Hi,
Even though your initial setup works, this kind of intermittent failure usually points to TCP connection handling issues, especially with simulators or tunnelling tools like NetToPLCSim.
Here are the most probable root causes:
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NetToPLCSim TCP port hangs or closes after the idle timeout or multiple reconnects.
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Too many open/half-open connections between Kepware and the simulator.
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The Simulator network interface (PLCSim V15.1) may enter an unusual state due to limitations in NetToPLCSim.
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The Kepware S7 TCP/IP driver retries too quickly after a dropped session, causing repeated connection denials.
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Windows firewall or antivirus temporarily blocks or throttles simulated interfaces after suspicious traffic.
NetToPLCSim is not production-grade software. It’s best for testing but not for long runtime reliability. If possible, switch to PLCSim Advanced, which offers better support for Ethernet-based communication.
Thanks,