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Useful information about the Siemens Simatic S7
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What is the Siemens Simatic S7?
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How are PLC controllers structured?
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Purchase criteria for PLC controllers
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What distinguishes the Siemens Simatic S7 controller?
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Which S7 system for what purpose?
What is the Siemens Simatic S7?
The Siemens Simatic S7 is a PLC controller. PLC stands for programmable logic controller. In the English-speaking area, these devices are known as Programmable Logic Controllers (PLC). A PLC controller is used to control and control a machine or plant, for example in industry for controlling conveyor belts. The controller is digitally programd to operate the machine to be controlled with software logic. A programmable logic controller thus works by means of software, while a conventional connection-programd controller (VPS) controls terminals hard-wired via relays and contactors. In contrast to the VPS, a PLC is very dynamic and can be individually adapted to the respective application scenario without having to convert it on the assembly side. A PLC has a fixed operating system and can load programs written by the user via a memory interface.
How are PLC controllers structured?
A PLC controller usually has inputs for sensors and outputs for the so-called actuators. The actuators are controlled via the CPU unit of the PLC controller based on the input values that the sensors supply to the PLC controller. For example, windows can be opened as soon as a certain temperature is reached, or conveyor belts can be started when a certain weight is applied. Analogue-to-digital converters (ADU) can be used to process signals from digital sensors as well as analogue sensors. Analogue output modules can in turn be controlled via digital-to-analogue converters (DAUs).
Today's PLC controllers are supplied with a supply voltage of 24 V DC. The CPU is the central computing unit of the PLC controller and comprises the necessary memory areas for firmware and data processing in addition to the processor.
Although there is a standard for PLC controllers, the IEC 61131, most PLC systems do not adhere strictly to it, so systems from different manufacturers are usually not compatible with each other.
A PLC controller usually has inputs for sensors and outputs for the so-called actuators. The actuators are controlled via the CPU unit of the PLC controller based on the input values that the sensors supply to the PLC controller. For example, windows can be opened as soon as a certain temperature is reached, or conveyor belts can be started when a certain weight is applied. Analogue-to-digital converters (ADU) can be used to process signals from digital sensors as well as analogue sensors. Analogue output modules can in turn be controlled via digital-to-analogue converters (DAUs).
Today's PLC controllers are supplied with a supply voltage of 24 V DC. The CPU is the central computing unit of the PLC controller and comprises the necessary memory areas for firmware and data processing in addition to the processor.
Although there is a standard for PLC controllers, the IEC 61131, most PLC systems do not adhere strictly to it, so systems from different manufacturers are usually not compatible with each other.
Purchase criteria for PLC controllers
Important features when purchasing a PLC controller are:
- Cycle time: This specifies the time for processing a cycle, including all communication tasks that occur. A cycle consists of the CPU startup routine, the reading in of all inputs and thus also of the data of the sensors present there, the serial processing of the program instructions, the writing of the program results into the memory and data areas of the controller and the control of the output modules or actuators. Normally, the CPU aborts a cycle if it takes too long, which can be the case, for example, if the program to be executed is poorly written.
- Response time: The response time specifies the duration between the change of an input signal and the reaction by changing the output signal. It therefore indicates how quickly the actuators connected to the outputs of the PLC controller can respond to newly measured values of the sensors.
- Bit memory area size: The bit memory area is used to store intermediate results. A bit memory contains data in the formats bit, byte, WORD or DWORD. The larger the area, the more data can be stored there. This affects the speed of the cycle processing and results in a lower cycle time for most programs.
- Usability of the HMI: HMI stands for Human Machine Interface in this context and is often implemented today in the form of a touch screen. The Simatic S7 system offers an advanced system with various panels with widescreen displays in sizes 4, 7, 9 and 12 inches. The HMI systems can be operated in combination with the help of keys as well as by touch input. Furthermore, mice, keyboards and bar code scanners can be connected via a USB interface and data can be transferred to a USB stick.
- Scalability of the PLC system: With its numerous expansion and control modules, Siemens offers a scalable system that can be mounted easily using a DIN rail system.
- Approvals of the PLC system: The fulfillment of national and international standards of the PLC system is becoming increasingly important. These include the standards cULus, FM, ATEX, CE as well as EN 61000-6-4, 60068-2-X and 61131-2
What distinguishes the Siemens Simatic S7 controller?
Software and spare parts
Siemens Simatic S7 is the most well-known and widely used PLC. In the older S7-200 series, the controller is delivered with the software Step7MicroWin, in the S7-300 and S7-400 series with the software Step7, which is now common. The successor of the S7-300 series, which is still available until 2020, is already available with the S7-1500 series. For S7-300, there will be a spare part backup for another ten years after the production stop.
Key features
From the S7-400 series, signal modules can be energized and inserted (hot swapping). Plant expansions and module replacement processes are thus possible during operation.
The Siemens S7 system is characterized by a very high capacity for I/O I/O, among other things because CPU performance can be dynamically scaled starting from the S/7-400 series. For this purpose, you can operate several CPUs in a single central unit. It is possible to allocate individual complex activities such as control, calculation or communication to individual CPUs in a dedicated manner, thereby achieving an optimization of the processor time. This results in better performance under high utilization.
In addition, efficient configuration and diagnostics of the programming work is possible thanks to the SIMATIC Engineering Tools. These offer various high-level languages such as SCL as well as graphical engineering tools for sequential controls, status graphs and technology plans.
With the Siemens own multipoint interface MPI (multipoint interface), it is also possible to overcome long distances in data communication. For this purpose, up to nine repeaters can be switched between two MPI nodes.
Programming
In contrast to the IEC 61131 standard, global data must be addressed by the Siemens Simatic S7 as a programr. Different memory areas are available for this purpose. Input and output signals are also integrated with operands I and A and their addresses, not by declaring variables to addresses, but by addressing the values in the process images PII and PPA. In contrast to PLC systems based on the IEC 61131 standard, the variable declaration in Step7 is therefore hardly important.
Which S7 system for what purpose?
- S7-300: For small control systems in the lower and middle power range. This includes, for example, textile machines, packaging machines, general machine and control engineering, machine tool construction, installation technology as well as applications of the electrical industry and in electrical trade.
- S7-400: For the middle and upper power range. The S7-400 series is characterized by the possibility of redundancy of the I/O I/O. This means, for example, that the failure of a CPU, a PROFIBUS or a signal module can be tolerated.
- S7-1500: This series has comprehensive integrated safety and security functions. This includes, for example, password-based know-how protection against unauthorized reading and modification of program blocks as well as copy protection against their unauthorized copying. In addition, this series provides for the first time an authorization concept with authorization levels as well as protection against manipulation of the engineering data. Ethernet connections can be secured by an integrated firewall and by establishing VPN connections. In addition, the system analysis is already integrated in the CPU in this series. This provides system information directly from the central unit and can then be read out on the HMI, TIA Portal, via display or via the integrated web server.