121
is a printer. each data line corresponds to one pin on the print
head. Thus each byte sent will fire up to eight pins.
But the printer has 9 pins available. So how do you tire the
ninth pin with only 8 data lines? In fact, do you really want
to bother with just one extra pin? For this purpose, your printer
has a special 9-pin graphics mode (it won’t, however, work
with 7-bit interface systems). In this mode the printer takes 2
bytes to fire all 9 pins as shown in Figure 7-8. For such
graphics-intensive applications as screen dumps, printing 9
pins at a time can speed up the process considerably.
g g 0 0 0 0 2 128
64
32
16
8
4
2
128
First byte
Second byte
(only the top bit is used)
Figure 7-8. The printer needs 2 bytes to fire all 9 pins in the 9-pin
graphics mode.
In addition, you can select the print density by the value
of n0. When nO is 0 the normal density is selected, and when
n0 is 1 the double density is selected.
Since computers are faster than printers, there is no signif-
icant time loss in printing a single line of graphics with 9 pins.
You get 9 dots per line in about the same time as you get 8
dots in the other graphics modes.