The Java Communications (a.k.a. javax.comm) API is a proposed standard extension that enables authors of communications applications to write Java software that accesses communications ports in a platform-independent way. This API may be used to write terminal emulation software, fax software, smart-card reader software, and so on.
Developing good software usually means having some clearly defined interfaces. The high-level diagram of the API interface layers are shown in this figure.
In this article we will show you how to use javax.comm to communicate with a serial device based on RS-232. We'll also discuss what the javax.comm API provides and what it doesn't provide. We'll present a small example program that shows you how to communicate to the serial port using this API. At the end of the article we'll briefly detail how this javax.comm API will work with other device drivers, and we'll go over the requirements for performing a native port of this API to a specific OS.
Unlike classical drivers, which come with their own models of communication of asynchronous events, the javax.comm API provides an event-style interface based on the Java event model (java.awt.event package). Let's say we want to know if there is any new data sitting on the input buffer. We can find that out in two ways -- by polling or listening. With polling, the processor checks the buffer periodically to see if there is any new data in the buffer. With listening, the processor waits for an event to occur in the form of new data in the input buffer. As soon as new data arrives in the buffer, it sends a notification or event to the processor.
Among the various serial interfaces available, two of the most popular are the RS-232C and RS-422 standards, which define the electrical signal levels and the meaning of various signal lines. Low-speed serial interfaces typically clock data out as a square wave, with clock coordination provided by start and stop bits.
RS-232 stands for Recommend Standard 232; the C simply refers to the latest revision of the standard. The serial ports on most computers use a subset of the RS-232C standard. The full RS-232C standard specifies a 25-pin "D" connector, of which 22 pins are used. Most of these pins are not needed for normal PC communications, and indeed, most new PCs are equipped with male D-type connectors having only 9 pins. For more on RS-232, see the Resources section.
Note: For an understanding of what other drivers have done in the past, take a look at the Unix
termio manual page or OpenBSD Unix, a variation of the BSD Unix driver source. This is available free on the Internet. Please see the Resources section for more information.
The javax.comm API: What is provided
The javax.comm API provides the following functionality to developers:
A complete API specification for serial and parallel communication ports. (In this article we consider serial ports only.) Without a common API in your development efforts, workload will increase because you'll have to provide support to serial devices.
Full control of all serial framing parameters (baud stop bits, parity, bits/frame) as well as manual or automatic control of the flow control lines. Normally, in RS-232, there are two signal lines and the rest are intended for control lines. Depending on the type of communication (synchronous or asynchronous), the number of control lines selected may vary. This API provides access to the underlying control signals.
A brief diversion here may help you understand something about parity and start and stop bits. Parity was added to RS-232 because communication lines can be noisy. Let's say we send ASCII 0, which in hex equals 0x30 (or 00110000 in binary), but along the way someone passes by holding a magnet, causing one of the bits to change. As a result, instead of sending 8 bits as intended, an additional bit is added to the first string of bits sent, making the sum total of bits sent even or odd. voilà! You've got parity.
Start and stop bits were added to the serial communication protocol to allow the receivers to synchronize on the characters being sent. One-bit parity does not allow error correction -- only detection. Solutions to this problem come from protocols that are layered on top of the serial APIs. Most serial communication these days uses block protocols with checksums (a mathematical function that can be generated on the receiver and compared to the transmitted checksum) that allow errors to be detected on larger groups of bits. When you are communicating with your ISP over PPP, packets may be 128 bytes per packet with a checksum. If they match, you are 99.999% sure the data is okay.
There are cases wherein this scheme doesn't work. For example, when sending critical commands to devices that are very far out in the solar system, forward correcting protocols can be used. Forward correcting protocols are needed because there may be no time for a retransmission, and space has a lot of electromagnetic noise.
Okay, back to the list of functionalities provided by the javax.comm API!
The basic I/O via a subclass of Java IO streams. For input and output, the javax.comm API uses streams; the concept of streams should be familiar to all Java programmers. It is important to reuse Java concepts when building new functionality or the APIs will become unwieldy.
Streams that can be extended to provide client flow control and threshold controls. For example, you may want an alert when there are 10 characters in the buffer or when there are only 10 locations left for characters. Flow control is important when the two devices connected via an interface cannot keep up with each other. Without flow control, you can have overruns or underruns. In the overrun condition, you received data before it was processed so it was lost; in the underrun, you were ready for data but it was not available. Usually these conditions occur at the USART (Universal Synchronous Asynchronous Receiver Transmitter), which is hardware that converts bytes to a serial wave form with timing to match the baud rate.
The javax.comm API uses the Java event model to provide notification of various signal line changes as well as buffer status. State changes refer to well-defined signals specified in the RS-232 standard. For example, carrier detect is used by a modem to signal that it has made a connection with another modem, or it has detected a carrier tone. Making the connection or detecting a carrier tone is an event. Event detection and notification of changes is implemented in this API.
What is not provided
The javax.comm API does not provide:
Line discipline type processing, dialer management, or modem management. Line discipline refers to additional processing of input or output characters. For example, one common post-processing option is the conversion of CR to CR LF. These terms have their origins in the early days of teletypes. CR (carriage return) means to simple-return the carriage to the left margin; in the Arabic world, this would be the right margin. LF (line feed) advances the printing area up by one. When bitmap screens and laser printers came along, these terms became less important.
Dialer management and modem management are additional applications that can be written using the javax.comm API. Dialer management typically provides an interface to the modem management's AT command interface. Almost all modems have an AT command interface. This interface is documented in modem manuals.
Perhaps a little example will make this concept clear. Suppose we have a modem on COM1 and we want to dial a phone number. A Java dialer management application will query for the phone number and interrogate the modem. These commands are carried by javax.comm, which does no interpretation. To dial the number 918003210288, for example, the dialer management probably sends an "AT," hoping to get back an "OK," followed by ATDT918003210288. One of the most important tasks of dialer management and modem management is to deal with errors and timeouts.
GUI for serial port management. Normally, serial ports have a dialog box that configures the serial ports, allowing users to set parameters such as baud rate, parity, and so on. The following diagram depicts the objects involved in reading and/or writing data to a serial port from Java.
- Support for X, Y, and Z modem protocols. These protocols provide support error detection and correction.
The programming basics
Too often, programmers dive right into a project and code interactively with an API on the screen without giving any thought to the problem they are trying to solve. To avoid confusion and potential problems, gather the following information before you start a project. Remember, programming devices usually requires that you consult a manual.
Get the manual for the device and read the section on the RS-232 interface and RS-232 protocol. Most devices have a protocol that must be followed. This protocol will be carried by the javax.comm API and delivered to the device. The device will decode the protocol, and you will have to pay close attention to sending data back and forth. Not getting the initial set-up correct can mean your application won't start, so take the time to test things out with a simple application. In other words, create an application that can simply write data onto the serial port and then read data from the serial port using the javax.comm API.
Try to get some code samples from the manufacturer. Even if they are in another language, these examples can be quite useful.
Find and code the smallest example you can to verify that you can communicate with the device. In the case of serial devices, this can be very painful -- you send data to a device connected to the serial port and nothing happens. This is often the result of incorrect conditioning of the line. The number one rule of device programming (unless you are writing a device driver) is to make sure you can communicate with the device. Do this by finding the simplest thing you can do with your device and getting that to work.
- If the protocol is very complicated, consider getting some RS-232 line analyzer software. This software allows you to look at the data moving between the two devices on the RS-232 connection without interfering with the transmission.
Using the javax.comm API successfully in an application requires you to provide some type of interface to the device protocol using the serial API as the transport mechanism. In other words, with the exception of the simplest devices, there is usually another layer required to format the data for the device. Of course the simplest protocol is "vanilla" -- meaning there is no protocol. You send and receive data with no interpretation.
Overview of suggested steps for using javax.comm
In addition to providing a protocol, the ISO layering model used for TCP/IP also applies here in that we have an electrical layer, followed by a very simple byte transport layer. On top of this byte transport layer you could put your transport layer. For example, your PPP stack could use the javax.comm API to transfer bytes back and forth to the modem. The role of the javax.comm layer is quite small when looked at in this context:
Give the javax.comm API control of some of the devices. Before you use a device, the javax.comm API has to know about it.
Open the device and condition the line. You may have a device that requires a baud rate of 115 kilobits with no parity.
Write some data and/or read data following whatever protocol the device you are communicating with requires. For example, if you connect to a printer, you may have to send a special code to start the printer and/or end the job. Some PostScript printers require you to end the job by sending CTRL-D 0x03.
- Close the port.
Initializing the javax.comm API registry with serial interface ports
The javax.comm API can only manage ports that it is aware of. The latest version of the API does not require any ports to be initialized. On start-up, the javax.comm API scans for ports on the particular host and adds them automatically.
You can initialize the serial ports your javax.comm API can use. For devices that do not follow the standard naming convention, you can add them explicitly using the code segment below.
// Register the device CommPort ttya = new javax.comm.solaris.SolarisSerial("ttya","/dev/ttya"); CommPortIdentifier.addPort(ttya,CommPortIdentifier.PORT_SERIAL); CommPort ttyb = new javax.comm.solaris.SolarisSerial("ttyb","/dev/ttyb"); CommPortIdentifier.addPort(ttyb,CommPortIdentifier.PORT_SERIAL);
Opening and conditioning devices
This next code sample demonstrates how to add, condition, and open a device. Details on the specific method calls are in the API pages for javax.comm. This example sets the device called XYZSerialDevice to be accessible with name
GenericSerialReader. The device connected on this line has a baud rate of 9600, 1 stop bit, a character of 8 bits (yes, they can be smaller), and no parity. The result of all of this is to provide two streams -- one for reading and another for writing.