Epson FX-80 User Manual page 162

The 7-bit limitation affects graphics in two ways. First, when you
enter a Graphics Mode, the first number (n
l )
you send can only go up
to 127. By changing n
z
from 0 to 1, 2, 3, etc., computers with this
limitation get figure widths with gaps: 0 to 127, 256 to 383, 512 to 639,
etc.
Let's look at an example. The usual way to reserve the entire width
of the page for graphics is:
LPRINT CHR$(27)"K"CHR$(225)CHR$(1);
This gives 225 dots plus 1 times 256 dots, or 480 total. But a 7-bit
computer cannot send the 225; the largest number it can send is 127.
So the maximum number attainable with n
2
=
1 is 127
+
1 x 256
=
383, which is less than 480. If you change n, to a 0 and n
z
to a 2, you
get 0
+
2 x 256
=
512. Now it's too high! See the problem?
So, what can these users do? Well, mainly try not to get discour-
aged. A lot can be done in 383 columns of dots, and if you must have
more than 383 columns of graphics, there is a solution. Just enter
normal-density Graphics Mode twice on the same line (see the sample
at the end of this chapter), once for 383 columns and a second time for
97 columns. This gives coverage of the full 480 columns. It's not ele-
gant, but it averts any crisis and gets the job done.
The second limitation is that 7-bit computers can't fire the top pin of
the print head (Figure 11-2). Since each of the top 8 pins corresponds
to 1 bit in Graphics Mode, 7-bit computers can only fire the middle 7
pins of the 9-pin print head. You'll see how that works shortly.
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