The PLC Instruction Set, Explained Instruction by Instruction

Every PLC on the market executes broadly the same instruction set, because every PLC models the same two ancestors: the hardwired relay panel (contacts, coils, latches) and the small computer (move, compare, math, subroutines). What differs between vendors is packaging and vocabulary. Allen-Bradley's Studio 5000 gives the instructions three-letter mnemonics — XIC, OTE, JSR — that date back to the PLC-5. Siemens TIA Portal and the IEC 61131-3 world (CODESYS, OpenPLC, Beckhoff) mostly drop the mnemonics: a normally-open contact is just a contact, and timers and counters are standard function blocks with defined pins. Mitsubishi's GX Works keeps its own compact instruction words — LD, OUT, SET, RST, PLS.

That means the real skill is not memorizing names — it is knowing what each instruction does in terms of bits, edges and the scan cycle. An XIC and an IEC normally-open contact are the same test (is the bit 1?). An Allen-Bradley ONS, a Siemens P contact, an IEC R_TRIG and a Mitsubishi PLS are the same rising-edge detector. Learn the behavior once and every vendor's manual becomes a naming table.

Each instruction below gets its own page: what it does during the scan, a ladder diagram showing it in context, a timing diagram where behavior is stateful, the vendor naming table, two worked examples and the mistakes that actually bite people in the field. Every one of them can be dropped on a rung and run live in the free browser simulator.