一個(gè)嵌入式系統(tǒng)通常需要通過(guò)串口與其主控系統(tǒng)進(jìn)行全雙工通訊，譬如一個(gè)流水線控制系統(tǒng)需要不斷的接受從主控系統(tǒng)發(fā)送來(lái)的查詢和控制信息，并將執(zhí)行結(jié)果或查詢結(jié)果發(fā)送回主控系統(tǒng)。本文介紹了一個(gè)簡(jiǎn)單的通過(guò)串口實(shí)現(xiàn)全雙工通訊的Java類庫(kù)，該類庫(kù)大大的簡(jiǎn)化了對(duì)串口進(jìn)行操作的過(guò)程。
本類庫(kù)主要包括：SerialBean.java (與其他應(yīng)用程序的接口), SerialBuffer.java (用來(lái)保存從串口所接收數(shù)據(jù)的緩沖區(qū)), ReadSerial.java (從串口讀取數(shù)據(jù)的程序)。另外本類庫(kù)還提供了一個(gè)例程SerialExample.java 作為示范。在下面的內(nèi)容中將逐一對(duì)這幾個(gè)部分進(jìn)行詳細(xì)介紹。
1. SerialBean
SerialBean是本類庫(kù)與其他應(yīng)用程序的接口。該類庫(kù)中定義了SerialBean的構(gòu)造方法以及初始化串口，從串口讀取數(shù)據(jù)，往串口寫入數(shù)據(jù)以及關(guān)閉串口的函數(shù)。具體介紹如下：
public SerialBean(int PortID)
本函數(shù)構(gòu)造一個(gè)指向特定串口的SerialBean，該串口由參數(shù)PortID所指定。PortID = 1 表示COM1，PortID = 2 表示COM2，由此類推。
public int Initialize()
本函數(shù)初始化所指定的串口并返回初始化結(jié)果。如果初始化成功返回1，否則返回-1。初始化的結(jié)果是該串口被SerialBean獨(dú)占性使用，其參數(shù)被設(shè)置為9600, N, 8, 1。如果串口被成功初始化，則打開(kāi)一個(gè)進(jìn)程讀取從串口傳入的數(shù)據(jù)并將其保存在緩沖區(qū)中。
public String ReadPort(int Length)
本函數(shù)從串口(緩沖區(qū))中讀取指定長(zhǎng)度的一個(gè)字符串。參數(shù)Length指定所返回字符串的長(zhǎng)度。
public void WritePort(String Msg)
本函數(shù)向串口發(fā)送一個(gè)字符串。參數(shù)Msg是需要發(fā)送的字符串。
public void ClosePort()
本函數(shù)停止串口檢測(cè)進(jìn)程并關(guān)閉串口。
SerialBean的源代碼如下：
package serial;
import java.io.*;
import java.util.*;
import javax.comm.*;
/**
*
* This bean provides some basic functions to implement full dulplex
* information exchange through the srial port.
*
*/
public class SerialBean
{
static String PortName;
CommPortIdentifier portId;
SerialPort serialPort;
static OutputStream out;
static InputStream in;
SerialBuffer SB;
ReadSerial RT;
/**
*
* Constructor
*
* @param PortID the ID of the serial to be used. 1 for COM1,
* 2 for COM2, etc.
*
*/
public SerialBean(int PortID)
{
PortName = "COM" + PortID;
}
/**
*
* This function initialize the serial port for communication. It startss a
* thread which consistently monitors the serial port. Any signal capturred
* from the serial port is stored into a buffer area.
*
*/
public int Initialize()
{
int InitSuccess = 1;
int InitFail = -1;
try
{
portId = CommPortIdentifier.getPortIdentifier(PortName);
try
{
serialPort = (SerialPort)
portId.open("Serial_Communication", 2000);
} catch (PortInUseException e)
{
return InitFail;
}
//Use InputStream in to read from the serial port, and OutputStream
//out to write to the serial port.
try
{
in = serialPort.getInputStream();
out = serialPort.getOutputStream();
} catch (IOException e)
{
return InitFail;
}
//Initialize the communication parameters to 9600, 8, 1, none.
try
{
serialPort.setSerialPortParams(9600,
SerialPort.DATABITS_8,
SerialPort.STOPBITS_1,
SerialPort.PARITY_NONE);
} catch (UnsupportedCommOperationException e)
{
return InitFail;
}
} catch (NoSuchPortException e)
{
return InitFail;
}
// when successfully open the serial port, create a new serial buffer,
// then create a thread that consistently accepts incoming signals from
// the serial port. Incoming signals are stored in the serial buffer.
SB = new SerialBuffer();
RT = new ReadSerial(SB, in);
RT.start();
// return success information
return InitSuccess;
}
/**
*
* This function returns a string with a certain length from the incomin
* messages.
*
* @param Length The length of the string to be returned.
*
*/
public String ReadPort(int Length)
{
String Msg;
Msg = SB.GetMsg(Length);
return Msg;
}
/**
*
* This function sends a message through the serial port.
*
* @param Msg The string to be sent.
*
*/
public void WritePort(String Msg)
{
int c;
try
{
for (int i = 0; i < Msg.length(); i++)
out.write(Msg.charAt(i));
} catch (IOException e) {}
}
/**
*
* This function closes the serial port in use.
*
*/
public void ClosePort()
{
RT.stop();
serialPort.close();
}
}
2. SerialBuffer
SerialBuffer是本類庫(kù)中所定義的串口緩沖區(qū)，它定義了往該緩沖區(qū)中寫入數(shù)據(jù)和從該緩沖區(qū)中讀取數(shù)據(jù)所需要的函數(shù)。
public synchronized String GetMsg(int Length)
本函數(shù)從串口(緩沖區(qū))中讀取指定長(zhǎng)度的一個(gè)字符串。參數(shù)Length指定所返回字符串的長(zhǎng)度。
public synchronized void PutChar(int c)
本函數(shù)望串口緩沖區(qū)中寫入一個(gè)字符，參數(shù)c 是需要寫入的字符。
在往緩沖區(qū)寫入數(shù)據(jù)或者是從緩沖區(qū)讀取數(shù)據(jù)的時(shí)候，必須保證數(shù)據(jù)的同步，因此GetMsg和PutChar函數(shù)均被聲明為synchronized并在具體實(shí)現(xiàn)中采取措施實(shí)現(xiàn)的數(shù)據(jù)的同步。
SerialBuffer的源代碼如下：
package serial;
/**
*
* This class implements the buffer area to store incoming data from the serial
* port.
*
*/
public class SerialBuffer
{
private String Content = "";
private String CurrentMsg, TempContent;
private boolean available = false;
private int LengthNeeded = 1;
/**
*
* This function returns a string with a certain length from the incomin
* messages.
*
* @param Length The length of the string to be returned.
*
*/
public synchronized String GetMsg(int Length)
{
LengthNeeded = Length;
notifyAll();
if (LengthNeeded > Content.length())
{
available = false;
while (available == false)
{
try
{
wait();
} catch (InterruptedException e) { }
}
}
CurrentMsg = Content.substring(0, LengthNeeded);
TempContent = Content.substring(LengthNeeded);
Content = TempContent;
LengthNeeded = 1;
notifyAll();
return CurrentMsg;
}
/**
*
* This function stores a character captured from the serial port to the
* buffer area.
*
* @param t The char value of the character to be stored.
*
*/
public synchronized void PutChar(int c)
{
Character d = new Character((char) c);
Content = Content.concat(d.toString());
if (LengthNeeded < Content.length())
{
available = true;
}
notifyAll();
}
}
3. ReadSerial
ReadSerial是一個(gè)進(jìn)程，它不斷的從指定的串口讀取數(shù)據(jù)并將其存放到緩沖區(qū)中。
public ReadSerial(SerialBuffer SB, InputStream Port)
本函數(shù)構(gòu)造一個(gè)ReadSerial進(jìn)程，參數(shù)SB指定存放傳入數(shù)據(jù)的緩沖區(qū)，參數(shù)Port指定從串口所接收的數(shù)據(jù)流。
public void run()
ReadSerial進(jìn)程的主函數(shù)，它不斷的從指定的串口讀取數(shù)據(jù)并將其存放到緩沖區(qū)中。
ReadSerial的源代碼如下：
package serial;
import java.io.*;
/**
*
* This class reads message from the specific serial port and save
* the message to the serial buffer.
*
*/
public class ReadSerial extends Thread
{
private SerialBuffer ComBuffer;
private InputStream ComPort;
/**
*
* Constructor
*
* @param SB The buffer to save the incoming messages.
* @param Port The InputStream from the specific serial port.
*
*/
public ReadSerial(SerialBuffer SB, InputStream Port)
{
ComBuffer = SB;
ComPort = Port;
}
public void run()
{
int c;
try
{
while (true)
{
c = ComPort.read();
ComBuffer.PutChar(c);
}
} catch (IOException e) {}
}
}
4. SerialExample
SerialExample是本類庫(kù)所提供的一個(gè)例程。它所實(shí)現(xiàn)的功能是打開(kāi)串口COM1，對(duì)其進(jìn)行初始化，從串口讀取信息對(duì)其進(jìn)行處理后將處理結(jié)果發(fā)送到串口。
import serial.*;
import java.io.*;
/**
*
* This is an example of how to use the SerialBean. It opens COM1 and reads
* six messages with different length form the serial port.
*
*/
class SerialExample
{
public static void main(String[] args)
{
//TO DO: Add your JAVA codes here
SerialBean SB = new SerialBean(1);
String Msg;
SB.Initialize();
for (int i = 5; i <= 10; i++)
{
Msg = SB.ReadPort(i);
SB.WritePort("Reply: " + Msg);
}
SB.ClosePort();
}
}
5. 編譯與調(diào)試
本類庫(kù)中使用了Java Communication API (javax.comm)。這是一個(gè)Java擴(kuò)展類庫(kù)，并不包括在標(biāo)準(zhǔn)的Java SDK當(dāng)中。如果你尚未安裝這個(gè)擴(kuò)展類庫(kù)的話，你應(yīng)該從Sun公司的Java站點(diǎn)下載這個(gè)類庫(kù)并將其安裝在你的系統(tǒng)上。在所下載的包里面包括一個(gè)安裝說(shuō)明，如果你沒(méi)有正確安裝這個(gè)類庫(kù)及其運(yùn)行環(huán)境的話，運(yùn)行這個(gè)程序的時(shí)候你會(huì)找不到串口。
正確安裝Java Communication API并將上述程序編譯通過(guò)以后，你可以按如下方法測(cè)試這個(gè)程序。如果你只有一臺(tái)機(jī)器，你可以利用一條RS-232電纜將COM1和COM2連接起來(lái)，在COM1上運(yùn)行SerialExample，在COM2上運(yùn)行Windows提供的超級(jí)終端程序。如果你有兩臺(tái)機(jī)器的話，你可以利用一條RS-232電纜將兩臺(tái)機(jī)器的COM1(或者是COM2)連接起來(lái)，在一端運(yùn)行例程，另外一端運(yùn)行Windows提供的超級(jí)終端程序。如果有必要的話，可以對(duì)SerialExample中所聲明的串口進(jìn)行相應(yīng)改動(dòng)。
本程序在Windows 2000 + Java SDK 1.3環(huán)境下編譯通過(guò)并成功運(yùn)行。
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