Why, in the case of a networked SIMATIC S7, does the connection to the OP clear down when you call diagnostics functions with the PG or transfer blocks?
Each connection requires connection resources for the end point or for the transition point (e.g. CP) on the stations concerned. The number of connection resources is CPU/CP-specific. The behavior described above indicates a resources bottleneck. You might not even be able to go online at all with your PG. Check the number of existing communication connections. Does you CPU have sufficient connection resources? The relevant information is available in the manuals...
- Automation System S7-400 Module Specifications (Entry ID 1117740)
- CPU 31xC and CPU 31x, Technical Data (Entry ID: 12996906)
- SIMATIC PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP (Entry ID 8860591).
Communication problems with OPs due to lack of connection resources occurred in SIMATIC S7-300 usually with old CPUs, which on the one hand had little connection resources and on the other the connection resources could not yet be reserved for PG, OP and S7 Basic communication. A table of the CPUs with which you can reserve resources is included in Entry ID: 299124. Your problem might well be cleared by reserving. Otherwise you should upgrade to a new CPU firmware version.