Modicon State Language

Modicon State Language was a successor to “ladder logic”, which had been used to program control systems since the 19th century.

Ladder logic is a system of drawing relays (one-bit sensors) and coils (one-bit actuators) in an appropriate sequence, enabling industrial machines to execute a series of actions to produce a new product from raw inputs.

Modicon was the inventor of the “Programmable Logic Controller” in the 1960s, adapting hardware relays to the computer age (if not yet completely digital).

MSL was a ground-up language design, a graphical state/transition programming system allowing control engineers to program machinery in a high-level language.

I wrote the runtime system and embedded interpreter for MSL, as well as a “control panel” Windows applet for downloading code to the machine controller.

After initial success, I replaced the interpreter with a native code generator.  Although the code was about 8 times larger, it also ran in about 8% of the time that the interpreter took.

Nicest of all for me, was that the code generator was just 112k of source code, in an obscure language called M4.  It fit the bill perfectly, accepting the stack-based machine codes used by the interpreter and putting out the corresponding 80×86 native code expansion.

MSL was ahead of its time and was superseded by the IEC 61131 series of specifications, as a result of the global market consolidation in industrial equipment.

The MSL baseball at right was an incentive for sales people to get out and “pitch” MSL.