PLC programming special knowledge and convenient operation interpretation

Siemens PLC is classified according to the scale of control

Can be divided into mainframes, medium-sized machines and minicomputers.

Minicomputer: The control point of the minicomputer is generally within 256 points, which is suitable for single-machine control or small system control.

The Siemens minicomputer has S7-200: processing speed 0.8~1.2ms; memory 2k; digital 248 points; analog 35 channels.

Medium-sized machine: The control point of the medium-sized machine is generally no more than 2048 points. It can be used for direct control of the equipment. It can also monitor multiple lower-level programmable controllers. It is suitable for medium or large control systems.

Siemens medium-sized machines have S7-300: processing speed 0.8~1.2ms; memory 2k; digital 1024 points; analog 128 channels; network PROFIBUS; industrial Ethernet; MPI.

Mainframe: The control point of the mainframe is generally greater than 2048 points, which not only can complete more complicated arithmetic operations.

Control mode

System Settings Remote / Local / Manual button:

Remote: The system can only be automatically started/stopped by the host computer. The local device has priority control. In the program control, the device can be soft-operated/automatically switched by the host computer, and the soft-handed operation starts/stops.

Local: The system can only be automatically started/stopped through the touch screen. The local device has priority control. In the program control, the device can be soft-operated/automatically switched through the touch screen, and the soft-handed operation starts/stops.

Manual: When the manual control is performed, the host computer/touch screen fails, and the start/stop of the device can only be controlled manually.

Single device control

A single device must have soft hands/automatic switching and soft start/stop functions. When switching from automatic to soft hands, the device cannot be stopped. When switching from soft hands to automatic, the device starts/stops depending on Automatic program.

Single device

(Pumps, fans and other large equipment) must be rotated after 24 hours of operation, and must have accumulated running time. If the start/stop sequence is set by the upper computer, the operator will set it himself.

Unified program structure

OB1: main program

OB100: Initialization procedure (no main program call required)

OB35: 100ms (modifiable) interrupt (no main program call required), you can call the PID module;

OB80, OB82, OB85, OB86, OB87, OB121, OB122: Fault diagnostics module (no main program call required, no programming required)

FC1: System mode FC2: Input processing

FC3: Output processing FC4: Run processing

FC5: Stop processing FC6: Hand automatic switching

FC100: Used to create some subroutines that can be called cyclically

FC105: The system comes with an analog input subroutine (can be called cyclically)

FC106: The system comes with an analog output subroutine (can be called cyclically)

Modbus communication (CP341): FB7: P_RCV_RK, FB8: P_SND_RK

Communication CP340: FB2: P_RCV, FB3: P_SND

General PID: use FB41

Temperature and humidity PID: use FB58

If the block is duplicated with the system block, please avoid it.

data block

DB1: AI data, type: REAL, interface with the host computer

DB2: AO data, type: REAL, interface with the host computer

DB3: DI data, type: BOOL, interface with the host computer

DB4: DO data, type: BOOL, interface with the host computer

DB5: device running time and traffic accumulation, type: REAL, interface with the host computer;

DB6: alarm message, type: BOOL, interface with the host computer

DB7: Type: REAL, intermediate register

DB8: Type: INT, intermediate register

DB9: Type: WORD, intermediate register

DB10: Type: BOOL, intermediate register

DB11: Used later for communication with devices, such as MODBUS communication, etc.

DB100: used as background data block when calling FB block

M area: also as an intermediate variable

DP slave fault diagnosis

The DP slave must do fault diagnosis, fault alarm, and use FB125.

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