Java IO.pdf - Nguyen Dang Binh

Java I/O
rate of the connection is a one bit. Sending between -3 and -25 volts for the same amount of
time is a bit. [2] These bits are grouped into serial data units, SDUs for short. Common SDU
lengths are 8 (used for binary data) and 7 (used for basic ASCII text). Most modern devices
use eight data bits per SDU. However, some older devices use seven, six, or even five data
bits per SDU. Once an SDU is begun, the rest of the SDU follows in close order. However,
there may be gaps of indeterminate length between SDUs.
One of the problems faced by asynchronous serial devices is determining SDU boundaries. If
a modem receives eight data bits, how is it to tell whether that's an entire SDU or the last four
bits of one SDU and the first four bits of another, especially if the connection has some noise
and isn't particularly reliable? To assist with this, each SDU is preceded by a single start bit
that's always 0, and followed by between one and two stop bits. Stop bits last longer than data
bits so they can always be identified.
In addition to the data and the start and stop bits, an SDU may have a parity bit. Parity is a
very simple error detection scheme that can detect (but not correct) single bit errors in an
SDU. There are two basic parity schemes. Even parity adds an extra one bit to the end of the
SDU if there are an even number of one bits in the data. Odd parity adds an extra one bit to
the end of the SDU if there are an odd number of one bits in the data. [3] No parity simply omits
the parity bit. The combination of data bits, parity scheme, and stop bits is abbreviated in
forms like 8N1 or 7E1. 8N1 means a connection uses eight data bits, no parity, and one stop
bit; 7E1 means seven data bits, even parity, and one stop bit. Virtually all modern systems use
8N1.
The baud rate is the number of times per second the state of the communication channel
changes. This is not the same as bits per second. Modern modems send multiple bits per baud.
Most U.S. phone lines, configured primarily for voice calls, have a maximum baud rate of
3200. Modems that send higher bit rates send multiple bits with each baud. A 28,800 bps
modem is a 3200 baud modem with nine states, for example. In fact, a standard 2400 bps
modem is really a 600 baud modem with four states.
The Java Comm API lets you set all of these parameters, including baud rate, data bits, stop
bits, and parity. They should all be familiar to anyone who's struggled with modem init strings
and terminal software in the bad old days before the Internet separated connectivity from
content. Four methods in the SerialPort class return the values of these settings. They are:
public abstract int getBaudRate()
public abstract int getDataBits()
public abstract int getStopBits()
public abstract int getParity()
A little surprisingly, you can't set these values independently. Instead, all four values (baud,
data bits, stop bits, and parity) are set at once with the setSerialPortParams() method:
public abstract void setSerialPortParams(int baud, int dataBits, int
stopBits, int parity) throws UnsupportedCommOperationException
2
Sending between 3 and -3 volts is a hardware error.
3
There are two more parity schemes you may encounter in brain-damaged hardware. Mark parity always adds a one bit for the parity; space parity
always adds a zero bit. These convey no useful information and are almost never used.
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