Accessing serial ports the easy way

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Every once in a while, I see people having a hard time accessing a RS232 or USB serial port from Java. There exist several solutions to do this in Java:

  • The Java Communications 3.0 API looks awfully old and unmaintained. It is available for Solaris SPARC, Solaris x86, and Linux x86.

  • RXTX is a mix between Java code and C code accessed through the Java native interface. It is hosted on a CVS server and it looks like the 2.2 release will never go out since it got stuck on version 2.2pre2 released in 2009. The last stable version is 2.1.7 from 2006.

  • PureJavaComm is a drop-in replacement for those two libraries, written in Java and using JNA to interface with the system. It is simpler to setup than RXTX, is hosted on GitHub and is actively maintained.

However, there exist at least one other solution which does not require the use of any external library. This is what I chose to interface a Scala program with a XBee Pro module through a serial interface to interact with my students devices. I also use it to interface the Factor programming language with the same XBee module.

Every language has a well-defined and well-maintained sockets library, right? So why not simply use socat, a multipurpose relay which is able to bridge many protocols and interfaces such as, in our case, TCP and a serial port?

I launch socat as

% socat TCP-LISTEN:4161,fork,reuseaddr FILE:/dev/ttyUSB0,b57600,raw

and what I immediately get is a TCP server listening onto port 4161 and ready to relay any incoming connection to the /dev/ttyUSB0 serial port with a 57600 baud rate. And not only do I have no more concern about accessing the serial port properly, but also I can access a port located on a remote computer as easily by launching socat there instead of locally.

But what if I want to spy on the TCP/serial relay to see that I send the right codes to the XBee module? socat offers you a choice of command-line options to dump the data in various formats.

What does the Scala interface look like? I have a XBee abstract class lacking an InputStream to receive the input from the XBee module and an OutputStream to send the output to it. This class is extended into a concrete class using simply:

import java.net.Socket

class TCPXBee(host: String, port: Int) extends XBee {

  private val socket: Socket = new Socket(host, port)

  override protected val inStream = socket.getInputStream
  override protected val outStream = socket.getOutputStream

  init()
}

socat makes my life easy. It is probably already packaged for your operating system, go and get it! Oh, and did I mention that it works with IPv6 too?

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