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Fundamentals of Dot Matrix Printing
Line 40 reveals that the variable M is used to control the
number of times line 90 is executed. When I is small, M is large.
When I is greater than 12, M is fixed to 2. Line 50 sets the vertical
line spacing (you may recall that ESC = CHR$(27), “3” =
CHR$(Sl)). So, in this program, line 50 sets the vertical spacing to
11144 inches.
Line 60 starts a loop that uses the value of M to count 25 times
when I equals 1, and counts 3 times when I is greater than 12. Line
70 prints a message telling us the value of the line spacing on the
next-to-last line of the M loop. Line 80 spaces over the printing
done by line 90 to column 30 by using the BASIC TAB statement.
Line 90 is where the action is. This line is similar to line 140
in the first reference program and prints 40 characters; its ASCII
code is 241. This character is a six-dot straight line that is part of
the block graphics set.
Line 100 and 110 end the J loops, and line 120 resets the
printer and ends the program.
Discussion of the Vertical Spacing Program
Let’s start by looking at how close the dots are vertically in Figure
6-5. Remember that the normal characters are seven dots high,
which is 141144ths (7/72nds) of an inch. Therefore, if you wanted
to print text, say in all uppercase letters, with each line resting atop
the next, you’d set the vertical spacing at 14/144th of an inch.
When we use block graphics (where each block is 6 dot-
spaced high), we set the vertical spacing to 6/72 (or 12/144) inches.
The maximum value of n allowed for the vertical spacing
control code is 127. The result is that almost one inch (127/144ths)
for the ESC “3” n code, and roughly l-314 inches (127/72nds) for
ESC “A” n.
In the sample program, we stopped I at 30, so the printout
would fit on a standard 66-line sheet of sprocket-fed paper. If you
wish to, you can replace the loop limit on line 20 with 127, and run
the program again. You might also try replacing the CHRS(51) with
CHR$(65) on line 50 and run the program using the n/72-inch
vertical line spacing mode.
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