The main serial port protocols include: MODBUS RTU communication protocol; PROFIBUS communication protocol; USS communication protocol; PPI communication protocol; MPI communication protocol; and free port.
The main Ethernet communication protocols include: MODBUS TCP/IP communication protocol; OPC communication protocol; ISO-ON-TCP communication protocol; UDP communication protocol; PROFINET communication protocol; and S7 protocol.
Based on specific models, they can be divided into two main categories: the S7-200 series and the S7-300/400 series.
The S7-200 series supports the following protocols: PPI, MPI, PROFIBUS, Ethernet, S7 protocol, AS-INTERFACE, USS, MODBUS, and freeport.
The S7-300/400 series supports protocols such as MPI, PROFIBUS, ETHERNET, ISO, ISO-ON-TCP, and MODBUS.
These two categories of protocols share the same physical transmission medium. For example, the S7 protocol can use DP or Ethernet as the transmission medium. The difference lies in the different configuration methods and procedures each protocol requires.
Below, we will show you several wireless communication examples under specific protocols to illustrate the working principles of the relevant protocols.
Siemens S7-1200 and DTD433F wireless Modbus communication
This is a self-organizing network wireless communication model based on the Modbus RTU protocol, with an S7-1200 PLC as the master station and a DTD433F as the slave station.
Test equipment and parameters:
1. Hardware environment setup
Master station equipment: Siemens S7-1200
Slave devices: 4 units of DTD433FC
Wireless data terminal (master station equipment) DTD434MC*1 unit
2. Test Parameters
Communication protocol: Modbus RTU protocol
Master-servant relationship: 1 master, 4 servants
Main station communication interface: RS485 interface
Slave interface: Analog signal
Weintek touchscreen communicates with two S7-200 SMART wireless PPI units.
The DTD434M wireless communication terminal, a dedicated wireless communication terminal for European and American PLCs, can stably and conveniently establish a wireless PPI network without changing network parameters or existing programs, directly replacing wired RS485 communication.
The Siemens S7-200 SMART is an outstanding PLC (Programmable Logic Controller) that has been widely used in various industrial automation fields due to its powerful functions and flexible configuration capabilities.
To better meet user needs, the Siemens S7-200 SMART not only has multiple communication interfaces, but also supports multiple communication protocols to achieve seamless integration with various devices and systems.
These communication protocols not only cover traditional fieldbus protocols, but also include some advanced Ethernet communication protocols, as well as some common industrial automation communication protocols.
This article will provide a detailed introduction to the protocols supported by Siemens S7-200 SMART, helping readers to better understand and apply this excellent PLC device.
The communication protocols supported by the 485 serial port include:
Modbus RTU Protocol: This protocol is a general-purpose standard protocol developed by Modicon and is widely used in small projects. The protocol uses a half-duplex data transmission mode, meaning it can only communicate with one device at a time, and sending and receiving data cannot occur simultaneously. A MODBUS communication network can have only one master station and a maximum of 31 slave stations. Programming software provides a MODBUS RTU communication library, making Modbus communication simple and convenient.
2. PPI protocol: This method was previously used for communication between Siemens 200 and 200 series devices. Now, this method can also be used for communication between the 200 SMART and the touchscreen, as well as for communication with the host computer.
3. USS Protocol: A proprietary communication protocol of Siemens, which can only be used for communication between Siemens PLCs and Siemens frequency converters, servo devices, etc.
Freeport Protocol: A protocol that is temporarily written by oneself when none of the above standard protocols are supported.
The communication protocols supported by the Ethernet communication interface include:
S7 Ethernet Protocol: An internal proprietary protocol developed by Siemens specifically for communication between S7 series products. The 200SMART programming software provides a wizard, making configuration simple and convenient.
Open Ethernet protocols TCP/IP, ISO, and UDP: These can communicate with other third-party devices. Libraries are provided in the programming software, but it is a two-way communication, meaning both parties need to write programs: one party writes a program to send data, and the other party writes a program to receive data. Programming utilizes the system-provided libraries, as shown in the figure below.
Modbus TCP Protocol: A Modbus protocol based on Ethernet ports, combining high speed and versatility. The software provides a free communication library, making Modbus TCP communication very simple and convenient. SMART can be used as both a client and a server.
4. PROFINET Protocol: A new generation of internationally recognized Ethernet communication protocol. SMART can be used as a controller and intelligent device. The software provides a PROFINET wizard.
The RS-232 interface can only perform one-to-one communication, and the transmission distance is short.
DP port: SMART can only be used as a slave station. You only need to add the module after the PLC and configure the module in the programming software. No program needs to be written on the SMART side.
The Siemens S7-200 SMART PLC supports a variety of communication protocols, providing not only flexibility and convenience but also paving new paths for innovation and development in industrial automation. By selecting the appropriate communication protocol, users can easily achieve seamless integration and efficient communication between devices, improving production efficiency, reducing costs, and achieving higher quality.
RS232: Can use three-core twisted pair cable, three-core shielded cable, etc.
RS485: Two-core twisted pair cable, two-core shielded cable, etc. can be used. In low-speed, short-distance, interference-free applications, ordinary twisted pair cable can be used. Conversely, in high-speed, long-distance transmission, impedance-matched (generally 120Ω) RS485 dedicated cable (STP-120Ω (for RS485 & CAN) one pair 18AWG) must be used. In environments with severe interference, armored twisted pair shielded cable (ASTP-120Ω (for RS485 & CAN) one pair 18AWG) should also be used.
Additional information: Since the RS232 transmission distance is only 15 meters, what is its purpose?
In fact, its applications are very wide; it can connect to various devices, such as for monitoring, upgrading or debugging other equipment. Its function is similar to USB, and as USB ports become more common, more conversion devices that convert USB to RS-232 or other interfaces will appear.
More RS-232 devices can be connected via the USB interface, which not only provides higher transmission speeds and true plug-and-play functionality, but also overcomes the limitation of USB interfaces in long-distance transmission (USB communication distance is limited to 5 meters).
RS485 is a physical interface, or simply put, hardware.
MODBUS is an international standard communication protocol used for exchanging data between devices from different manufacturers (generally for industrial use); a protocol can also be understood as the "language" mentioned above, or simply put, software.
Generally, two devices transmit data via the MODBUS protocol: the earliest hardware interface used was RS232 (which is the serial communication port on a regular computer); RS422 is also used, and RS485 is commonly used. This type of interface has a long transmission distance and is more commonly used in general industrial sites.
The MODBUS protocol is further divided into three modes: MODBUS RTU, MODBUS ASCII, and the later-developed MODBUS TCP.
The first two types (MODBUS RTU and MODBUS ASCII) use serial communication interfaces (RS232, RS422, RS485). MODBUS TCP, however, was developed to adapt to the current global trend of using Ethernet or the Internet to connect and transmit data. Therefore, the MODBUS TCP mode was created, and its hardware interface is the Ethernet port, which is the network port commonly used on computers.
Then we can understand the several communication protocols supported by the S7-200 PLC through a picture.
