Character-mode printers are rapidly disappearing. Macintosh
PCs rarely put printers in character mode. PC DOS and Microsoft
Windows are merging, and OS/2 and UNIX are developing graphics
interfaces. This means that future printers will spend more
of their lives in graphics mode. Unfortunately, the graphics-mode
bottleneck could be the PC software, printer, or cable.
Furthermore, different PC software packages may cause the
bottleneck' to change. This makes the bottleneck identification
in graphics mode much more difficult.
The printer cable may be the bottleneck when you are working
in graphics mode. Suppose that a laser or inkjet printer
is printing in 300-by-300 dotsper-inch resolution. A full
page of graphic information would equal 8,415,000 bits (300
x 300 x 8.5 x 11). If these bits are transmitted at 9,600
bps, it will take 877 seconds or 15 minutes to send just
the data. Most of the time, the page is smaller than 8 1/2-by-11
inches, and just certain dots not all dots are sent. Therefore,
the amount of data is much smaller.
A dot-matrix printer's graphics mode speed is computed
in a different way. For a 9-pin dot-matrix printer, assume
that the cell size is 9-by-9 bits. If the printer is printing
at 100 cps, this means that the printer could theoretically
print at 8,100 bps (9 x 9 x 100). If the cable is not the
bottleneck, there is a good chance that the printer is the
bottleneck. Following the same logic, a 24-pin dot-matrix
printer could print at 57,600 bps (24 x 24 x 100). But,
a 24-pin printer has to print seven times as much data 24
x 24/(9 x 9) as a 9-pin printer. This means that graphics
printing speed (if the cable is not the bottleneck) should
be the same for 24-pin printers as for 9-pin printers. Unfortunately,
experiments show that 24-pin dot-matrix printers in graphics
mode print at about half the speed of 9-pin printers. The
manufacturer has had to slow down the 24-pin printers in
order to keep the heads from heating up.
Ink cartridges printers and Laser toner printers can print dots so fast in graphics
mode that the paper movement is still the bottleneck within
the printer. More often, how-ever, the cable is the bottleneck.
Recall that at 300 dpi, it would take 15 minutes to send
a full page in graphics mode at 9,600 bps. The laser or
the inkjet printer is rarely the bottleneck in graphics
mode. To relieve the cable bottleneck, the parallel port
has been redesigned to transmit data up to 10 times faster
than the serial port. This means that the entire page can
be transmitted in 1.5 minutes compared to 15 minutes. Because
lasers can print eight pages per minute, the cable is still
the bottleneck. Inkjet printers, how-ever, can print an
effective page-per-minute rate of only 1.2 pages per minute,
so they can turn into the bottleneck.
Besides the cable and the printer bottlenecks, a common
bottleneck is the PC software. Because the PC software has
to compute the dots to be sent to the printer, the time
it takes to do this can be the bottleneck. In fact, in most
applications, PCs and XTs are almost certainly the bottleneck.
In other words, when printing in graphics mode with an IBM
PC or IBM XT, it really doesn't matter how fast the printer
or the cable is. The PC will slow down the whole process.
Most PCs are fast enough today that graphics-mode printing
is a toss-up. Sometimes it is the PC software; sometimes
it is the cable, and sometimes it is the printer.
You can determine easily whether the software is the bottleneck
by looking at the on-line or ready lights on the printer.
These lights usually flash when the printer is receiving
data. If the lights are flashing constantly, the printer
is the bottleneck. If the lights flash with pauses, the
cable or the PC is the bottleneck.
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