to my PunchBot, both with this sequence, the robot will run whatever blocks I have in the 9th tab of the Switch block TWICE. You can submit as many cards to the PunchBot as you like - it stores the cards' values in a file and then reads them back in sequence to determine the order that your sub-programs will run.

Building instructions and the complete program are provided in the book, along with some expanded exercises and suggested modifications to improve the robot.

I hope you enjoy it!

4 comments:

Sam said...

Brilliant!
How about a few steps farther back and create a Bletchley Park Bombe/Fish that can crack Nazi Enigma codes? I think they used long strips of punched paper rather than cards.

Jim Kelly said...

Sam,

If you build and program an NXTigma, please let us know - we'd love to have you provide a write-up and photos for the blog.

Jim

London said...

Very very cool, now I'm going to have to scrape up some money to buy the book, or *gasp* build my own version

Anonymous said...

ah yes, those punch cards. Fondly recall dropping a huge stack of them all over the floor while on route to feeding them into the mainframe. Nightmare!

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