S7-200 Series: Modular (the main micro PLC is an integrated unit, expandable with modules). It is a small programmable logic controller (PLC) suitable for automation of detection, monitoring, and control in various industries and applications. The powerful functions of the S7-200 series enable it to perform complex control functions whether operating independently or networked. Therefore, the S7-200 series offers an extremely high performance/price ratio.
The Siemens S7-200 PLC features high speed, communication capabilities, and high productivity in real-time mode. Its consistent modular design facilitates the creation of low-performance customized products and scalable solutions. The Siemens S7-200 micro PLC can be used as a standalone micro PLC solution or in combination with other controllers.
The S7-200 series PLC offers eight CPUs across four different basic models. There are also six expansion units, which do not have their own CPU and must be connected to the basic units to expand the number of I/O points.
The dedicated programming software for the S7-200 series PLC is STEP7-Micro/WIN.
Detailed answers to 15 common questions about Siemens S7-200 PLC:
1. Why use a PC/PPI interface?
Because the S7200 CPU uses RS485 while the PC's COM port uses RS232, their electrical specifications are incompatible, requiring an intermediate circuit for matching. The PC/PPI cable is essentially an RS485/RS232 matching cable.
2. What are the advantages of transistor output versus relay output?
Transistors cannot drive AC 220V loads; they can only drive low-voltage DC. They have poor overload and overvoltage resistance. However, they can output at high frequencies, making them suitable for high-frequency applications such as pulse control. Relays can drive both AC 220V and DC loads. However, due to the inherent characteristics of relays, they cannot output at high frequencies. Furthermore, the switching life of relays is generally around 100,000 cycles. Therefore, transistors are also suitable for applications requiring frequent switching.
3. What is the communication distance of the communication port on the S7-200 CPU?
The S7-200 system manual provides a network segment distance of 50m, which represents the guaranteed communication distance under compliant network conditions. For distances exceeding 50m, repeaters should be added. Adding a repeater extends the communication network by 50 meters. If a pair of repeaters are added, and there are no S7-200 CPU stations between them (EM277 can be used), the distance between the repeaters can reach 1000 meters. Meeting these requirements ensures highly reliable communication. In reality, some users have achieved communication distances exceeding 50m without repeaters. Siemens cannot guarantee the success of such communication.
4. How to set the communication port parameters?
By default, the S7-200 CPU's communication port is in PPI slave mode, with an address of 2 and a communication rate of 9.6K . To change the communication port's address or communication rate, you must set it in the Communication Port tab of the system block, and then download the system block to the CPU for the new settings to take effect.
5. What if there aren't enough addresses in the M region?
Some users are accustomed to using the M area as an intermediate address, but the M area address space in the S7-200 CPU is very small, only 32 bytes, which is often insufficient. The S7-200 CPU provides a large V area storage space, i.e., the user data space. The V area storage is relatively large, and its usage is similar to the M area; data in the V area can be accessed bit by bit, byte by byte, word by word, or double word by double word. Examples include V10.1 , VB20, VW100, VD200, etc.
6. What are the long-distance communication methods of S7-200?
1) RS-485 network communication: PPI, MPI, and PROFIBUS-DP protocols can all communicate on an RS-485 network. With the addition of a repeater, the distance can reach up to 9600 meters.
2) Fiber Optic Communication: In addition to its anti-interference capabilities and high speed, fiber optic communication also boasts a long communication distance. S7-200 products do not directly support fiber optic communication; an additional fiber optic conversion module is required.
3) Telephone Network: The S7-200 supports telephone network communication via the EM241 audio modem module. The EM241 requires a standard audio telephone line as the communication endpoint, regardless of the inter-exchange communication method. Global communication is possible via the EM241.
4) Wireless communication: The communication distance of S7-200 through radio stations depends on factors such as the frequency, power, and antenna of the radio station; the communication distance of S7-200 through GSM networks depends on the coverage of the network service; the communication distance of S7-200 through infrared devices also depends on their specifications.
7. Which communication protocols supported by S7-200 are publicly available, and which are not?
1) PPI Agreement: An internal Siemens agreement, not publicly disclosed.
2) MPI Protocol: An internal Siemens protocol, not publicly disclosed.
3) S7 Agreement: An internal Siemens agreement, not publicly disclosed.
4) PROFIBUS-DP protocol: Standard protocol, publicly available
5) USS Protocol: Siemens' universal serial communication protocol for drives. For details, please refer to the manual of the corresponding drive. 6) MODBUS-RTU (Slave): Publicly available.
8. How to use the high-speed inputs and outputs of the S7-200?
The high-speed input and output terminals on the S7-200 CPU are wired in the same way as ordinary digital I/O. However, high-speed pulse output requires a CPU with DC transistor output (i.e., DC/DC/DC type).
9. Can the rotary encoder (and other sensors) with NPN/PNP output be connected to the S7-200 CPU?
Both are possible. The digital inputs on the S7-200 CPU and expansion modules can be connected to either source or sink sensor outputs; simply change the wiring of the common terminal accordingly.
10. Methods for mixing NPN and PNP sensors in a 200 PLC: As we all know, Japanese PLCs such as Mitsubishi and OMRON usually use NPN sensors when the common terminal is a + signal input.
European PLCs typically have a common terminal that is -, and most use PNP sensors for signal input. For example, the 200/300 series. When using a 200 PLC system, how do you solve the problem of providing both PNP and NPN sensors? Method 1: Use an intermediate relay to convert the NPN sensor input. Method 2: While it's common to connect all inputs [M] of the 200 PLC to 24V-, the 200 PLC can also accept - signals. Connect the 1M input to 24V+, and connect I0.0-0.7 to NPN sensors . Connect the 2M input to 24V-, and connect the PNP sensors to I1.0-1.7 . This allows for a mixed connection of NPN and PNP sensors within the PLC. The reason is simple: the 200 PLC supports both signal inputs, and internally uses bidirectional diodes with opto-isolation for signal transmission.
11. How do high-speed counters occupy output points?
High-speed counters occupy digital input points on the CPU as needed, based on their defined operating modes. Each counter occupies a fixed number of input points according to its operating mode. Input points not used in a certain mode can still be used as ordinary input points; input points occupied by counters (such as external reset) are still accessible in the user program.
12. Why does the high-speed counter fail to work properly?
The HDEF instruction must be called using the initial scan memory bit SM0.1 in the program, and it can only be called once. Calling it with SM0.0 or executing the HDEF instruction a second time will cause a runtime error, and the counter settings from the first HDEF instruction execution cannot be changed.
13. How does a high-speed counter address? Why can't the current count value be read from SMDx?
The current value can be read directly by addressing different high-speed counters using HC0; HC1; HC2; HC3; HC4; HC5, or the current value of the high-speed counter can be monitored directly by entering the above addresses in the status table. SMDx does not store the current value. The count value of the high-speed counter is a 32-bit signed integer.
14. How to reset a high-speed counter to 0?
A high-speed counter with an external reset mode is selected. When the external reset input signal is valid, the high-speed counter is reset to 0. It can also be reset using an internal program, which sets the high-speed counter to an updateable initial value and sets the initial value to 0. After executing the HSC instruction, the high-speed counter is reset to 0.
15. Why do the initial and preset values assigned to the high-speed counter not work, or why do they have unexpected effects?
High-speed counters can have their settings changed during initialization or operation, such as initial values and preset values. The operation steps should be: 1) Set the update options for the control byte. Set the corresponding control bit in the control byte to "1" for the setting data that needs to be updated; for settings that do not need to be changed, the corresponding control bit should not be set.
2) Then, send the required values into the initial and preset value control registers. 3) Execute the HSC instruction.
