Graphtrax II

Now for the Bad News

Before racing off to create a HI-RES forgery of the Mona Lisa, be aware that the above is how is it SUPPOSED to work. Reality is brutal - there are some exceptions to almost every rule.

Code numbers 9 and 13 for the Apple, and 0, 10, 11, and 12 for the TRS-80 create havoc when used as N1 or N2. (It’s deja vu from the last chapter). Other computers may have trouble with different codes. Even worse, the printer accepts code numbers from 0 to 255 to determine the dot pattern, but many computers are not able to send code numbers greater than 127. It’s the old “missing eighth bit” caper. Like the “lost chord.”

Sigh! Once again the printer can out perform a computer. In the Apple’s case, the Epson Parallel Interface card deactivates the eighth bit so Apple users can control only 7 pins. If it didn’t, bit 8 from the Apple would be on all the time, firing pin 8 every time. Most TRS-80s can control all 8 pins without difficulty.

Back to the Welcome Program

We’ve seen that each pin is associated with a number. The numbers 1, 2, 4, 8, 16, 32, 64, I28 all relate to the mathematical powers of 2. Binary math, and all that stuff. Here’s the relationship:

27

= 128

23

= 8

26

=

64

22

= 4

25

=

32

2 1 = 2

24

= 16

20

= 1

Let’s see if we can fire the pins, in sequence, from the bottom up. Make the program read:

9 PR #1(Apple)

10 PRINT CHR$ (27) “K” CHR$ (50) CHR$ (0) ;

20FOR P = 0 TO 6

30PRINT CHR$ (2^P) ;

40 NEXT P

50 GOT0 2 0

45