In order to improve accessibility of the IEC 61131-3 virtual machine for external developers, I have committed code which provides a basic Modbus TCP protocol interface for the IEC 61131-3 virtual machine. The protocol was originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs). Since this time, this protocol has become the de facto standard for communication with a range of industrial devices including programmable logic controllers, data loggers and smart instrumentation. See http://www.modbus.org/ and http://en.wikipedia.org/wiki/Modbus.
While this interface is still under development, the basic interface that is in place now allows for 16-bit integer values to be read from the virtual machine environment by external data visualisation packages – The package that I have used for evaluation purposes above is from http://thesz.diecru.eu/content/medford.php. This interface in turn provides a greater level flexibility for the monitoring of byte code under evaluation within the IEC 61131-3 virtual machine environment.