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Some things that might be useful IMHO:

The ladder program tagname, instruction comment and rung comment databases should be integrated into the PLC itself, rather than stored only on the PLC ladder programming application database (I've had to work on many machines that have had the orginal database file lost - what a major pain it is to try to reconstruct someone's old code from raw ladder and seriously out of date
electrical drawings!). Long tagnames with strange characters should also be supported - like "clampSolenoid1.open( )" for instance.

Many different types of data types should be supported in the ladder language (reals, shorts, longs, string, structures, arrays as well as the usual int and bool).

The machine integrator should have the ability to create their own special purpose ladder instructions from their own C code with going through undo coding hardships.

The ladder language should support many ladder program tasks, each of which should have its own local variables (by alias, not hard data file reference!), its own data type (maybe a structure named the same as the task name) to support
identification of attributes and methods, and each task should be configurable as to its scan time and relative priority to other ladder tasks.

For HMI/SCADA functions on non-speed critical machinery/processes, how about providing the machine integrator the option of using a stripped down web server running java servlets on the PLC. Web browsers are everywhere and can make a
fair (and cheap!) HMI/data collection window into certain types of machines and processes.

On another note, it might be wise to keep in mind that the open source PLC being developed here could also be used for the PLC side of a CNC. This would require fair amounts of memory to be shared synchronously with the CNC at relatively fast speeds (5 to 10 milliseconds for at least a portion of the ladder). It would also require fairly sophisticated floating point ladder

A number of posts have mentioned that they are a little rusty on C coding to be able to support this project directly, but that they have been coding on the integrators side of things for some time. There could be a parallel effort to
develop a large library of open source ladder example code which would be provided with the open source ladder programming IDE. How many times have we coded up ladder to control a cylinder of one type or another? A little thought
to object definition and encapsulation would create some very useful code libraries. That would be a great feature for machine integrators.

Alan Locke
Controls engineer, Boeing

"My opinions are my own and not necessarily that of my employer"

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