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Factors Affecting Communication Speed between Siemens HMI and PLC
2023-12-08
Customers often complain that the communication speed between Siemens' panel and PLC is slow. In fact, in many cases, it is caused by improper settings. Some settings listed below will affect the communication speed between the panel and PLC.
1. Refresh cycle of variables
The minimum refresh cycle that the panel can support is 100ms for normal communication mode.
Is the shorter the setting, the faster the communication? When there are few variables, this is basically the case. But if there are many variables, it is not. Because the execution time of a large number of communication tasks far exceeds the refresh cycle, the communication will be blocked. The performance is that the communication speed will decrease instead. Therefore, the variable refresh cycle should be long and short according to the actual needs of the project.
For some special applications, such as direct keys. The panel can be configured as a DP slave. So as to ensure the fast transmission of some key data
2. Collection mode of variables
Variables have three acquisition modes, continuous cycle, cyclic use and according to command.
By default, we should set it to recycle. This means that only when a variable is used in the current screen, the variable will be refreshed and the communication load will be generated. If it is wrongly set as continuous cycle, the panel will constantly refresh the variable in the background regardless of whether the variable is used in the current screen, thus generating unnecessary communication load and reducing the overall communication speed of the panel.
A common application that is set to cyclic continuous mode is that this variable is configured with a value change event.
3. Communication baud rate
Of course, the faster the better, provided that the line quality is good, the interference is less or the grounding shielding measures are appropriate. When communication conditions are poor, high baud rate will lead to higher probability of communication failure, which will reduce the speed of successful communication. If the baud rate is reduced at this time, the probability of communication packet errors will be reduced, thus improving the communication speed.
4. Number of I/O fields in a single screen
Each panel will show the maximum number of objects allowed to be configured in a single screen in the performance parameters, such as the maximum number of fields, variables, and complex objects. These are limit parameters. In practical projects, it is not recommended to make these parameters reach the limit value, unless the project is relatively simple and there are few communication variables. Therefore, if possible, split the multi I/O domain pictures into sub pictures.