Controlling Ifos Using Jamod
Up to now I’ve been controlling my Ifos slaves with a simple Modbus master I wrote in Ruby. In preparation for the move to TINI, though, I thought I’d better investigate Modbus master code actually capable of running there.
jamod is the logical first choice, since it’s Java-based and fairly mature. TINI runs Java, and if jamod complies with the Modbus application protocol specification, everything should just work.
Before jumping straight to the TINI, though, I thought it best to get things working from the Mac—it’s just so much easier to develop there. Here’s my basic jamod-based test program:
import javax.comm.SerialPort;
import net.wimpi.modbus.Modbus;
import net.wimpi.modbus.facade.ModbusSerialMaster;
import net.wimpi.modbus.procimg.Register;
import net.wimpi.modbus.procimg.SimpleRegister;
import net.wimpi.modbus.util.SerialParameters;
/**
* Simple test to determine if jamod and Ifos speak the same language.
* Takes a port name, slave id, and a value to write to register 0.
*/
public class JamodTest
{
private static void printUsage()
{
System.out.println( "java -cp .:jamod.jar JamodTest <port> <slave> <value>" );
}
public static void main( String[] args )
{
ModbusSerialMaster msm = null;
String port = "/dev/ttyIfos";
int slave = 3;
int value = 1;
try
{
// Initialize some parameters from the command line.
if( 3 != args.length )
{
printUsage();
System.exit( 1 );
}
else
{
try
{
port = args[ 0 ];
slave = Integer.parseInt( args[ 1 ] );
value = Integer.parseInt( args[ 2 ] );
}
catch( NumberFormatException nfe )
{
nfe.printStackTrace();
printUsage();
System.exit( 1 );
}
}
// Get ready to initialize the hardware serial port and write a register.
Register reg = new SimpleRegister( value );
SerialParameters params = new SerialParameters( port,
19200,
SerialPort.FLOWCONTROL_NONE,
SerialPort.FLOWCONTROL_NONE,
SerialPort.DATABITS_7,
SerialPort.STOPBITS_1,
SerialPort.PARITY_EVEN,
false );
// Create a master in ASCII mode, attach it to the serial port, then
// write out the value. Since Ifos maps its registers directly to the
// LEDs, writing most non-zero values to register 0 will cause the first
// multicolor LED to light up.
params.setEncoding( Modbus.SERIAL_ENCODING_ASCII );
msm = new ModbusSerialMaster( params );
msm.connect();
msm.writeSingleRegister( slave, 0, reg );
}
catch( Exception e )
{
e.printStackTrace();
}
finally
{
// Make sure we relinquish control of the harware port.
if( null != msm )
msm.disconnect();
}
}
}Modbus in ASCII mode uses an LRC to verify packets, and using the program above I immediately discovered a problem in how I handled this checksum. The algorithm is dead simple, however, and I quickly verified my PIC assembler implementation produced the correct results; it turns out my interpretation of the spec was just wrong. I still think my interpretation is the correct one—someday I’ll list all the things Modbus does wrong.
After a quick-and-dirty (and temporary) update to ignore checksum errors, Ifos started responding to jamod! Checksums on return messages are still generated incorrectly, so jamod complains about them, but at least Ifos is processing the incoming messages. jamod actually behaves as though no response is received at all, which is a little worrisome (I would expect something like, “Bad Checksum,” or similar), but I’m keeping my fingers crossed that fixing the LRC will make everything rainbows and unicorns.