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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 provides the following functionality to developers:
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 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.
The javax.comm API does not provide:
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.