Video interfaces are a feature of laser printers. Laser
printers grew out of copier machine technology. Early on,
the copier machine manufacturers needed a partner to help
turn them into printers. So, they turned to computer manufacturers.
The computer companies had to write some software, design
a front panel, and get the entire system to work with computers.
Today, the copier machine part of the laser printer is called
the engine. The computer part is called the engine interface.
The interface between the printer logic board and the engine
is called the video interface. An external video interface
is really a method of bypassing the printer logic hoard.
The external video interface is a data pipe directly into
the engine. The cable has to be extremely well-built and
short. The data transfer rate is at bus speeds supposedly
around 2.5M per second or 20,000,000 bps. When the video
interface is active, the printer's logic board is disabled.
The first video interfaces appeared for the Hewlett-Packard
LaserJet II. The LaserJet II was the first laser printer
with an actual video interface available out the back, along
with the RS-232/RS-422 and parallel interfaces. The Hewlett-Packard
manuals do not consistently call this interface the video
interface. Stenciled on the face plate covering the video
interface are the words Optional I/O. In the manuals it
is called the Expansion Interface Slot,which is different
from the Expansion Memory Slot. Today, it is most often
called the video interface slot. To connect to the video
interface, you need a completely new engine inter-face for
the print engine. Because there is not room in the printer
for another circuit board, you have to place the new engine
interface inside the personal computer and take up a slot.
The special video interface cable attaches to the external
video interface on one end and to a connector mounted on
the new engine interface board in the PC. The video interface
is really not an interface at all in the sense of GPIB,
SCSI, parallel, or serial. In reality, a video interface
is more like an umbilical cord. The printer is actually
part of the PC!
There are advantages and disadvantages to this type of
system. Advantages are that the transfer of data to the
printer imaging system is performed through the PC bus rather
than a serial or parallel cable. Unloading fonts and downloading
font caches are much faster. The bottleneck is how fast
the new engine interface can do its job.
The disadvantages arise when you attempt to share the printer.
Because part of the printer is now inside the PC, there
is no way to share the printer. Normal switch boxes do not
work. The PC has to be shared if the printer is shared.
This is the function of a IAN. In fact, because all PC printers
are "dumb," they need a "smart" PC to
manage all the users that want to print. Because PC LANs
have to negotiate the sharing of a printer, you need soft-ware
to share this non-standard engine interface board with those
on the LAN. Unfortunately, it is not the responsibility
of the LAN software vendors to write this software, because
the engine interface board is a non-standard interface,
meaning that the manufacturer of the engine interface board
in the PC would have to write special drivers for all types
of LANs.
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