In order to complete the control strategy,in order to replace the relay and enable users to complete the control system ladder diagram similar to the relay circuit,a set of control algorithm function blocks(or subroutines),called the command system,are compiled,which are solidified in the memory ROM.It can be called by the user when programming an application.The instruction system can be roughly divided into two categories,namely basic instructions and extended instructions.The instruction system of subdivided general PLC includes:basic instruction,timer/counter instruction,shift instruction,transfer instruction,comparison instruction,conversion instruction,BCD operation instruction,binary operation instruction,increment/decrement instruction,logical operation instruction,Special operation instructions,etc.,these instructions are mostly similar to assembly language.In addition,PLC has also improved sufficient internal resources such as timers,counters,internal relays,registers and storage areas,which brings great convenience to programming.
Windows-based programming language standard—IEC61131-3
Due to the differences in the instruction system and the user requirements of the programming methods of the products of various PLC manufacturers,in recent years IEC has formulated the Windows-based programming language standard IEC61131-3(IEC1131,the international standard for programmable controllers promulgated by IEC in 1993).Five programming languagesare specified:Instruction List(IL),Ladder Diagram(LD),Sequential Function Chart(SFC),Function Block Diagram(FBD),and Structured Text(ST).This includes two aspects of textual programming(IL,ST)and graphic programming(LD,FBD),while SFC can be used in both programming languages.The IEC Technical Committee(TC65)came in and carried out the IEC61499 project,extending IEC61131-3,which is a standard for the architecture of modular distributed systems interconnected through communication networks,and will improve IEC61131-3.This is a standardization document that the programmable logic control device based on digital technology moves towards openness at a high level,and it is a major trend in the development of PLC.
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