Frequent disconnection of analog data in Siemens PLC 485 communication

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The model is S7-200 226 series; it has 3 additional digital input modules and 4 additional analog input modules. When no analog input modules are connected, the 485 communication is carried out using the MODSCAN332 device. The digital inputs and outputs work properly, while all analog inputs show a value of 0, with no loss of connection. Once the analog input modules are connected, the digital inputs and outputs still function properly without any loss of connection. There are 16 analog inputs in total; the first 3 and the last 3 do not experience any loss of connection, while those in the middle lose connection frequently. The digital inputs, on the other hand, remain stable with no loss of connection. What’s the solution? ? ? ? ? ? ? ? ? ? ? ? ? The PLC suspects that the port is damaged; after replacing it with a new one, the same problem persists. The grounding has also been checked