PLC program issue
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How to enable reading variables from one PLC’s program block in another. It is one PLC that needs to read the constraints of another PLC.
The currently popular approach is to achieve this using fieldbus technology! Implementing this function using PROFIBUS-DP in SIEMENS S7-300/400 is quite simple!
Communication between PLCs to exchange data.
Use DP network communication or Ethernet communication to transfer the required interlock conditions from another PLC to your PLC. If real-time performance is required, it is recommended to use DP network communication. If it’s not that strict, Ethernet can be used. When establishing a communication link, it is advisable to include a heartbeat counter in the communication data in order to determine whether the communication with both PLCs is functioning properly. Even if the communication is lost, emergency actions can still be taken (such as calling the police, locking things down, or shutting off valves, etc.).
I’d like to introduce to you a new way of passing variables: MMS and CMS. You might want to take a look at them
Our company uses DP/DP cards for data transfer; one DP card is defined in each of the two PLCs, and the write address of one card corresponds to the read address of the other card, and vice versa. This method can enable communication between two sets of Siemens PLCs. One set in our unit is S7400, and the other is S700; communication works properly using this method.