Java IO.pdf - Nguyen Dang Binh

Java I/O
standard messages on other wires. Each return wire corresponded to a particular message, like
"Out of paper" or "Printer busy." Modern parallel ports allow full, bidirectional
communication.
The javax.comm.ParallelPort class is a concrete subclass of javax.comm.CommPort that
provides various methods and constants useful for working with parallel ports and devices.
The main purposes of the class are to allow the programmer to inspect, adjust, and monitor
changes in the settings of the parallel port. Simple input and output are accomplished with the
methods of the superclass, CommPort. ParallelPort has a single public constructor, but that
shouldn't be used by applications. Instead, you should simply call the open() method of a
CommPortIdentifier that maps to the port you want to communicate with, then cast it to
ParallelPort:
CommPortIdentifier cpi = CommPortIdentifier.getPortIdentifier("LPT2");
if (cpi.getType() == CommPortIdentifier.PORT_PARALLEL) {
try {
ParallelPort printer = (ParallelPort) cpi.open ();
}
catch (PortInUseException e) {}
}
Methods in the ParallelPort class fall into roughly four categories:
• Methods that adjust the port mode
• Methods to control the port
• Methods to inspect the state of the port
• Methods that listen for changes in the state of the port
17.5.1 Parallel Port Modes
Like most other computer hardware, parallel ports have evolved over the last two decades.
Modern parallel ports support bidirectional communication and other features never
envisioned for the original parallel port that was only supposed to send data to a daisy-wheel
printer. However, older peripherals may not work with newer parallel ports, so they can, if
necessary, be downgraded to any of several various compatibility modes. All of these are
available as named int constants in the javax.comm.ParallelPort class:
ParallelPort.LPT_MODE_ANY // Use the most advanced mode possible.
ParallelPort.LPT_MODE_SPP // Original lineprinter mode. Unidirectional
// transfer from PC to printer. Most
compatible
// with older peripherals.
ParallelPort.LPT_MODE_PS2 // Byte at a time, bidirectional mode as
// introduced in the IBM PS/2 family.
ParallelPort.LPT_MODE_EPP // Extended parallel port.
ParallelPort.LPT_MODE_ECP // Enhanced capabilities port.
ParallelPort.LPT_MODE_NIBBLE // Nibble (4 bits, half a byte) at a time
mode,
// bidirectional, used by some Hewlett Packard
// equipment.
The mode the parallel port uses is returned by the getMode() method and set by passing the
appropriate constant to the setMode() method:
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