The Epson/IBM parallel port is almost universally available
in the IBM PC world. Parallel ports are sprinkled everywhere.
All printers, inkjet cartridges printer, laserjet printers, sold into the IBM PC market today have a parallel
port. The cable is standard, and the PC DOS operating system
defaults to it. The Shift-PrtSc key is designed to send
a copy of the characters currently displayed on-screen out
the parallel port of the PC. For ease of use, a printer
in the IBM PC world must use the parallel port.
One drawback of the parallel port is that most other computing
systems are not using it. The reasons are significant. First
of all, the parallel port is slow; most 9,600-bps serial
ports are faster. Hewlett-Packard speeded up the printer
side of the parallel data transfer in the parallel port
of the LaserJet line of printers. Sometimes, this works;
other times, nothing is printed. Some-times, making the
parallel printer cable longer, shorter, cheaper, or more
expensive solves the problem. Try doing all the preceding
and see what works if you have parallel port troubles.
If you have trouble with a PC talking to an HP LaserJet
II through the parallel port, try the serial port. If the
serial port works, try activating the parallel port with
shorter or longer cables. The shorter or longer cables are
acting like a delay line delaying or speeding up the electrical
energy representing bits.
The future of the parallel port looks bleak. The cable has
to be less than 15 feet. Hooking a manual AB switch box
in the middle of this cable (which enables two computers
to share one printer using a manual switch) can kill more
sensitive circuitry in the printer interface, especially
in the HP LaserJet II. In fact, the HP LaserJet II parallel
port will not be repaired under warranty if you used it
with a manual AB switch box.
The most significant drawback is that the parallel port
is fundamentall a one-way communication device. Information
flows from the computer to the printer. The computer cannot
ask the printer questions: "Are you at the top of the
page? How wide are the letters on a line? What size of paper
is in use? Have the fonts already been downloaded, or do
I have to do it?" Similarly, the printer cannot ask
the computer "Do you have the fonts?"
In the parallel port, there is a tiny- bit of information
that trickles back from the printer; therefore, some two-way
communication is possible. But, the hardware design philosophy
and software implementation of most parallel ports is fundamentally
one directional.
Successful two-way use of parallel ports is possible. Most
laptop IBM PC clones have a parallel port that can talk
one way to a printer or two ways with a floppy disk drive
or a CD. When designing the IBM portable PC, IBM built a
parallel port that temporarily disabled all slots (and circuit
hoards in them) when transforming the two-way bus into a
parallel port. IBM also released a data migration kit to
move data from old PC, XT, and AT parallel ports to parallel
ports in PS/2s. The data migration kit consisted of a cable
hooking up parallel ports to software. You can use parallel
ports on some machines for bi-directional communication.
But, none of these applications has featured a printer talking
back. Instead, USB flash disk, floppy disk drives, circuit boards, and file
transfer software has talked back through parallel ports.
And, the two-way com munication is just between two devices
not multiple devices. No company is going to design a printer
that talks back and shares a cable that has a maxi-mum distance
limitation of 15 feet.
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