OBA Developer s Guide, Doc.no: 3/ KRD Uen, Rev B

Transcription

1 2%$'HYHORSHU V *XLGH &RQWHQWV 1 Introduction Who Should Read this Document? Updates Since Last Revision Terminology Mobitex Network Information MASC/OBA Application Areas M3000 OBA API Accessible Interfaces Required Software Creating an Application Restrictions and Limitations Application Development Test of Application Supplied Batch Files Example Applications Radio Demo Serial Port Demo Binary Ports Demo Analog Port Demo...22 OBA Developer s Guide, Doc.no:

2 2 (23) OBA Developer s Guide &RS\ULJKW Ericsson AB 2001, All rights reserved 0RELWH[ is a trademark owned by Ericsson AB 'LVFODLPHU 7KHFRQWHQWVRIWKLVGRFXPHQWDUHVXEMHFWWRUHYLVLRQZLWKRXWQRWLFHGXHWR FRQWLQXHGSURJUHVVLQPHWKRGRORJ\GHVLJQDQGPDQXIDFWXULQJ (ULFVVRQVKDOOKDYHQROLDELOLW\IRUDQ\HUURURUGDPDJHVRIDQ\NLQGUHVXOWLQJIURP WKHXVHRIWKLVGRFXPHQW

3 OBA Developer s Guide 3 (23),QWURGXFWLRQ This manual describes the concept of onboard applications for the M3000 series Mobitex radio modem and provides some background information on the Mobitex network. It also contains guidelines for creating applications, and step by step instructions for a number of example applications. :KR6KRXOG5HDGWKLV'RFXPHQW" This manual is intended for engineers who plan and develop onboard applications for the M3000 series Mobitex radio modem. It is assumed that the reader is familiar with Java programming as well as the basics of radio and telecommunications technology. 8SGDWHV6LQFH/DVW5HYLVLRQ The document has been updated to reflect the new software that support Java. A number of minor corrections and clarifications has also been made, as well as format and layout changes. 7HUPLQRORJ\ API AVL CLDC IDE Application Program Interface The specific method prescribed by a computer operating system or by another application program by which a developer can make requests of the operating system or another application. Automatic Vehicle Location Technology used for tracking vehicles and mobile assets such as containers. Connected Limited Device Configuration The Java 2 Micro Edition configuration best suited for the M3000 series Mobitex radio modem. Integrated Development Environment Development platform normally offering a source code editor and a compiler.

4 4 (23) OBA Developer s Guide I 2 C JVM M3000 OBA MASC MPAK OBA POS Terminal SDK Inter IC A bidirectional 2-wire bus for communication with general purpose circuits, for example, remote ports or data convertors. Java Virtual Machine An abstract computing machine reading and executing Java bytecode instructions. M3000 series Mobitex radio modem with Java support Mobitex Asynchronous Communication A protocol used for communication between a M3000 series Mobitex radio modem and a host. MASC is not normally used together with OBA. A data packet, containing a maximum of 512 bytes of user data, sent over the Mobitex network. Onboard Application A Java application executed on top of the radio modem code. Point of Sales Terminal Terminal for credit card payments. Software Developer s Kit A set of programs used by a computer programmer to write application programs.

5 OBA Developer s Guide 5 (23) 0RELWH[1HWZRUN,QIRUPDWLRQ In Mobitex (see Figure 1), data and text messages can be transferred wireless. The system can, for example, provide field personnel with direct access to a centralized computer without any dispatcher involvement. Mobitex is a system primarily designed for dispatch traffic, which offers great potential for intercompany traffic and closed user groups. It provides nationwide coverage and keeps track of all mobile terminals so that calls and messages can be automatically routed to the correct radio base station. However, this also means that host/client systems demanding intensive or constant data exchange (offered by, for example, the Internet) will run into problems regarding efficiency and scaling, if used on a wireless network like Mobitex. The network is installed and operated by a network operator. It is specified as an open system and provides a transparent communications link between subscriber data terminals. Each terminal can be customized. By specifying his own terminal or the application software and using Mobitex as a link between them, the user can design his own communication system. )LJXUH 0RELWH[QHWZRUNZLWKIL[HGDQGPRELOHWHUPLQDOV A typical network consists of at least one main exchange (MHX), a number of area exchanges (MOX), radio base stations (BAS), fixed terminals (FST) and mobile terminals (MOB). The network is controlled and monitored via the NCC, Network Control Center.

6 6 (23) OBA Developer s Guide 0$6&2%$ This chapter describes the two different ways of using the M3000 series Mobitex radio modem, MASC and OBA. The M3000 series Mobitex radio modem may be used as a MASC modem (see Figure 2), using the MASC protocol to communicate with an external processor through the serial port. In this case, the external processor handles all data processing and the M3000 series Mobitex radio modem is used only as a radio modem. See the 0RELWH[,QWHUIDFH6SHFLILFDWLRQ0,6for further information regarding how to use the MASC protocol. The MIS can be downloaded from the Mobitex homepage, )LJXUH 0ZLWKWKH0$6&3URWRFRO This document, however, describes the OBA concept (see Figure 3), which is able to handle a number of utilities in addition to the basic radio function. In the two use cases MASC and OBA, the hardware (M3000) is the same, but the software differs. To implement OBA, the MASC functionality is removed and replaced by a JVM that makes it possible to write applications for downloading onto the radio modem. )LJXUH 0ZLWKWKH2%$XWLOLW\ A Java programmed M3000 OBA can run a number of applications, without the

7 OBA Developer s Guide 7 (23) need for an external processor. The same operations, such as measurements, performed with a MASC radio modem would require an external unit, for instance a PC, for processing data. Thus the OBA concept offers a number of benefits compared to MASC: In most cases less equipment is needed, which decreases costs. Less power is needed compared to using an external processor. Start-up-time is shortened, since the developer will not have to learn and implement MASC. A number of applications can be fitted into physically smaller areas. This facilitates integration in existing machines and vehicles. $SSOLFDWLRQ$UHDV The M3000 OBA has a wide range of application areas. Listed below are some examples of them. AVL Fleet Management POS terminals Telematic solutions Vendor Machines System for locating and monitoring vehicles and returning vital information about the vehicle. Advanced vehicle systems, used for transport logistics, etc. Handling of credit card payments on delivery routes or in shopping centres. Solutions for different kinds of teleoperating device. Alarm systems for service and support on stationary machines, such as soda machines, copying machines, etc.

8 8 (23) OBA Developer s Guide 02%$ $3, The M3000 OBA API gives access to the interfaces described in ³$FFHVVLEOH,QWHUIDFHV, as well as to useful radio and Mobitex network parameters. All the interfaces are accessed through a framework called the Generic Connection Framework which supports input/output and networking in a generalized, extensible fashion. On the implementation level this means that a static method in the system class Connector is used to access the interfaces, using a single string parameter. See the API documentation for a complete description of the M3000 OBA API classes. $FFHVVLEOH,QWHUIDFHV This section describes the different interfaces which can be accessed and handled via the M3000 OBA API. See the 0RELWH[05DGLR0RGHP,QWHJUDWRU V0DQXDO for information regarding electrical specifications.,qwhuidfh Two digital inputs Three digital outputs One analog input One analog output One asynchronous serial port One radio interface 'HVFULSWLRQ One of these can be used as event-controlled input. The inputs can also be set to be outputs. Two of these can handle more current than the other digital output. This has 8-bit resolution. This has 8-bit resolution. For connection of serial units such as GPS, PC, bar-code readers, etc. For sending and receiving the different types of messages Mobitex offers.

9 OBA Developer s Guide 9 (23) 5HTXLUHG6RIWZDUH In order to develop onboard applications for the M3000 OBA, the following software is required: Ericsson M3000 API Can be downloaded from Select Product Portfolio. MobiOne This is an Ericsson program for downloading of firmware files, Java byte-code and configuration data for a number of radio modems (radio modems for Mobitex). See the 0REL2QH8VHU V*XLGH for more information. MobiOne can be downloaded free of charge from Sun s Java 2 SDK, Standard Edition, v 1.3 May be downloaded from Sun s Preverify tool The M3000 Integrator s Kit, includes all necessary software according to the list above.

10 10 (23) OBA Developer s Guide &UHDWLQJDQ$SSOLFDWLRQ This chapter will provide you with general information about how to create an application on your M3000 OBA. The M3000 series Mobitex radio modem has an embedded JVM which enables it to execute Java onboard applications. The Java 2 technology (see Figure 4) is divided into three editions: Java 2 Enterprise Edition (J2EE), Java 2 Standard Edition (J2SE), Java 2 Micro Edition (J2ME) J2Me addresses embedded and consumer devices, and is in turn divided into two configurations; Connected Device Configuration (CDC) and Connected Limited Device Configuration (CLDC), where CLDC addresses devices such as cell phones, pagers and PDA s. The M3000 OBA supports J2ME CLDC. )LJXUH -DYDHGLWLRQVDQGFRQILJXUDWLRQV In order to start developing onboard applications, the software in ³5HTXLUHG 6RIWZDUH must be installed. If using the M3000 Integrator s Kit, install the software according to the instructions on the enclosed CD. Before starting to write applications, see ³5HVWULFWLRQVDQG/LPLWDWLRQV. In the development instructions, it is assumed that the reader has access to a development board, see Figure 5. If not, see the,qwhjudwru V0DQXDO06HULHV 0RELWH[5DGLR0RGHP for instructions on handling the M3000 OBA.

11 OBA Developer s Guide 11 (23) /RESET /WAKE SERIAL RS 232 /WAKE ON OFF /DOWN- LOAD ON OFF MOUNT M3000-MODEM HERE! SWITCH BETWEEN 6-9 OR 12VDC 12 VDC INPUT SUPPLY - 6 TO 9 VDC + SDA SCL I2C OBA OnBoard Application Area VDD GND PARALLEL I/O-CONNECTOR EXTERN INPUT BIN1 IN BIN1_IN EXT INT INPUT SOURCE AD_IN INT INPUT AD_OUT LOW HIGH EXTERN INPUT BIN2_IN BIN3_OUT BIN2_IN EXT INT INPUTSO BIN4_OUT INT INPUT LOW HIGH DCD ANTENNA- CORD HOLDER 48/LZE R1 )LJXUH 'HYHORSPHQWERDUG In the lower right-hand corner of the development board, there is an area which is called the OBA OnBoard Application Area. This section contains a drawn-out access points for the interfaces which can be controlled/monitored by onboard applications. These are as follows: 3,1180%(5,16<67(0 &217$&7$1'1$0( 02%$$3,&211(&725 1$0( 12 BIN1_IN binary:1 9 BIN2_IN binary:2 10 BIN3_OUT binary:3 7 BIN4_OUT binary:4 19 DCD binary:5 2 AD_IN analog:1 3 AD_OUT analog:1 11 VDD - 1 GND - The D-SUB connector located in the same area contains the same connections. See the,qwhjudwru V0DQXDO06HULHV0RELWH[5DGLR0RGHP for information regarding the pin configuration of the D-SUB. If you are using a serial port in an OBA, connect your equipment to the D-SUB connector on the top left-hand side of the board. See the,qwhjudwru V0DQXDO06HULHV0RELWH[5DGLR0RGHP, for further

12 12 (23) OBA Developer s Guide information regarding electrical specifications. 5HVWULFWLRQVDQG/LPLWDWLRQV General information regarding the M3000 OBA development: Approximately 1 MB flash memory is available for the application (excluding the CLDC and M3000 libraries). Approximately 64 KB RAM is available. Only one application at the time can be stored in the M3000 OBA. The application is automatically started when the radio modem is powered on and activated. From the application developer s point of view, the J2ME CLDC differs from the J2EE and the J2SE in a number of areas. Observe that J2ME CLDC has, for example: No floating point support No user-defined class loaders No Java native interface No reflection A smaller class library Observe these limitations when creating an application for the M3000 OBA. See the CLDC specification for further information. Another restriction has been added mainly in order to minimize start-up time: The class holding thesxeolfvwdwlfyrlgpdlq6wulqj>@dujvmethod must be named 0DLQ&ODVV.

13 OBA Developer s Guide 13 (23) $SSOLFDWLRQ'HYHORSPHQW This section describes a development process, step by step, from editing the source code, until running the created application. The following description explains the necessary steps of the process. However, it may be even easier to utilize the batch files supplied, and follow the instructions in ³6XSSOLHG%DWFK)LOHV, when developing. (GLWWKHVRXUFHFRGH The Java source code can be edited in any editor or IDE. &RPSLOHWKHVRXUFHFRGH Use a Java 2 compatible compiler, for instance the Java compiler included in Sun s Java 2 SDK, Standard Edition. Set the compile options FODVVSDWK and ERRWFODVVSDWK to PMDU. This includes the Ericsson M3000 API classes as well as the classes defined in J2ME CLDC. Note! The syntax for setting compiler options may vary between different compilers. 3UHYHULI\WKHFODVVHV Preverify all classes that you intend to run on the M3000 OBA, by running them through the Preverifier tool from Sun. This is necessary due to security reasons. Instructions on how to use the tool are available at There is also information in the enclosed batch files which are described in ³6XSSOLHG%DWFK )LOHV. -DUWKHSUHYHULILHGFODVVHV Place the preverified classes in a single jar called FODVVHVMDU. You can use, for instance, the jar tool included in Sun s Java 2 SDK, Standard Edition. 'RZQORDGWR02%$ Set the M3000 download switch to on (see Figure 5). Restart the radio modem. Download FODVVHVMDU to the M3000 OBA by using the MobiOne tool. Instructions can be found in the 0REL2QH8VHU V*XLGH. 5XQWKHDSSOLFDWLRQ Set the M3000 download switch to off and restart the radio modem (see Figure 5). The application is now running.

14 14 (23) OBA Developer s Guide 7HVWRI$SSOLFDWLRQ This section describes how to test an onboard application using the development board in Figure 5. We will use the example application in &KDSWHU%LQDU\ 3RUWVGHPR. When the onboard application is downloaded and the radio modem is activated according to the description in,qwhjudwru V0DQXDO06HULHV0RELWH[5DGLR 0RGHP,the application is tested in the following way. 1. Set the switch INPUTSOURCE in the BIN1_IN area to INT 2. Set the switch INT INPUT in the same area to LOW. The diode at the BIN3_OUT will now go out as BIN1_IN is being fed. BIN3_OUT in this application will only follow the input signal to BIN1_IN. 3. Set the switch INT INPUT to HIGH. The diode at the BIN3_OUT will now light up which, according to the application logic, means that the input signal to BIN1_IN is high. It is also possible to set the switch INPUTSOURCE to EXT and feed HIGH (+3.3 VDC) or LOW (0 VDC) signal level the pin EXTERN INPUT to BIN1_IN. The output on the BIN3 diode will be the same. It is also possible to read the level at BIN3_OUT by connecting, for example, a voltmeter between one of the pins at BIN3_OUT and the ground to the pins at GND. These tests can, of course, also be performed without having access to a development board. The user can check how the connection to the system contact occurs in this instance by referring to the,qwhjudwru V0DQXDO06HULHV 0RELWH[5DGLR0RGHP. 6XSSOLHG%DWFK)LOHV For convenience, three batch files are supplied, that perform some of the steps in ³$SSOLFDWLRQ'HYHORSPHQW. The batch files require Java 2 SDK v.1.3 to be installed in C:\jdk1.3\. Starting from these batch files, most IDEs can be suited to develop for M3000 OBA. &RPSLOHEDW - compiles the source files in.\source\ and outputs to.\classes\ 3UHYHULI\EDW - preverifies the class files in.\classes\ and outputs to.\preverified\ -DUEDW - adds the classes in.\preverified\ to a jar file and outputs to classes.jar

15 OBA Developer s Guide 15 (23) Thus, in order to develop an application, perform these simple actions: Place your source files (.java) in.\source\ Execute &RPSLOHEDW Execute 3UHYHULI\EDW Execute -DUEDW Download and run your application. After each executed batch file, ensure that there are no error messages. Compilation errors after the first batch file must be corrected before the next batch files are run. ([DPSOH$SSOLFDWLRQV In this section you will find a couple of example applications, provided to be of help when developing your own onboard applications. Note! The numbering is not a part of the code. The number and the dot are added in order to explain the meaning and function of each line. The table in the end of each section contains more information. Some lines are broken in the code, for example, line 14 in 6HULDOSRUWGHPR and line 13 and 14 in %LQDU\SRUWVGHPR, due to lack of space. These lines are meant to continue on the original row with no space in between.

16 16 (23) OBA Developer s Guide 5DGLR'HPR This application shows a simple use of the M3000 OBA capabilities to connect to the Mobitex network by radio. When the application receives a status message, it adds 1 to the status byte and sends it back to the sender. ([DPSOH If a status message with the status byte set to 65 is received via the radio, a radio message with status byte set to 66 will be returned to the sender. 1. import java.io.*; 2. import javax.microedition.io.*; 3. import se.ericsson.erv.mobitex.modem.*; 4. import se.ericsson.erv.mobitex.modem.io.*; 5. public class MainClass implements RadioListener { 6. private MobitexConnection mc; 7. private Mpak mpak = new Mpak(); 8. public static void main(string args[]) { 9. new MainClass(); 10. } 11. public MainClass() { 12. try { 13. mc = (MobitexConnection) Connector.open("mobitex:"); 14. } catch (IOException ioe) {}

17 OBA Developer s Guide 17 (23) 7DEOH$SSOLFDWLRQ'HVFULSWLRQ /LQH 'HVFULSWLRQ 1-4 The necessary packages are imported. 5 The class MainClass is defined. The class implements the interface RadioListener, which contains the declarations of the methods that are invoked when radio events occur. 8 The main() method. This is where the application starts. 9 An object of the class MainClass is created. Execution continues in the objects constructor. 12 The code on line 13 can throw an IOException, so it has to be enclosed in a try block. 13 A connection according to the Generic Connection Framework is opened to the Mobitex network. 14 Exception handling code should be added here. 15 The current object is registered to listen for radio events. Only classes that implements RadioListener can register to listen for radio events. 17 The method messagereceived() is invoked when a message has been received. 18 Get the MPAK. 19 Check that it is a status message. 20 Set the addressee to the sender of the incoming message. 21 Set the status byte to the incoming status byte Send the MPAK. 25 The method messagesent() is invoked when the radio modem has sent a message. 26 The method outofnetworkcontact() is invoked when the radio modem loses network contact. 27 The method innetworkcontact() is invoked when the radio modem is in network contact.

18 18 (23) OBA Developer s Guide 6HULDO3RUW'HPR This application shows a simple use of the M3000 OBA serial port. The application reads in one byte from the serial port and writes incoming byte + 1 to the serial port. ([DPSOH If byte 65 is received on the serial port, byte 66 will be transmitted on the serial port. 1. import java.io.*; 2. import javax.microedition.io.*; 3. import se.ericsson.erv.mobitex.modem.*; 4. import se.ericsson.erv.mobitex.modem.io.*; 5. public class MainClass implements SerialPortListener { 6. private StreamConnection sc; 7. private InputStream is; 8. private OutputStream os; 9. public static void main(string args[]) { 10. new MainClass(); 11. } 12. public MainClass() { 13. try { 14. sc = (StreamConnection) Connector.open("comm:0;baudrate=9600, parity=even,bitsperchar=7"); 15. is = sc.openinputstream(); 16. os = sc.openoutputstream(); 17. } catch (IOException ioe) {} 18. SerialPort.addEventListener(this); 19. } 20. public void serialportdataavailable() { 21. int i; 22. try { 23. while (is.available() > 0) { 24. i = is.read(); 25. os.write(++i); 26. } 27. } catch (IOException ioe) {} 28. } 29. public void serialporttxempty() {} 30. public void serialportdtrchanged(boolean dtr) {} 31. public void serialporterror(int error) {} 32. }

19 OBA Developer s Guide 19 (23) $SSOLFDWLRQGHVFULSWLRQ /LQH 'HVFULSWLRQ 1-4 The necessary packages are imported. 5 The class MainClass is defined. The class implements the interface SerialPortListener, which contains the declarations of the methods that are invoked when serial port events occur. 9 The main() method. This is where the application starts. 10 An object of the class MainClass is created. Execution continues in the objects constructor. 13 The code on line can throw an IOException, so it has to be enclosed in a try block. 14 A connection according to the Generic Connection Framework is opened to the serial port. The connection is opened with baud rate = 9600, even parity and 7 databits. 15 An input stream is opened to the connection that enables reading from the serial port. 16 An output stream is opened to the connection that enables writing to the serial port. 17 Exception handling code should be added here. 18 The current object is registered to listen for serial port events. Only classes that implements SerialPortListener can register to listen for serial port events. 20 The method serialportdataavailable() is invoked when there is incoming data available at the serial port. 22 The code on line can throw an IOException, so it has to be enclosed in a try block. 23 Loop while there is more data available at the serial port. 24 Read 1 byte from the serial port. 25 Write the incoming byte + 1 to the serial port. 27 Exception handling code should be added here. 29 The method serialporttxempty() is invoked when the serial port output queue is empty. 30 The method serialportdtrchanged(boolean dtr) is invoked when the DTR level changes. 31 The method serialporterror(int error) is invoked when a serial port error has occurred.

20 20 (23) OBA Developer s Guide %LQDU\3RUWV'HPR This application shows a simple use of the M3000 OBA binary ports. The application reads the level of binary input port one and sets binary output port three to the same level. 1. import java.io.*; 2. import javax.microedition.io.*; 3. import se.ericsson.erv.mobitex.modem.*; 4. import se.ericsson.erv.mobitex.modem.io.*; 5. public class MainClass implements BinaryPortListener { 6. private StreamConnection inconn, outconn; 7. private OutputStream os; 8. public static void main(string args[]) { 9. new MainClass(); 10. } 11. public MainClass() { 12. try { 13. inconn = (StreamConnection) Connector.open("binary:1; direction=in"); 14. outconn = (StreamConnection) Connector.open("binary:3; direction=out"); 15. os = outconn.openoutputstream(); 16. } catch (IOException ioe) {} 17. BinaryPorts.addEventListener(this, BinaryPorts.PORT_1); 18. } 19. public void binaryportlevelchanged(byte portnumber, byte level) { 20. if(portnumber == BinaryPorts.PORT_1) { 21. try { 22. os.write(level); 23. } catch(ioexception ioe) {} 24. } 25. } 26. }

21 OBA Developer s Guide 21 (23) $SSOLFDWLRQ'HVFULSWLRQ /LQH 'HVFULSWLRQ 1-4 The necessary packages are imported. 5 The class MainClass is defined. The class implements the interface BinaryPortListener, which contains the declarations of the methods that are invoked when binary port events occur. 8 The main() method. This is where the application starts. 9 An object of the class MainClass is created. Execution continues in the objects constructor. 12 The code on line can throw an IOException, so it has to be enclosed in a try block. 13 A connection according to the Generic Connection Framework is opened to binary port number one. It is opened as an input port. 14 A connection according to the Generic Connection Framework is opened to binary port number three. It is opened as an output port. 15 An output stream is opened to the connection that enables writing to binary port three. 16 Exception handling code should be added here. 17 The current object is registered to listen for events on binary port number one. Only classes that implement BinaryPortListener can register to listen for binary port events. 19 The method binaryportlevelchanged(byte portnumber, byte level) is invoked when the level on an input port changes. 20 Check that it is the level of port one that has changed. 21 The code on line 21 can throw an IOException, so it has to be enclosed in a try block. 22 Write the level of port one to port three. 23 Exception handling code should be added here.

22 22 (23) OBA Developer s Guide $QDORJ3RUW'HPR This application shows a simple use of the M3000 OBA analog ports. The application reads the level of the analog input port and sets the analog output port to the same level. 1. import java.io.*; 2. import javax.microedition.io.*; 3. import se.ericsson.erv.mobitex.modem.*; 4. import se.ericsson.erv.mobitex.modem.io.*; 5. public class MainClass { 6. private StreamConnection sc; 7. private InputStream is; 8. private OutputStream os; 9. public static void main(string args[]) { 10. MainClass mc = new MainClass(); 11. mc.startpolling(); 12. } 13. public MainClass() { 14. try { 15. sc = (StreamConnection) Connector.open("analog:1"); 16. is = sc.openinputstream(); 17. os = sc.openoutputstream(); 18. } catch (IOException ioe) {} 19. } 20. public void startpolling() { 21. while(true) { 22. try { 23. os.write(is.read()); 24. } catch(ioexception ioe) {} 25. } 26. } 27. }

23 OBA Developer s Guide 23 (23) $SSOLFDWLRQ'HVFULSWLRQ /LQH 'HVFULSWLRQ 1-4 The necessary packages are imported. 5 The class MainClass is defined. 9 The main() method. This is where the application starts. 10 An object of the class MainClass is created. Execution continues in the objects constructor. 11 The method startpolling() is invoked. 14 The code on line can throw an IOException, so it has to be enclosed in a try block. 15 A connection according to the Generic Connection Framework is opened to the analog ports. 16 An input stream is opened to the connection that enables reading from the analog input port. 17 An output stream is opened to the connection that enables writing to the analog output port. 18 Exception handling code should be added here. 21 The application loops forever. 22 The code on line 23 can throw an IOException, so it has to be enclosed in a try block. 23 Read the level of the analog input port and write the same level to the analog output port. 24 Exception handling code should be added here.