With regard to the impact on WinCC communication quality, we all talked about various methods of how to optimize WinCC configuration projects. Such as the number of process variables archived by WinCC, access speed, simplified script, etc. Now I analyze the communication quality from the perspective of several common protocol modes such as industrial Ethernet, PROFIBUS, MPI and PROFINET and the selection of transmission media such as twisted pair, optical fiber and coaxial cable. In addition, when the communication mode and quality are determined, it will also affect the communication quality. The specific analysis is as follows:
1。 Industrial Ethernet protocol and twisted pair medium are adopted – this is a popular and common way of WinCC communication at present, with high cost performance and good compatibility. It adopts the TCP / IP protocol used by the Internet, and can also connect to the Internet network. However, this communication mode adopts competitive transmission, conflict detection and carrier listening mechanism, and the rate is uncertain, so it can not achieve real-time performance. Depending on the current communication flow of the network, due to the use of twisted pair, the bandwidth is limited and the anti-interference ability is poor. Therefore, when communicating in this way, do not connect with the Internet network or the computer irrelevant to industrial control, which can not only reduce the network virus attack, but also reduce the network packet traffic.
In WinCC configuration project, the cyclic dynamic connection shall be minimized (for example, after too many control objects in the picture are connected with variables in PLC, they will make dynamic displacement movement, so as to achieve visual and intuitive effect), so as to reduce the communication burden between WinCC and PLC.
However, when WinCC works normally, the data packet flow is very large (that is, the TXD indicator on cp443-1 module is always on and not off at almost any time), which indicates that there are multiple WinCC servers communicating with one PLC, or one PLC communicating with another and multiple PLCs in the network, and the number of agreed communication bytes is also large. The solutions are as follows: 1. Optical fiber media can be used to improve the network bandwidth and improve the anti-interference ability of transmission; 2: Add a central archive server of WinCC to reduce the number of WinCC servers, which can greatly reduce the communication burden of PLC.
2. PROFIBUS protocol and PROFIBUS cable are adopted – this is the standard mode of Siemens industrial control network, which is adopted by most older Siemens industrial control users. PROFIBUS protocol is divided into DP, FMS and PA, among which DP protocol is more common. In this way, a CP5611 or cp5613 network card is required on the OS station with WinCC installed. At this time, after each WinCC monitoring station is opened, it will communicate as a DP remote slave station of PLC. However, when the WinCC monitoring station computer fails, or the CP5611 / cp5613 network card is broken and needs to be shut down for replacement or maintenance, it is very easy to interrupt the communication of other remote DP stations and affect the production. Therefore, the cables in this way should be connected into a ring network as far as possible.
Where production requires high reliability, FMS Protocol shall be adopted at this time. However, if FMS protocol is adopted, a PROFIBUS communication module (such as cp443-5 or cp342-5) shall be added to the PLC master station. In this way, each WinCC monitoring station communicates with the Profibus communication module, which physically separates the connection with the remote DP slave station, which not only improves the safety and reliability of production, but also reduces the number of remote DP slave stations, thus reducing the communication burden of the PLC CPU processing DP slave station. The communication with each WinCC monitoring station is completed by the Profibus communication module. However, this FMS protocol mode needs to add a PROFIBUS communication module (increasing hardware cost).
PA protocol is applicable to field intelligent equipment (such as the connection between some intelligent transmitters, intelligent sensors and PLC). In short, because the communication between Profibus master stations adopts token ring (token ring) rotation, the real-time performance of WinCC communication is better than that of industrial Ethernet, the communication time interval is relatively stable, the possibility of network being attacked by virus is small, and the network security is also higher than that of industrial Ethernet.
3. Adopt MPI protocol – this is a point-to-point connection mode, and the communication rate is only 187.5kbps, which is similar to serial communication. Moreover, WinCC can only communicate with a single PLC, so the speed is slow and the communication range is narrow. It is only applicable to the communication of a single field control equipment and local range.
However, at present, the modules of Siemens supporting PROFINET communication function are more expensive than those of industrial Ethernet communication function, and the market consumption is small. Therefore, if the management users do not need to control the field level equipment status in real time in the office, this communication method may not be necessary. The real-time status of field level equipment should be more controlled and handled by operators and engineers, which is more conducive to hierarchical management and equipment operation safety.
Responsible editor: LQ