Digital sealing

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I'm trying to figure out a way to make an automatic digital sealing for a NX-controller.

Functionality: Each time I do a remote connect and/or change to the program on the PLC via VPN/Tosibox I want a counter to automatically increment. This counter is the digital sealing (checked by the authorities from time to time), and is displayed on a HMI. When the sealing breaks, the new value of the counter has to be reported.

I've been thinking about incrementing the counter each time the Ethernet port on the PLC registers a rising edge on online establishment. But this will not work if there is a power failure (then the counter will increment unitented), or someone by mistake unplugs and replug the Ethernet cable.

Antoher idea was to ping a specified IP-address, but there seems to be no PING-functionality.

I've also been thinking about using some program data (i.e. last modified), but it seems there's no system variables on the PLC that contains this kind of information.

Any ideas how to solve this in the PLC? Basically I want to know if the PLC program has been changed or if there a specified IP-address has been connected to the PLC-network.

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I replaced Tosibox model from Lock 100/150 to Lock 200/250. The 200/250 (and higher) has two digital outputs. These outputs can be set to active when VPN connection is active. Then I connected one of these outputs to an digital input on the PLC. Each time a rising edge occurs on PLC input -> increment counter.

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Thanks for posting the solution. This has been asked before.

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