Java IO.pdf - Nguyen Dang Binh

Java I/O
the DTR pin is connected to the DCD and DSR pins. This allows two PCs to communicate
using a communications program and a direct serial connection without any modem. This is
why not all serial cables are created equal, and the cable that works for one device may not
work for another.
Figure 17.3. PC null modem cable
Data is sent from computer to device across the TD line and from device to computer across
the RD line. You access these lines through the output and input streams returned by
CommPort's getOutputStream() and getInputStream() methods. You do not directly
manipulate these pins. The ground pins are used only to maintain a common reference voltage
between the devices. No program ever sends voltage over these lines. This leaves six pins you
may want to read or write. These are:
• DTR
• RTS
• CTS
• DSR
• RI
• CD
Each of these has an effectively boolean value: true if it's showing voltage relative to ground,
false if it isn't. The SerialPort class provides methods to read the current state of all these
pins. It provides methods to write to those pins that would normally be written to by the
computer end of the connection.
17.4.3.1 DTR
Data Terminal Ready, DTR, means the computer is ready to send or receive data. CR,
Computer Ready, would be more likely true nowadays, but the RS-232 standard was
developed in the days of dumb terminals, when personal computers were still an oddity.
public abstract void setDTR(boolean dtr)
public abstract boolean isDTR()
17.4.3.2 RTS
Request To Send, RTS, is one-half of hardware handshaking. The computer raises voltage on
the RTS line to tell the modem it's waiting to send.
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