Peripheral devices are usually external to the computer.[26] Printers, mice, video cameras, scanners, data/fax modems, plotters, robots, telephones, light switches, weather gauges, Palm Computing Platform devices, and many others exist “out there,” beyond the confines of your desktop or server machine. We need a way to reach out to them.
The Java Communications API not only gives us that, but cleverly unifies the programming model for dealing with a range of external devices. It supports both serial (RS232/434, COM, or tty) and parallel (printer, LPT) ports. We’ll cover this in more detail later, but briefly, serial ports are used for modems and occasionally printers, and parallel ports are used for printers and sometimes (in the PC world) for Zip drives and other peripherals. Before USB (Universal Serial Bus) came along, it seemed that parallel ports would dominate for such peripherals, as manufacturers were starting to make video cameras, scanners, and the like. Now, however, USB has become the main attachment mode for such devices. One can imagine that future releases of Java Communications might expand the structure to include USB support (Sun has admitted that this is a possibility) and maybe other bus-like devices.
This chapter[27] aims to teach
you the principles of controlling these many kinds of devices in a
machine-independent way using the Java Communications API, which is
in package javax.comm
.
I’ll start this chapter by showing you how to get a list of available ports and how to control simple serial devices like modems. Such details as baud rate, parity, and word size are attended to before we can write commands to the modem, read the results, and establish communications. We’ll move on to parallel (printer) ports, and then look at how to transfer data synchronously (using read/write calls directly) and asynchronously (using Java listeners). Then we build a simple phone dialer that can call a friend’s voice phone for you -- a simple phone controller, if you will. The discussion ends with a serial-port printer/plotter driver.
The
Communications API is centered around the
abstract class
CommPort
and its two subclasses,
SerialPort
and ParallelPort
,
which describe the two main types of ports found on desktop
computers. CommPort
represents a general model of
communications, and has general methods like getInputStream( )
and getOutputStream( )
that allow you to use the information from Chapter 9 to communicate with the device on that port.
However, the constructors for these classes are intentionally
non-public. Rather than constructing them, you instead use the static
factory method CommPortIdentifier.getPortIdentifiers( )
to get a list of ports,
let the user choose a port from this list, and call this
CommPortIdentifier
’s open( )
method to receive a CommPort
object.
You cast the CommPort
object to a non-abstract
subclass representing a particular communications device. At present,
the subclass must be either SerialPort
or
ParallelPort
.
Each of these subclasses has some methods that apply only to that
type. For example, the SerialPort
class has a
method to set baud rate, parity, and the like, while the
ParallelPort
class has methods for setting the
“port mode” to original PC mode, bidirectional mode, etc.
Both subclasses also have methods that allow you to use the standard Java event model to receive notification of events such as data available for reading, output buffer empty, and type-specific events such as ring indicator for a serial port and out-of-paper for a parallel port -- as we’ll see, the parallel ports were originally for printers, and still use their terminology in a few places.
Java Communication is a standard extension. This means that it is not a required part of the Java API, which in turn means that your vendor probably didn’t ship it. You may need to download the Java Communications API from Sun’s Java web site, http://java.sun.com, or from your system vendor’s web site, and install it. If your platform or vendor doesn’t ship it, you may need to find, modify, compile, and install some C code. Try my personal web site, too. And, naturally enough, to run some of the examples you will need additional peripheral devices beyond those normally provided with a desktop computer. Batteries -- and peripheral devices -- are not included in the purchase of this book.
Elliotte Rusty Harold’s book Java I/O contains a chapter that discusses the Communications API in considerable detail, as well as some background issues such as baud rate that we take for granted here. Rusty also discusses some details that I have glossed over, such as the ability to set receive timeouts and buffer sizes.
This book is about portable Java. If you want the gory low-level details of setting device registers on a 16451 UART on an ISA or PCI PC, you’ll have to look elsewhere; there are several books on these topics. If you really need the hardware details for I/O ports on other platforms such as Sun Workstations and Palm Computing Platform, consult either the vendor’s documentation and/or the available open source operating systems that run on that platform.
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