94
peripheral devices using eight data lines. When the peripheral 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 fire the
ninth pin with only 8 data lines? In fact, do you really want to
bother with just one extra pin? Well, for such graphics-intensive
applications as screen dumps, printing 9 pins at a time can
speed up the process considerably. For this purpose, your
printer has a special g-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 6-8.
0
:
Ij:
0
0
E
128
64
32
16 First byte
8
4
2
1 J
128 Second byte
(Only the top bit is used)
Figure 6-8. The printer needs 2 bytes to fire all 9 pins in the g-pin
graphics mode.
In addition, you can select the print density by the value of no.
When n0 is 0 the normal density is selcted, and when n0 is 1 the
double density is selected.
Since computers are faster than printers, there is no signifi-
cant 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.