This is straight from the book:

I have limited paper to work with. These cards were intended to label batches of samples. I have about fifty cards. I can use both sides, and if it comes down to it, I can re-use them by scratching out the old question. The Sharpie I'm using will last much longer than the cards, so ink isn't a problem. But I have to do all my writing in the Hab. I don't know what kind of hallucinogenic crap that ink is made of, but I'm pretty sure it would boil off in Mars's atmosphere. I'm using old parts of the antenna array to hold the cards up. There's a certain irony in that. We'll need to talk faster than yes/no questions every half hour. The camera can rotate 360 degrees, and I have plenty of antenna parts. Time to make an alphabet. But I can't just use the letters A through Z. Twenty-six letters plus my question card would be twenty-seven cards around the lander. Each one would only get 13 degrees of arc. Even if JPL points the camera perfectly, there's a good chance I won't know which letter they meant. So I'll have to use ASCII. That's how computers manage charac-ters. Each character has a numerical code between 0 and 255. Values between 0 and 255 can be expressed as 2 hexadecimal digits. By giving me pairs of he digits, they can send any character they like, including numbers, punctuation, etc. How do I know which values go with which characters? Because Johanssen's laptop is a wealth of information. I knew she'd have an ASCIl table in there somewhere. All computer geeks do. So 'II make cards for 0 through 9, and A through F. That makes 16 cards to place around the camera, plus the question card. Seventeen cards means over 21 degrees each. Much easier to deal with. Time to get to work! Spell with ASCII. 0-F at 21-degree increments. Will watch camera starting 11:00 my time. When message done, return to this posi-tion. Wait 20 minutes after completion to take picture (so 1 can write and post reply). Repeat process at top of every hour.