Page 67 - Hacking Roomba

48 Part I — Interfacing

for a clear representation of this. When physically laying out an LED, the flat part on side of
the LED corresponds to the “bar” part of the schematic.

Understanding MAX232 RS-232 Transceivers

The MAX232 transceiver IC originally developed by Maxim (not the men’s magazine, but the
creator of some of the coolest interfacing ICs out there) performs the magic of converting the
0–5V positive logic signals from the microcontroller in the Roomba to the approximately +/-
12V negative logic signals that are part of the RS-232 standard. Instead of accomplishing this
conversion with a tricky circuit using several transistors, resistors, and capacitors, you just plop
down the MAX232 and a few capacitors and the problem is solved.

Virtually every microcontroller has a serial port on it, so many hackers are familiar with the
MAX232. If you want your little gadget to talk to your computer, chances are you’ve used
a MAX232. There are many circuit schematics on the Internet and in books with the MAX232,
but they tend to vary regarding which value of capacitors to use. Some use 10 µF capacitors,
some use 1 µF, and others use 0.1 µF. Which is the right value? Why do people use different
values?
The pedantic but true answer is that the datasheet for the MAX232 tells what capacitor values
to use. The trick is that there are slightly different versions of these transceivers that can take
different capacitors. One variant, the MAX233, has internal capacitors, so no extra parts are
needed. (It’s expensive though.) Some parts are MAX232 clones and are also called MAX232
but are slightly redesigned. If you have the datasheet for the exact part being used, use the
capacitors described in the datasheet. If unsure, use 1µF capacitors.

The MAX232 works by using the capacitors to create a charge pump that boosts the input
voltage from 5V to either -12V or +12V. The capacitors store the charge needed to make this
voltage. Since it takes more charge to drive long serial cable lines, generally the longer the
cable, the larger the capacitors will need to be. And in RS-232, long means several hundred
feet, not the 10-feet cable you’ll be using here.
Maxim will help you use their parts by sending you free samples. Just go to the Maxim web site
(www.maxim-ic.com), find the part you want, and click sample. This is really handy if you’re a
starving student and want to try out a few interesting parts. If you’re in a hurry or need many
Maxim parts, it’s quicker and easier to buy them from a place like Digikey or Jameco. Most of
their parts are only a few dollars.

Building the Serial Tether

Now that you have some understanding of the circuit, it’s time to build it.

It’s easy to burn yourself with a soldering iron. Be careful, always know where it’s at, and always
make sure to turn it off when done. Also be sure to be properly grounded so you don’t zap any-
thing. See Appendix A for some guidelines on soldering techniques.